Lin, Yuchun, and Leo Oey, February 2020: Global trends of sea surface gravity wave, wind and coastal wave set-up. Journal of Climate, 33(3), DOI:10.1175/JCLI-D-19-0347.1. Abstract
Assessing trends of sea surface wave, wind and coastal wave set-up is of considerable scientific and practical importance in view of recent and projected long-term sea level rise due to global warming. Here we analyze global significant wave height (SWH) and wind data from 1993 to 2015, and a wave model, to (i) calculate wave age and explain the causal, or the lack thereof, relationship between wave and wind trends; and (ii) estimate trends of coastal wave set-up and its contributions to secular trends of relative sea level at coastal locations around the world. We show in-phase, increasing SWH and wind trends in regions dominated by younger waves, and decreasing SWH trends where older waves dominate and are unrelated to the local wind trends. In central North Pacific where wave age is transitional, in-phase decreasing wave and wind trends are found over the west-northwestern region, but wave and wind trends are insignificantly correlated in the south-southeastern region; here, a reversed, upward momentum flux from wave to wind is postulated. We show that coastal wave set-up depends primarily on open-ocean SWH but only weakly on wind, varying approximately like SWH/(wind speed)1/5. The wave-setup trends are shown to be increasing along many coastlines where the local relative sea level trends are also increasing: the North & Irish Seas, Mediterranean Sea, East and South Asian seas, and eastern USA, exacerbating the potential for increased floods along these populated coastlines.
Huang, S-M, and Leo Oey, June 2019: Malay Archipelago forest loss to cash crops and urban contributes to weaken the Asian summer monsoon: an atmospheric modeling study. Journal of Climate, 32(11), DOI:10.1175/JCLI-D-18-0467.1. Abstract
In the Malay Archipelago (Indonesia and Malaysia), forest is lost on large scales to cash-crop plantation (oil palm, rubber and acacia, including fallow lands) and urban expansion. Deforestation changes land surface properties and fluxes, which modifies wind and rainfall. Despite the expansive land-cover change over a climatically sensitive region of the tropics, the resulting impact on the Asian summer monsoon has not been studied. Here we study the atmospheric response caused by the island surface change due to deforestation into cash-crop plantation and urban. Using a large ensemble of Atmospheric model experiments with observed and idealized land-cover-change specifications, we show that the deforestation warms the Malay Archipelago, caused by an increase in soil warming due to decreased evapotranspirative cooling. The island warming agrees well with in situ and satellite observations; it causes moisture to converge from the surrounding seas into Sumatra and Malaya, and updraft, rainfall and cyclonic circulation to spread northwestward into southern India and the Arabian Sea, as well as drying anticyclonic circulation over the Indo-Gangetic plains, Indochina and the South China Sea, weakening the Asian summer monsoon. The modeled monsoon-weakening agrees well with, and tends to enhance the observed long-term trend, suggesting the potential for continued weakening with protracted cash-crop plantation and urban expansion.
Sun, Jingru, F-H Xu, Leo Oey, and Yanluan Lin, March 2019: Monthly variability of Luzon Strait tropical cyclone intensification over the Northern South China Sea in recent decades. Climate Dynamics, 52(5-6), DOI:10.1007/s00382-018-4341-x. Abstract
A number of tropical cyclones (TCs) in the western North Pacific (WNP) pass through Luzon Strait (LS) into the South China Sea (SCS) from June to November every year. The monthly variability of the ratio of TC intensity change, Rtc, shows that majority of the LSTCs achieve their lifetime maximum intensity (LMI) over the northern SCS (WNP) during August–September (June–July and October–November). Furthermore, compared to August, LSTCs in September are more easily intensified, suggesting that atmospheric and/or oceanic environments over the northern SCS in September are more favorable for TC development. The monthly-averaged oceanic and atmospheric environmental factors, including sea surface temperature, upper-ocean warm layer depth, vertical wind shear, relative humidity and large-scale low-level vorticity, are compared. The comparison between August and September is mainly studied because of the higher LSTCs frequency in these 2 months. The intensification tendency of LSTCs in September is primarily attributed to the relative thick upper-ocean warm layer and weak vertical wind shear. The transition of East Asian summer monsoon to winter monsoon tends to provide more favorable environmental conditions in September than in August for TC intensification in the northern SCS.
Zhang, L, and Leo Oey, January 2019: Young ocean waves favor the rapid intensification of tropical cyclones - a global observational analysis. Monthly Weather Review, 147(1), DOI:10.1175/MWR-D-18-0214.1. Abstract
Identifying the condition(s) of how tropical cyclones intensify, in particular rapid intensification, is challenging, due to the complexity of the problem involving internal dynamics, environments and mutual interactions; yet the benefit to improved forecasts may be rewarding. To make the analysis more tractable, an attempt is made here focusing near the sea surface, by examining 23-year global observations comprising over 16,000 cases of tropical cyclone intensity change, together with upper-ocean features, surface waves and low-level atmospheric moisture convergence. Contrary to the popular misconception, we found no statistically significant evidence that thicker upper-ocean layers and/or warmer temperatures are conducive to rapid intensification. Instead, we found in storms undergoing rapid intensification significantly higher coincidence of low-level moisture convergence and a dimensionless air-sea exchange coefficient closely related to the youth of the surface waves under the storm. This finding is consistent with the previous modeling results, verified here using ensemble experiments, that higher coincidence of moisture and surface fluxes tends to correlate with intensification, through greater precipitation and heat release. The young waves grow to saturation in the right-front quadrant due to trapped-wave resonance for a group of Goldilocks cyclones that translate neither too slowly nor too quickly, which 70% of rapidly-intensifying storms belong. Young waves in rapidly-intensifying storms also produce relatively less (compared to the wind input) Stokes-induced mixing and cooling in the cyclone core. A reinforcing coupling between tropical cyclone wind and waves leading to rapid intensification is proposed.
Liao, E, and Leo Oey, et al., July 2018: The Deflection of the China Coastal Current over the Taiwan Bank in Winter. Journal of Physical Oceanography, 48(7), DOI:10.1175/JPO-D-17-0037.1. Abstract
In winter, an off-shore flow of the coastal current can be inferred from satellite and in-situ data over the western Taiwan Bank. The dynamics related to this off-shore flow are examined here using observations as well as analytical and numerical models. The currents can be classified into three regimes. The downwind (i.e. southward) cold coastal current remains attached to the coast when the northeasterly wind stress is stronger than a critical value depending on the upwind (i.e. northward) large-scale pressure gradient force. By contrast, an upwind warm current appears over the Taiwan Bank when the wind stress is less than the critical pressure gradient force. The downwind coastal current and upwind current converge and the coastal current deflects off-shore onto the bank during a moderate wind. Analysis of the vorticity balance shows that the off-shore transport is a result of negative bottom stress curl that is triggered by the positive vorticity of the two opposite flows. The negative bottom stress curl is reinforced by the gentle slope over the bank, which enhances the off-shore current. Composite analyses using satellite observations show cool waters with high chlorophyll in the off-shore current under the moderate wind. The results of composite analyses support the model findings and may explain the high productivity over the western bank in winter.
Oey, Leo, et al., March 2018: Fish Catch Is Related to the Fluctuations of a Western Boundary Current. Journal of Physical Oceanography, 48(3), DOI:10.1175/JPO-D-17-0041.1. Abstract
In eastern boundary upwelling ecosystems, substantial variance of biological productivity (~50%) can often be related to physical forcing such as winds and ocean temperatures. Robust biophysical connections are less clear-cut in western boundary currents. Here the authors show that interannual variation of fish catch along the western boundary current of the North Pacific, the Kuroshio, significantly correlates (r = 0.67; p < 0.001) with the current’s off-slope (more fish) and on-slope (less fish) sideways shifts in the southern East China Sea. Remotely, transport fluctuations and fish catch are related to the oscillation of a wind stress-curl dipole in the tropical–subtropical gyre of the western North Pacific. Locally, the current’s sideways fluctuations are driven by transport fluctuations through a feedback process between along-isobath pressure gradients and vertical motions: upwelling (downwelling) during the off-slope (on slope) shift, which in turn significantly enhances (depresses) the chlorophyll-a (Chl-a) concentration in winter and early spring. The authors hypothesize that changes in the phytoplankton biomass as indicated by the Chl-a lead to changes in copepodites, the main food source of the fish larvae, and hence also to the observed variation in fish catch.
Liang, Alice T., and Leo Oey, et al., March 2017: Long-term trends of typhoon-induced rainfall over Taiwan: In situ evidence of poleward shift of typhoons in western North Pacific in recent decades. Journal of Geophysical Research: Atmospheres, 122(5), DOI:10.1002/2017JD026446. Abstract
A significant poleward shift of tropical cyclones (TCs or typhoons) and TC-induced storm surge in the western North Pacific has occurred in recent decades. Here we use 64 year rainfall observations around Taiwan to provide an independent evidence of the shift. We show that, due to the island's unique location relative to typhoon tracks, TC-induced rainfall trends are significantly rising west and north of the island but are insignificant east and southeast, caused by a preference in recent decades for TCs to veer more poleward. Analyses of large-scale fields indicate that the TCs' poleward shift is caused by the weakening of the steering flow and western North Pacific subtropical high, which in turn is due to tropic-subtropical Indo-Pacific warming and a weakened monsoon, consistent with the expansion of the tropics due to climate change.
Xu, F-H, Y Yuan, Leo Oey, and Yanluan Lin, August 2017: Impacts of preexisting ocean cyclonic circulation on sea surface Chlorophyll-a concentration off northeastern Taiwan following episodic typhoon passages. Journal of Geophysical Research: Oceans, 122(8), DOI:10.1002/2016JC012625. Abstract
Off northeastern Taiwan, enhancement of sea surface chlorophyll-a (Chl-a) concentration is frequently found after typhoon passages. From 1998 to 2013, forty-six typhoon events are analyzed to examine the variations in Chl-a concentration from satellite ocean color data. On average, Chl-a concentration increased by 38% after a typhoon passage. Noticeably, four remarkable Chl-a increases after typhoons coincide with pre-existing oceanic cyclones in the study area. The Chl-a increase is significantly anti-correlated (p<0.01) with relative SSH, defined as the difference of SSH in the study area and in the surrounding area. To assess the impact of pre-existing cyclones on the upper ocean response to typhoons, we conduct a series of numerical experiments to simulate the oceanic response to Typhoon Kaemi (2006) with or without a pre-existing oceanic cyclone, and with or without strong typhoon winds. The results show that the experiment with a pre-existing oceanic cyclone produces the largest upwelling due to cyclone intensification, mainly induced by the positive wind stress curl dipole northeast of Taiwan.
Lin, Yuchun, and Leo Oey, et al., February 2016: Rossby waves and eddies observed at a temperature mooring in Northern South China Sea. Journal of Physical Oceanography, 46(2), DOI:10.1175/JPO-D-15-0094.1. Abstract
Annual Rossby waves in Northern South China Sea had previously been studied using altimetry and model data; how they connect to sub-surface temperature fluctuations have not been examined, however. Here we analyzed a 22-month, surface~-500m temperature time-series at (115.5°E,18.3°N), together with satellite and other data, to show the arrivals near z≈-300 m and deeper of cool (warm) Rossby waves after their generation near the Luzon Strait in winter (summer). Temperature fluctuations with time scales of a few weeks, and with maximum anomalies near z≈-100 m, were also found embedded in the smooth Rossby wave, and caused by propagating eddies. Eddy fluctuations and propagation past the mooring were of two types: southwestward from southwestern Taiwan, triggered by Kuroshio intrusion that produced anticyclone-cyclone pairs in late fall~winter, and eddies propagating westward from Luzon forced by annual anomalies of wind stress curl and Kuroshio path in the Luzon Strait.
Oey, Leo, and Simon Chou, July 2016: Evidence of rising and poleward shift of storm surge in western North Pacific in recent decades. Journal of Geophysical Research: Oceans, 121(7), DOI:10.1002/2016JC011777. Abstract
Recently, there has been considerable interest in examining how sea-level extremes due to storm surge may be related to climate change. Evidence of how storm-surge extremes have evolved since the start of the most recent warming of mid-1970s and early 1980s has not been firmly established however. Here we use 64 years (1950–2013) of observations and model simulations, and find evidence of a significant rise in the intensity as well as poleward-shifting of location of typhoon surges in the western North Pacific after 1980s. The rising and poleward-shifting trends are caused by the weakening of the steering flow in the tropics, which is related to climate warming, resulting in slower-moving and longer-lasting typhoons which had shifted northward.
Huang, S-M, and Leo Oey, August 2015: Right-side cooling and phytoplankton bloom in the wake of a tropical cyclone. Journal of Geophysical Research: Oceans, 120(8), DOI:10.1002/2015JC010896. Abstract
The rightward tendency (in northern hemisphere) of enhanced phytoplankton bloom often observed in the wake of a tropical cyclone has commonly been attributed to the rightward bias of mixing due to stronger wind and wind-current resonance. We demonstrated using a high-resolution biophysical model that vertical mixing alone resulted only in weak asymmetry after the passage of the storm. The enhanced bloom was caused instead by decreased turbulence due to re-stratification by sub-mesoscale recirculation cells preferentially produced on the right side, rightward shift of cool isotherms, and spin-up of a subsurface jet. We showed using a two-time scale asymptotic expansion that these slower evolving features were forced by resonance Reynolds stresses of the energetic and rapidly oscillating near-inertial internal waves. This article is protected by copyright. All rights reserved.
Oey, Leo, et al., October 2015: The influence of shelf-sea fronts on winter monsoon over East China Sea. Climate Dynamics, 45(7), DOI:10.1007/s00382-014-2455-3. Abstract
Strong sea surface temperature fronts in open seas are known to affect the atmosphere. Shelf-sea fronts in winter have comparable strengths, yet their impacts on winds have not been studied. In January of 2012, a persistent, narrow band of cloud stretching 600–1,000 km was observed along the front of East China Sea (ECS). Numerical and analytical models show that the cloud was formed atop a recirculating cell induced by the front and, more generally, that β-plumes of low and high pressures emanate and spread far from fronts. Consistent with the theory, observations show that in ECS at inter-annual time scales, strong fronts co-vary with on-shelf convergent wind, strong northeasterly monsoon, and alongshelf alignment of clouds with low clouds near the coast and higher clouds offshore. Our results suggest that shelf-sea fronts are potentially an important dynamic determinant of climate variability of East Asia.
Sun, Jingru, and Leo Oey, et al., May 2015: Ocean response to typhoon Nuri (2008) in western Pacific and South China Sea. Ocean Dynamics, 65(5), DOI:10.1007/s10236-015-0823-0. Abstract
Typhoon Nuri formed on 18 August 2008 in the western North Pacific east of the Philippines and traversed northwestward over the Kuroshio in the Luzon Strait where it intensified to a category 3 typhoon. The storm weakened as it passed over South China Sea (SCS) and made landfall in Hong Kong as a category 1 typhoon on 22 August. Despite the storm’s modest strength, the change in typhoon Nuri’s intensity was unique in that it strongly depended on the upper ocean. This study examines the ocean response to typhoon Nuri using the Princeton Ocean Model. An ocean state accounting for the sea-surface temperature (SST) and mesoscale eddy field prior to Nuri was constructed by assimilating satellite SST and altimetry data 12 days before the storm. The simulation then continued without further data assimilation, so that the ocean response to the strong wind can be used to understand processes. It is found that the SST cooling was biased to the right of the storm’s track due to inertial currents that rotated in the same sense as the wind vector, as has previously been found in the literature. However, despite the comparable wind speeds while the storm was in western Pacific and SCS, the SST cooling was much more intense in SCS. The reason was because in SCS, the surface layer was thinner, the vorticity field of the Kuroshio was cyclonic, and moreover a combination of larger Coriolis frequency as the storm moved northward and the typhoon’s slower translational speed produced a stronger resonance between wind and current, resulting in strong shears and entrainment of cool subsurface waters in the upper ocean.
Sun, Jingru, and Leo Oey, November 2015: The influence of ocean on Typhoon Nuri (2008). Monthly Weather Review, 143(11), DOI:10.1175/MWR-D-15-0029.1. Abstract
Typhoon Nuri (2008) was one of approximately 120 typhoons in the past 60 years that passed through a narrow gap, the Luzon Strait, connecting the western North Pacific and South China Sea (SCS). Seventy % of these storms reached their maximum intensities over the warm waters east of Luzon and in the Kuroshio, then rapidly weakened in SCS. We conducted numerical experiments to understand the intensity change of Nuri. Westward across the Kuroshio in the Luzon Strait, the 26°C-isotherm shallows rapidly by half. This and stronger mixing by wind-ocean resonance preferentially cooled sea-surface temperature and weakened the typhoon in SCS. We then described a positive-feedback mechanism to explain the intensification of Nuri over the western North Pacific.
Xu, S, X Huang, Y Zhang, H Fu, and Leo Oey, et al., September 2015: POM.gpu-v1.0: a GPU-based Princeton Ocean Model. Geoscientific Model Development, 8(9), DOI:10.5194/gmd-8-2815-2015. Abstract
Graphics processing units (GPUs) are an attractive solution in many scientific applications due to their high performance. However, most existing GPU conversions of climate models use GPUs for only a few computationally intensive regions. In the present study, we redesign the mpiPOM (a parallel version of the Princeton Ocean Model) with GPUs. Specifically, we first convert the model from its original Fortran form to a new Compute Unified Device Architecture C (CUDA-C) code, then we optimize the code on each of the GPUs, the communications between the GPUs, and the I / O between the GPUs and the central processing units (CPUs). We show that the performance of the new model on a workstation containing four GPUs is comparable to that on a powerful cluster with 408 standard CPU cores, and it reduces the energy consumption by a factor of 6.8.
Xu, F-H, and Leo Oey, June 2015: Seasonal SSH variability of the Northern South China Sea. Journal of Physical Oceanography, 45(6), DOI:10.1175/JPO-D-14-0193.1. Abstract
The seasonal response of sea surface height anomaly (SSHA) to wind stress curl (WSC) in northern South China Sea (NSCS) and Kuroshio intrusion through the Luzon Strait is analyzed using observations and models. The dominant response to WSC is through simple Ekman pumping while effects of β appear as the weaker second Empirical Orthogonal Function mode. The Luzon Strait intrusion is shown to be largely deterministic using a model forced by realistic wind in the North Pacific Ocean, and it contributes significantly to the SSH variability in the NSCS. The WSC accounts for 62% while intrusion 38% of the total forcing but the latter alters the forced Rossby wave response. Without the intrusion, westward propagation is too fast, resulting in incorrect balance and erroneous annual SSH variability in NSCS.
Chang, Y-L, and Leo Oey, February 2014: Analysis of STCC eddies using the Okubo – Weiss parameter on model and satellite data. Ocean Dynamics, 64(2), DOI:10.1007/s10236-013-0680-7. Abstract
The North Pacific Subtropical Counter Current (STCC) is a weak zonal current comprising of a weak eastward flow near the surface (with speeds of less than 0.1 m/s and a thickness of approximately 50–100 m) and westward flow (the North Equatorial Current) beneath. Previous studies (e.g., Qiu J Phys Oceanogr 29: 2471–2486, 1999) have shown that the STCC is baroclinically unstable. Therefore, despite its weak mean speeds, nonlinear STCC eddies with diameters ~300 km or larger and rotational speeds exceeding the eddy propagation speeds develop (Samelson J Phys Oceanogr 27: 2645–2662, 1997; Chelton et al. Prog Oceanogr 91: 167–216, 2011). In this study, the authors present numerical experiments to describe and explain the instability and eddy-generation processes of the STCC and the seasonal variation. Emphasis is on finite-amplitude eddies which are analyzed based on the parameter of Okubo (Deep-Sea Res 17: 445–454, 1970) and Weiss (Physica D 48: 273–294, 1991). The temperature and salinity distribution in March and April offer the favorable condition for eddies to grow, while September and October are unfavorable seasons for the generation of eddies. STCC is maintained not only by subsurface front but also by the sea surface temperature (SST) front. The seasonal variation of the vertical shear is dominated by the seasonal surface STCC velocity. The SST front enhances the instability and lead to the faster growth of STCC eddies in winter and spring. The near-surface processes are therefore crucial for the STCC system.
Chang, Y-L, and Leo Oey, March 2014: Instability of the North Pacific Subtropical Countercurrent. Journal of Physical Oceanography, 44(3), DOI:10.1175/JPO-D-13-0162.1. Abstract
The North Pacific Subtropical Countercurrent (STCC) has a weak eastward velocity near the surface, but the region is populated with eddies. Studies have shown that the STCC is baroclinically unstable with a peak growth rate of 0.015 day−1 in March, and the ~60-day growth time has been used to explain the peak eddy kinetic energy (EKE) in May observed from satellites. It is argued here that this growth time from previously published normal-mode instability analyses is too slow. Growth rates calculated from an initial-value problem without the normal-mode assumption are found to be 1.5 to 2 times faster and at shorter wavelengths, due to the existence of (i) nonmodal solutions and (ii) sea surface temperature front in the mixed layer in winter. At interannual time scales it is shown that because of rapid surface adjustments, the STCC geostrophic shear, hence also the instability growth, is approximately in phase with surface forcing, leading to EKE modulation that peaks approximately 10 months later. However, the EKE can only be partially explained by this mechanism of modulation by baroclinic instability. It is suggested that the unexplained variance may be caused additionally by modulation of the EKE by dissipation.
Oey, Leo, and Y-L Chang, et al., March 2014: Cross flows in the Taiwan Strait in winter. Journal of Physical Oceanography, 44(3), DOI:10.1175/JPO-D-13-0128.1. Abstract
In winter a branch of the China Coastal Current can turn in Taiwan Strait to join the poleward-flowing Taiwan coastal current. The associated cross-strait flows have been inferred from hydrographic and satellite data, from observed abundances off northwestern Taiwan of cold-water copepod species Calanus sinicus, and in late March of 2012, also from debris found along the northwestern shore of Taiwan of a ship that broke 2 weeks earlier off the coast of China. The dynamics related to such cross flows has not been previously explained, and is the focus of this study using analytical and numerical models. It is shown that the strait’s currents can be classified into 3 regimes depending on the strength of the winter monsoon: equatorward (poleward) for northeasterly winds stronger (weaker) than an upper (lower) bound, and cross-strait flows for relaxing northeasterly winds between the two bounds. These regimes are related to the formation of stationary Rossby wave over the Changyun Ridge off mid-western Taiwan. In weak (strong) northeasterly-wind regime, weak (no) wave is produced. In the relaxing-wind regime, cross-strait currents are triggered by imbalance between pressure gradient and wind, and amplified by finite-amplitude meander downstream of the ridge where a strong cyclone develops.
Wang, J, and Leo Oey, December 2014: Inter-annual and decadal fluctuations of the Kuroshio in East China Sea and connection with surface fluxes of momentum and heat. Geophysical Research Letters, 41(23), DOI:10.1002/2014GL062118. Abstract
Despite attempts in the literature to link large-scale wind to long-term variations of the Kuroshio in East China Sea (ECS), the driving mechanism(s) are unknown. Here we use satellite altimetry data, wind, surface heat fluxes and sea-surface temperatures (SST) to explain the low-frequency fluctuations of Kuroshio path (KP) in ECS. The dominant fluctuations occur northeast of Taiwan. The KP correlates best with the PTO index of Chang and Oey [2012], less with the PDO index and a Kuroshio transport index, and poorly with other climate indices. The forcing are wind stress curl and surface heat flux northeast of Taiwan, which produce a thermocline tilt along the Kuroshio. Shelf's SST warms and cools in response to onshore and offshore KP, but prominent change occurs at a localized coastal zone shoreward of the above dominant KP-fluctuations. Over the past 2 decades, the KP has shifted onshore, coincident with a coastal warming trend.
Chang, Y-L, and Leo Oey, March 2013: Loop Current Growth and Eddy Shedding Using Models and Observations: Numerical Process Experiments and Satellite Altimetry Data. Journal of Physical Oceanography, 43(3), DOI:10.1175/JPO-D-12-0139.1. Abstract
Recent studies on Loop Current’s variability in the Gulf of Mexico suggest that the system may behave with some regularity forced by the biannually-varying trade winds. The process is analyzed here using a reduced-gravity model and satellite data. The model shows that a biannual signal is produced by vorticity and transport fluctuations in the Yucatan Channel due to the piling-up and retreat of warm water in the northwestern Caribbean Sea forced by the biannually-varying trade wind. The Loop grows and expands with increased northward velocity and cyclonic vorticity of the Yucatan Current, and eddies are shed when these are near minima. Satellite sea-surface-height (SSH) data from 1993-2010 is analyzed. These show, consistent with the reduced-gravity experiments and previous studies, a (statistically) significant asymmetric biannual variation of the growth and wane of Loop Current: strong from summer to fall and weaker from winter to spring; the asymmetry being due to the asymmetry that also exists in the long-term observed wind. The biannual signal is contained in the two leading EOF modes which together explain 47% of the total variance, and which additionally describe the eddy shedding and westward propagation in summer~fall. The EOF’s also show connectivity between Loop Current and Caribbean Sea’s variability by mass and vorticity fluxes through the Yucatan Channel.
Chang, Y-L, and Leo Oey, July 2013: Coupled response of the trade wind, SST-gradient and SST in the Caribbean Sea, and the potential impact on Loop Current’s interannual variability. Journal of Physical Oceanography, 43(7), DOI:10.1175/JPO-D-12-0183.1. Abstract
Air-sea coupling in the Intra-American Sea (IAS: Caribbean Sea & Gulf of Mexico) is studied through analyses of observational data from satellite, reanalysis products and in situ measurements. A strong coupling is found between the easterly trade wind (-U) and meridional SST gradient (∂T/∂y) across a localized region of the south central Caribbean Sea from seasonal, interannual to decadal time scales. The ∂T/∂y-anomaly is caused by variation in the strength of coastal upwelling off the Venezuelan coast by the wind which in turn strengthens (weakens) for stronger (weaker) ∂T/∂y. Wind speeds and seasonal fluctuations in IAS have increased in the past 2 decades with transition near 1994 coincident approximately with when the Atlantic Multidecadal Oscillation (AMO) turned from cold to warm phases. In particular, the seasonal “swing” of summer’s strong to fall’s weak trade wind has become larger. The ocean’s upper-layer depth has also deepened, by as much as 50% on average in the eastern Gulf of Mexico. These conditions favor the shedding of eddies from the Loop Current, making it more likely to shed at a biannual frequency, as has been observed from altimetry data.
Ezer, Tal, and Leo Oey, March 2013: On the dynamics of strait flows: an ocean model study of the Aleutian passages and the Bering Strait. Ocean Dynamics, 63(2-3), DOI:10.1007/s10236-012-0589-6. Abstract
A high-resolution numerical ocean circulation model of the Bering Sea (BS) is used to study the natural variability of the BS straits. Three distinct categories of strait dynamics have been identified: (1) Shallow passages such as the Bering Strait and the Unimak Passage have northward, near barotropic flow with periodic pulses of larger transports; (2) wide passages such as Near Straits, Amukta Pass, and Buldir Pass have complex flow patterns driven by the passage of mesoscale eddies across the strait; and (3) deep passages such as Amchitka Pass and Kamchatka Strait have persistent deep return flows opposite in direction to major surface currents; the deep flows persist independent of the local wind. Empirical orthogonal function analyses reveal the spatial structure and the temporal variability of strait flows and demonstrate how mesoscale variations in the Aleutian passages influence the Bering Strait flow toward the Arctic Ocean. The study suggests a general relation between the barotropic and baroclinic Rossby radii of deformations in each strait, and the level of flow variability through the strait, independent of geographical location. The mesoscale variability in the BS seems to originate from two different sources: a remote origin from variability in the Alaskan Stream that enters the BS through the Aleutian passages and a local origin from the interaction of currents with the Bowers Ridge in the Aleutian Basin. Comparisons between the flow in the Aleutian passages and flow in other straits, such as the Yucatan Channel and the Faroe Bank Channel, suggest some universal topographically induced dynamics in strait flows.
Oey, Leo, et al., December 2013: Decadal warming of coastal China Seas and coupling with winter monsoon and currents. Geophysical Research Letters, 40(23), DOI:10.1002/2013GL058202. Abstract
In recent decades, wintertime sea surface temperatures off the eastern coast of China have steadily increased. The warming is accompanied by on-coast wind convergence across East China Sea, and by stronger northeasterly wind which is spatially inhomogeneous being greatest in the Taiwan Strait. Strong winds favor more frequent cross-shelf currents and vigorous spreading of heat from the Kuroshio, which warms the coastal sea in a positive feedback loop. The process also weakens the East Asian winter monsoon over eastern China, contributing to its decoupling from the recent rebound of the Siberian High.
Xu, F-H, Y-L Chang, Leo Oey, and P Hamilton, May 2013: Loop Current Growth and Eddy Shedding Using Models and Observations: Analyses of the July 2011 Eddy-Shedding Event. Journal of Physical Oceanography, 43(5), DOI:10.1175/JPO-D-12-0138.1. Abstract
Recent studies suggest that as trade wind in the Caribbean Sea weakens from summer to fall, conditions become more favorable for Loop Current in the Gulf of Mexico to shed an anticyclonic ring. This idea originated with observations showing a preference for more eddies in summer through fall, and it was confirmed using multi-decadal model experiments. Here the hypothesis is further tested by studying the dynamics of a specific eddy-shedding event in summer 2011 using a model experiment initialized with observation-assimilated reanalysis, and forced by the NCEP wind. Eddy shedding in July of 2011 is shown to follow the weakening of the trade wind and Yucatan transport in late June. The shedding time is significantly earlier than can be explained based on reduced-gravity Rossby wave dynamics. Altimetry and model data are analyzed to show that Empirical Orthogonal Function modes1+2 dominate the reduced-gravity process, while higher modes contain the coupling of the Loop Current with deep layer underneath. The Loop’s westward expansion at incipient shedding induces in the eastern Gulf a deep cyclonic gyre, embedded within which are small cyclones due to baroclinic instability of the strongly sheared current north of Campeche Bank. The associated deep upwelling and upper-layer divergence due to these cyclonic circulations accelerate eddy shedding.
Xu, F-H, Leo Oey, Y Miyazawa, and P Hamilton, September 2013: Hindcasts and Forecasts of Loop Current & Eddies in the Gulf of Mexico using Local Ensemble Transform Kalman Filter and Optimum-Interpolation Assimilation Schemes. Ocean Modelling, 69, DOI:10.1016/j.ocemod.2013.05.002. Abstract
The Local Ensemble Transform Kalman Filter (LETKF) is applied to the parallelized version of the Princeton Ocean Model to estimate the states of Loop Current and eddies in the Gulf of Mexico from April/20 to July/21, 2010 when detailed in situ current measurements were available. Tests are conducted to explore the sensitivity of the LETKF estimates to different parameters, and to systematic additions of different observational datasets which include satellite sea surface height anomaly (SSHA), satellite sea surface temperature (SST), and moored ADCP’s. The results are compared against observations to assess model skills, and also against estimates based on a simpler optimum interpolation (OI) assimilation scheme. With appropriate values of parameters and observational errors, LETKF provides improved estimates of Loop Current and eddy. In particular, the Loop Current in the late spring to summer of 2010 underwent a shedding-reattachment-shedding process. It is shown that such a nonlinear behavior is more accurately captured by LETKF, but not by OI, due to the former’s time-evolving error covariance. Finally, the accuracies of 8-week forecasts initialized from the OI and LETKF analyses and forced by reanalysis winds are compared. This period is particularly challenging to forecast because, instead of a more easily simulated westward propagation at approximately the first-mode baroclinic Rossby wave speed, the newly-shed eddy propagated very slowly, stalled, and finally decayed in the eastern Gulf. Both OI and LETKF beat persistence, but the LETKF significantly improves the eddy’s position and strength throughout the 8-week forecast.
Chang, Y-L, and Leo Oey, March 2012: The Philippines-Taiwan Oscillation: Monsoon-like interannual oscillation of the subtropical-tropical Western North Pacific wind system and its impact on the ocean. Journal of Climate, 25(5), DOI:10.1175/JCLI-D-11-00158.1. Abstract
Tide-gauge and satellite data reveal an interannual oscillation of the ocean's thermoclines east of Philippines and Taiwan forced by a corresponding oscillation in the wind stress curl. This so called “Philippines-Taiwan Oscillation” (PTO) is shown to control the interannual variability of the circulation of the subtropical and tropical western North Pacific. The PTO shares some characteristics of known Pacific indices, e.g. Nino3.4. However, unlike PTO, these other indices explain only portions of the Western North Pacific circulation. The reason is because of the nonlinear nature of the forcing in which mesoscale (ocean) eddies play a crucial role. In years of positive PTO, thermoclines east of Philippines rise while those east of Taiwan deepen. This results in northward shift of the North Equatorial Current (NEC), increased vertical shear of the Subtropical Counter Current (STCC)/NEC system, increased eddy activity dominated by warm eddies in the STCC, increased Kuroshio transport off the northeastern coast of Taiwan into the East China Sea, increased westward inflow through the Luzon Strait into the South China Sea, and cyclonic circulation and low sea-surface-height anomalies in South China Sea. The reverse applies in years of negative PTO.
Chang, Y-L, and Leo Oey, March 2012: Why does the Loop Current tend to shed more eddies in summer and winter?Geophysical Research Letters, 39, L05605, DOI:10.1029/2011GL050773. Abstract
The observed seasonal preferences of Loop Current eddy shedding, more in summer and winter and less in fall and spring, are shown for the first time to be due to a curious combination of forcing by the seasonal winds in the Caribbean Sea and the Gulf of Mexico. The conditions are favorable for the Loop to shed eddies in summer and winter when strong trade winds in the Caribbean produce large Yucatan transport and Loop's intrusion, and concurrently when weak easterlies in the Gulf offer little impediment to eddy shedding. The conditions are less favorable in fall and spring as the trade winds and Yucatan transport weaken, and the strengthening of the Gulf's easterlies impedes shedding.
Tamura, H, Y Miyazawa, and Leo Oey, August 2012: The Stokes drift and wave induced-mass flux in the North Pacific. Journal of Geophysical Research: Oceans, 117, C08021, DOI:10.1029/2012JC008113. Abstract
Stokes drift and wave induced-mass flux in realistic wave fields and ocean currents in the North Pacific Ocean are studied using a third generation wave model with ambient geostrophic currents estimated from satellite altimetry data to directly estimate the Stokes drift for random directional waves. Comparison with in situ buoy data shows that the model performed well in representing the Stokes drift field and total wave momentum. In the North Pacific, the annual mean surface Stokes drift ranges from 2 to 10 cm/s and the mean Stokes e-folding depth is 1 to 2 m. The Stokes drift fields estimated using bulk wave parameters compare poorly against buoy data, and are shown to overestimate (underestimate) the Stokes e-folding depth (the surface Stokes drift) computed directly from wave spectra by as much as 5-20 times larger (2-10 times smaller). The spatial distributions of mean wave height and mass transport approximately follow the synoptic scale associated with atmospheric forcing, and the divergence of wave induced-mass flux is significantly modified by local fetch, the coast and ocean currents. Due to strong wave refraction along the Kuroshio Extension, surface vertical velocity induced by Stokes divergence is comparable to the Ekman velocity, and may alter the meso-scale dynamics of the front.
Chang, Y-L, and Leo Oey, March 2011: Loop Current Cycle: Coupled response of the Loop Current with deep flows. Journal of Physical Oceanography, 41(3), DOI:10.1175/2010JPO4479.1. Abstract
While the upper-layer dynamics of Loop Current and eddies in the Gulf of Mexico are well-studied, our understanding of how they are coupled to the deep flows is limited. In this work, results from a numerical model are analyzed to classify the expansion, shedding, retraction and deep-coupling cycle (the Loop Current Cycle) according to the vertical mass flux across the base of the Loop. Stage A is “Loop-reforming” period with downward flux and deep divergence under the Loop Current. Stage B is “incipient-shedding” with strong upward flux and deep convergence. Stage C is “eddy-migration” with waning upward flux and deep throughflow from the western Gulf into the Yucatan Channel. Because of the strong deep coupling between the eastern and western Gulf, the Loop's expansion is poorly correlated with deep flows through the Yucatan Channel. Stage A is longest and the mean vertical flux under the Loop Current is downward. Therefore, since the net circulation around the abyssal basin is zero, the abyssal gyre in the western Gulf is cyclonic. The gyre's strength is strongest when the Loop Current is reforming, and weakest after an eddy is shed. The result suggests that the Loop Current Cycle can force a low-frequency (time scales ~ shedding periods; O(months)) abyssal oscillation in the Gulf of Mexico.
Chang, Y-L, and Leo Oey, April 2011: Interannual and seasonal variations of Kuroshio transport east of Taiwan inferred from 29 years of tide-gauge data. Geophysical Research Letters, 39, L08603, DOI:10.1029/2011GL047062. Abstract
Twenty-nine years of tide-gauge data are analyzed in conjunction with wind and satellite-derived sea-surface height and ocean velocity data to study the interannual and seasonal variations of the Kuroshio transport off the northeastern coast of Taiwan. The data reveals an interannual variation of ±0.1 m (transport-variation of approximately ±3.5 Sv; 1 Sv = 106 m3 s−1), and a much weaker (5–10 times weaker) seasonal fluctuation that is minimum in May and maximum in November. The interannual fluctuations are not directly wind-driven by linear dynamics; rather, the Kuroshio strengthens in years of abundant eddies of the Subtropical Counter Current, which is related to the current's instability state driven by the slow fluctuations of the large-scale wind stress curl in the western Pacific. The seasonal transport fluctuation is also eddy-forced, but has weaker amplitude because the seasonal time scale is of the same order as the eddy-propagation time scale, and transport-producing eddy signals tend to overlap east of Taiwan.
Chang, Y-L, Leo Oey, F-H Xu, H-F Lu, and A Fujisaki, June 2011: 2010 oil spill: trajectory projections based on ensemble drifter analyses. Ocean Dynamics, 61(6), DOI:10.1007/s10236-011-0397-4. Abstract
An accurate method for long-term (weeks to months) projections of oil spill trajectories based on multi-year ensemble analyses of simulated surface and subsurface (z = −800 m) drifters released at the northern Gulf of Mexico spill site is demonstrated during the 2010 oil spill. The simulation compares well with satellite images of the actual oil spill which show that the surface spread of oil was mainly confined to the northern shelf and slope of the Gulf of Mexico, with some (more limited) spreading over the north/northeastern face of the Loop Current, as well as northwestward toward the Louisiana–Texas shelf. At subsurface, the ensemble projection shows drifters spreading south/southwestward, and this tendency agrees well with ADCP current measurements near the spill site during the months of May–July, which also show southward mean currents. An additional model analysis during the spill period (Apr–Jul/2010) confirms the above ensemble projection. The 2010 analysis confirms that the reason for the surface oil spread to be predominantly confined to the northern Gulf shelf and slope is because the 2010 wind was more southerly compared to climatology and also because a cyclone existed north of the Loop Current which moreover was positioned to the south of the spilled site.
Chang, Y-L, and Leo Oey, September 2011: Frontal circulation induced by up-front and coastal downwelling winds. Ocean Dynamics, 61(9), DOI:10.1007/s10236-011-0435-2. Abstract
Two-dimensional (cross-shelf and depth) circulation by downwelling wind in the presence of a prograding front (with isopycnals that slope in the same direction as the topographic slope) over a continental shelf is studied using high-resolution numerical experiments. The physical process of interest is the cross-shelf circulation produced by northeasterly monsoon winds acting on the Kuroshio front over the East China Sea outer shelf and shelfbreak where upwelling is often observed. However, a general problem is posed and solved by idealized numerical and analytical models. It is shown that upwelling is produced shoreward of the front. The upwelling is maintained by (1) a surface bulge of negative vorticity at the head of the front; (2) bottom offshore convergence beneath the front; and (3) in the case of a surface front that is thin relative to water depth, also by upwelling due to the vorticity sheet under the front. The near-coast downwelling produces intense mixing due to both upright and slant-wise convection in regions of positive potential vorticity. The analytical model shows that the size and on-shore propagating speed of the bulge are determined by the wind and its shape is governed by a nonlinear advection–dispersion equation which yields unchanging wave-form solutions. Successive bulges can detach from the front under a steady wind. Vertical circulation cells develop under the propagating bulges despite a stable stratification. These cells can have important consequences to vertical exchanges of tracers and water masses.
Chiang, T-L, C-R Wu, and Leo Oey, January 2011: Typhoon Kai-Tak: An Ocean’s Perfect Storm. Journal of Physical Oceanography, 41(1), DOI:10.1175/2010JPO4518.1. Abstract
An unusually intense sea surface temperature drop (ΔSST) of about 10.8°C induced by the Typhoon Kai-Tak is observed in the northern South China Sea (SCS) in July 2000. Observational and high-resolution SCS model analyses were carried out to study the favorable conditions and relevant physical processes that cause the intense surface cooling by Kai-Tak. Upwelling and entrainment induced by Kai-Tak account for 62% and 31% of the ΔSST, respectively, so that upwelling dominates vertical entrainment in producing the surface cooling for a subcritical storm such as Kai-Tak. However, wind intensity and propagation speed alone cannot account for the large ΔSST. Prior to Kai-Tak, the sea surface was anomalously warm and the main thermocline was anomalously shallow. The cause was a delayed transition of winter to summer monsoon in the northern SCS in May 2000. This produced an anomalously strong wind stress curl and a cold eddy capped by a thin layer of very warm surface water west of Luzon. Kai-Tak was the ocean’s perfect storm in passing over the eddy at the “right time,” producing the record SST drop and high chlorophyll-a concentration.
Ezer, Tal, Leo Oey, H Xue, and X-H Wang, September 2011: Editorial—The 2nd International Workshop on Modeling the Ocean (IWMO-2010). Ocean Dynamics, 61(9), DOI:10.1007/s10236-011-0470-z.
Fujisaki, A, and Leo Oey, October 2011: Formation of ice bands by winds. Journal of Geophysical Research: Oceans, 116, C10015, DOI:10.1029/2010JC006655. Abstract
A mechanism for the formation of ice bands is proposed as a coupled response of
ice edge and lee waves to wind under the hydrostatic approximation. A high-resolution
ice-ocean coupled model is used in an x-z domain with grid sizes (x,z)= (250 m,1 m).
Under an along-ice-edge wind, such that the Ekman transport is away from the ice edge,
the nearly discontinuous surface stress between the ice-covered and open seas generates
lee waves. A thin layer of high-potential vorticity fluid under the ice is produced by the
Ekman forcing, enabling the ice edge to rapidly slip over less stratified water. This is
favorable for supercritical conditions when lee waves are generated. Ice bands are
formed by the corresponding convergences and divergences. The flow becomes
subcritical farther behind the ice-edge but secondary lee waves and ice bands form
because of the secondary stress discontinuity behind the lead ice band. An analytical
solution is derived to show that ice bands have longer widths than the lee-wavelengths
because the ice-ocean stress creates the smoothing effect. Vertical motions associated
with the lee waves have speed of the order of 10 m/day, extend to the bottom (300 m),
and contribute to deep vertical mixing and the subsequent melting of the ice. These small-scale features are not modeled well with horizontal grids coarser than
approximately 2.5 km.
Xu, F-H, and Leo Oey, September 2011: The origin of along-shelf pressure gradient in the Middle Atlantic Bight. Journal of Physical Oceanography, 41(9), DOI:10.1175/2011JPO4589.1. Abstract
It is quite widely accepted that the along-shelf pressure gradient (ASPG) contributes in driving shelf currents in the Middle Atlantic Bight (MAB) off the U.S. northeastern coast; its origin, however, remains a subject for debate. Based on analyses of sixteen-year (1993–2008) satellite data, tide-gauge, rivers, wind and numerical experiments the authors suggest that rivers and Coastal Labrador Sea Water (CLSW) transport contribute to a positive mean ASPG (tilt up northward) approximately in the ratio 1:7 (i.e. CLSW dominates), whereas wind and Gulf Stream tend to produce a negative mean ASPG, approximately 1:6.
Data also indicate seasonal and inter-annual variations of ASPG that correlate with the Gulf Stream's shift and eddy-kinetic-energy (N-EKE) north of the Gulf Stream due to warm-core rings. A southward shift in the Gulf Stream produces sea-level drop north of Cape Hatteras which is most rapid in winter. The N-EKE peaks in late spring to early summer, and is larger in some years than others. A process model is used to show that ring-propagation along the MAB slope and ring-impingement upon the shelf break north of Cape Hatteras generate along-isobath density gradients and cross-shelfbreak transports that produce sea-level change on the shelf; the dominant ageostrophic term in the depth-integrated vorticity balance is the JEBAR term. In particular, shelf's sea-surface slopes down to the north when rings approach Cape Hatteras.
Berntsen, J, and Leo Oey, April 2010: Estimation of the internal pressure gradient in σ-coordinate ocean models: comparison of second-, fourth-, and sixth-order schemes. Ocean Dynamics, 60(2), DOI:10.1007/s10236-009-0245-y. Abstract
Sigma-coordinate ocean models are attractive because of their abilities to resolve bottom and surface boundary layers. However, these models can have large internal pressure gradient (IPG) errors. In this paper, two classes of methods for the estimation of the IPGs are assessed. The first is based on the integral approach used in the Princeton Ocean Model (POM). The second is suggested by Shchepetkin and McWilliams (2003) based on Green’s theorem; thus, area integrals of the pressure forces are transformed into line integrals. Numerical tests on the seamount problem, as well as on a northwestern Atlantic grid using both classes of methods, are presented. For each class, second-, fourth-, and sixth-order approximations are tested. Results produced with a fourth-order compact method and with cubic spline methods are also given. The results show that the methods based on the POM approach in general give smaller errors than the corresponding methods given in Shchepetkin and McWilliams (2003). The POM approach also is more robust when noise is added to the topography. In particular, the IPG errors may be substantially reduced by using the computationally simple fourth-order method from McCalpin (1994).
Chang, Y-L, and Leo Oey, November 2010: Why can wind delay the shedding of Loop Current eddies?Journal of Physical Oceanography, 40(11), DOI:10.1175/2010JPO4460.1. Abstract
We first show that wind in the Gulf of Mexico can delay the shedding of Loop current eddies. We analyze a time-dependent three-dimensional numerical experiment forced by a spatially and temporally constant westward wind stress within the Gulf, compare it with an otherwise identical no-wind run, and confirm the result with reduced-gravity experiments. We show that the wind produces westward transports over the northern and southern shelves of the Gulf, convergence in the west and a returned (i.e. eastward) upper-layer flow over the deep central basin towards the Loop Current. We then use Pichevin and Nof's (1997) and Nof's (2005) theory to explain that the returned flow constitutes a zonal momentum flux that delays eddy-shedding. Mass-balance analysis shows that wind also forces larger Loop Current and rings (because the delayed shedding allows more mass to be accumulated in them) and produces more efficient mass exchange between the Gulf and the Caribbean Sea. Finally, it is shown that eddies alone (without wind stress curl) can force a boundary current and downward flow in the western Gulf, and a corresponding deep flow from western to eastern Gulf.
Chang, Y-L, Leo Oey, C-R Wu, and H-F Lu, June 2010: Why are there upwellings on the northern shelf of Taiwan under northeasterly winds?Journal of Physical Oceanography, 40(6), DOI:10.1175/2010JPO4348.1. Abstract
Upwellings are observed on the northern shelf of Taiwan during northeasterly winds. Analytical and realistic numerical models are used to explain how vertical motions are created by divergence and convergence produced by wind acting on the vorticity field of two strong jets: the Kuroshio and the Taiwan Warm Current. The seaward increase in cyclonic vorticity near the Kuroshio’s western edge favors a stronger Ekman transport away from the jet, producing upwelling at the shelfbreak under a northeasterly wind. A similar mechanism for generating vertical motions is found across the Taiwan Warm Current. The numerical model results indicate that the vorticity effects can account for up to 30%–50% of the total variation in the surface Ekman transport. Except during summer’s weak southwesterlies, northeasterly wind is dominant over the East China Sea, suggesting that the vorticity effects may be prominent in the observed shelfbreak upwelling in nonsummer months.
Chang, Y-L, and Leo Oey, December 2010: Eddy and wind-forced heat transports in the Gulf of Mexico. Journal of Physical Oceanography, 40(12), DOI:10.1175/2010JPO4474.1. Abstract
The Gulf of Mexico (GOM) receives heat from the Caribbean Sea via the Yucatan–Loop Current (LC) system, and the corresponding ocean heat content (OHC) is important to weather and climate of the continental United States. However, the mechanisms that affect this heat influx and how it is distributed in the Gulf have not been studied. Using the Princeton Ocean Model, the authors show that a steady, uniform westward wind in the Gulf increases (100 KJ cm−2) the upper OHC (temperature T > 18°C) of the Gulf. This is because wind increases the water exchange between the Gulf and the Caribbean Sea, and the heat input into the Gulf is also increased, by about 50 TW. The westward heat transport to the western Gulf is 30 TW, and a substantial portion of this is due to wind-induced shelf currents, which converge to produce downwelling near the western coast. Finally, eddies are effective transporters of heat across the central Gulf. Wind forces larger LC and rings with deeper isotherms. This and downfront-wind mixing on the southern side of anticyclonic rings, northward spread of near-zero potential vorticity waters, and downwelling on the northern shelf break result in wide and deep eddies that transport large OHCs across the Gulf.
Ezer, Tal, and Leo Oey, April 2010: The role of the Alaskan Stream in modulating the Bering Sea climate. Journal of Geophysical Research: Oceans, 115, C04025, DOI:10.1029/2009JC005830. Abstract
A numerical ocean circulation model with realistic topography, but with an idealized forcing that includes only lateral transports is used to study the role of the Alaskan Stream (AS) in modulating the Bering Sea (BS) variability. Sensitivity experiments, each one with a different strength of the AS transport reveal a nonlinear BS response. An increase of AS transport from 10 to 25 Sv causes warming (∼0.25°C mean, ∼0.5°C maximum) and sea level rise in the BS shelf due to increased transports of warmer Pacific waters through the eastern passages of the Aleutian Islands, but an increase of AS transport from 25 to 40 Sv had an opposite impact on the BS shelf with a slight cooling (∼−0.1°C mean, ∼−0.5°C maximum). As the AS transport increases, flows through passages farther downstream in the western Aleutian Islands are affected and the variability in the entire BS is reduced. Transport variations of ∼0.1Sv in the Bering Strait are found to be correlated with mesoscale variations of the AS and associated transport variations in the Aleutian Islands passages. These results have important implications for understanding the observed variations in the Bering Strait and potential future climate variations in the Arctic Ocean.
Oey, Leo, Y-C Hsin, and C-R Wu, April 2010: Why does the Kuroshio northeast of Taiwan shift shelfward in winter?Ocean Dynamics, 60(2), DOI:10.1007/s10236-009-0259-5. Abstract
Observations indicate that off the northeastern coast of Taiwan a branch of the Kuroshio intrudes farther northward in winter onto the shelf of the East China Sea. We demonstrate that this seasonal shift can be explained solely by winter cooling. Cooling produces downslope flux of dense shelf water that is compensated by shelfward intrusion. Parabathic isopycnals steepen eastward in winter and couple with the cross-shelf topographic slope (the “JEBAR” effect) to balance the enhanced intrusion. The downslope flow also increases vortex stretching and decreases the thickness of the inertial boundary layer, resulting in a Kuroshio that shifts closer to the shelf break.
Oey, Leo, Y Miyazawa, and C-R Wu, April 2010: Editorial—International Workshop on Modeling the Ocean (IWMO) special issue in Ocean Dynamics. Ocean Dynamics, 60(2), DOI:10.1007/s10236-010-0281-7.
Oey, Leo, Tal Ezer, Y Miyazawa, and C-R Wu, October 2010: Editorial—International Workshop on Modeling the Ocean (IWMO) special issue part 2 in ocean dynamics. Ocean Dynamics, 60(5), DOI:10.1007/s10236-010-0338-7.
Sheu, W-J, C-R Wu, and Leo Oey, October 2010: Blocking and westward passage of eddies in the Luzon Strait. Deep-Sea Research, Part II, 57(19-20), DOI:10.1016/j.dsr2.2010.04.004. Abstract
Satellite observations have shown the abundance of generally westward-propagating eddies in the subtropical regions in the North Pacific Ocean, especially north of 10°N. Eddies transport mass, and can significantly impact the circulation as well as the heat, salt and nutrient balances of the western Pacific marginal seas. This paper uses a numerical model to examine the conditions when eddies can or cannot freely propagate westward through the Luzon Strait into the South China Sea (SCS). Composite analyses on the 10-year model data show that the fates of eddies depend on the strength and path of the Kuroshio. In one path that exists mostly during fall and winter, the Kuroshio loops westward into the SCS, the potential vorticity (PV) across the current is weak, and eddies are likely to propagate freely through the Luzon Strait. In another path, which exists mostly during spring and summer, the Kuroshio tends to leap directly northward bypassing the SCS, the PV across it strengthens, and eddies are then blocked and are constrained to also follow the northward path. Nonlinear eddy-current interaction and the existence of a cyclone north of the Luzon Island during the looping phase explain why eddies of both signs can pass through the strait. It is shown also that the upstream state of the Kuroshio in the western tropical Pacific plays an important role in dictating the different paths of the Kuroshio. The looping (leaping) path is caused by a weakened (stronger) Kuroshio transport related to the northward (southward) shift of the North Equatorial Current in wintertime (summertime).
Chang, Y-L, C-R Wu, and Leo Oey, 2009: Bimodal behavior of the seasonal upwelling off the northeastern coast of Taiwan. Journal of Geophysical Research, 114, C03027, DOI:10.1029/2008JC005131. Abstract
Observations over the outer shelf and shelf break off the northeastern coast of Taiwan indicate a curious seasonal variability of upwelling. At deeper levels 100 m below the surface, upwelling is most intense in summer but weaker in winter. Nearer the surface at approximately 30 m below the surface, the opposite is true and the upwelling is stronger in winter than in summer. Results from a high-resolution numerical model together with observations and simple Ekman models are used to explain the phenomenon. It is shown that the upwelling at deeper levels (∼100 m) is primarily induced by offshore (summer) and onshore (winter) migrations of the Kuroshio, while monsoonal change in the wind stress curl, positive in winter and negative in summer, is responsible for the reversal in the seasonal variation of the upwelling near the surface (∼30 m). This mechanism reconciles previous upwelling data.
Deep (not, vert, similar2000 m) observations near the Sigsbee escarpment in the Gulf of Mexico show short-period (approximately 5–12 days) energetic currents due to topographic Rossby waves (TRW’s). We suggest that the phenomenon is due to the focusing and accumulation of TRW energy by the slopes coupled with a bend in isobaths, in a topographic caustic (topocaustic). The idea draws on a simple mathematical equivalence between the propagation of internal waves and of TRW’s. Topocaustics occur near regions of maximum NT = N|backward differenceh| (N = Brunt–Väisälä frequency; h = water depth). Because of the one-sided propagation property of TRW’s, energy also tends to accumulate at the “western” end of closed contours of NT. The process is demonstrated here using a nonlinear primitive-equation numerical model with idealized bathymetry and forcing. A Gulf of Mexico simulation initialized with a data-assimilated analysis covering the period of the Sigsbee observation is then conducted. The mooring is near a localized maximum NT, and Intrinsic Mode Functions confirm the existence of energetic bursts of short-period deep-current events. The strong currents are locally forced from above, either by an extended Loop Current or a warm ring.
Ezer, Tal, R Hobbs, and Leo Oey, December 2008: On the movement of Beluga whales in Cook Inlet, Alaska. Oceanography, 21(4), 186-195.
Mellor, George L., M A Donelan, and Leo Oey, October 2008: A surface wave model for coupling with numerical ocean circulation models. Journal of Atmospheric and Oceanic Technology, 25(10), DOI:10.1175/2008JTECHO573.1. Abstract
A surface wave model is developed with the intention of coupling it to three-dimensional ocean circulation models. The model is based on a paper by Mellor wherein depth-dependent coupling terms were derived. To be compatible with circulation models and to be numerically economical, this model is simplified compared to popular third-generation models. However, the model does support depth and current refraction, deep and shallow water, and proper coupling with depth-variable currents.
The model is demonstrated for several simple scenarios culminating in comparisons of model calculations with buoy data during Hurricane Katrina and with calculations from the model Simulating Waves Nearshore (SWAN); for these calculations, coupling with the ocean was not activated.
Oey, Leo, M Inoue, R Lai, X-H Lin, S E Welsh, and L J Rouse, Jr, June 2008: Stalling of near-inertial waves in a cyclone. Geophysical Research Letters, 35, L12604, DOI:10.1029/2008GL034273. Abstract
Observations at the edge of the Loop Current after hurricane Katrina show inertial energy amplified at a depth of approximately 600∼700 m. Ray-analysis using the eddy field obtained from a numerical simulation with data assimilation suggests that the amplification is due to inertial motions stalled in a deep cyclone.
In contrast to the Loop Current and rings, much less is known about deep eddies (deeper than 1000 m) of the Gulf of Mexico. In this paper, results from a high-resolution numerical model of the Gulf are analyzed to explain their origin and how they excite topographic Rossby waves (TRWs) that disperse energy to the northern slopes of the Gulf. It is shown that north of Campeche Bank is a fertile ground for the growth of deep cyclones by baroclinic instability of the Loop Current. The cyclones have horizontal (vertical) scales of about 100 km (1000~2000 m) and swirl speeds ~0.3 m s−1. The subsequent development of these cyclones consists of two modes, A and B. Mode-A cyclones evolve into the relatively well-known frontal eddies that propagate around the Loop Current. Mode-A cyclone can amplify off the west Florida slope and cause the Loop Current to develop a “neck” that sometimes leads to shedding of a ring; this process is shown to be the Loop Current’s dominant mode of upper-to-deep variability. Mode-B cyclones are “shed” and propagate west-northwestward at speeds of about 2–6 km day−1, often in concert with an expanding loop or a migrating ring. TRWs are produced through wave–eddy coupling originating primarily from the cyclone birthplace as well as from the mode-B cyclones, and second, but for longer periods of 20~30 days only, also from the mode-A frontal eddies. The waves are “channeled” onto the northern slope by a deep ridge located over the lower slope. For very short periods (10 days), the forcing is a short distance to the south, which suggests that the TRWs are locally forced by features that have intruded upslope and that most likely have accompanied the Loop Current or a ring.
Wang, D-P, and Leo Oey, April 2008: Hindcast of waves and currents in Hurricane Katrina. Bulletin of the American Meteorological Society, 89(4), DOI:10.1175/BAMS-89-4-487. Abstract
Hurricane Katrina caused extensive damage to offshore oil and gas production facilities. In this study, the state-of-the-art ocean circulation (the Princeton Ocean Model) and surface wave (Wave Watch III) models, together with high-resolution analyzed winds from NOAA/Hurricane Research Division, are used to simulate the current and wave conditions during Katrina. The model simulation shows large (>15 m) surface waves and strong (>2 m s−1) wind-driven and inertial currents superposed on the Loop Current and Loop Current eddy. The simulated wave fields are verified with surface buoy and satellite altimetry observations; the agreement generally is better than 0.5 m, and the correlation coefficient is above 0.95. Also, while the observed 55-ft significant wave heights on National Data Buoy Center (NDBC) buoy 42040 surpassed the previous record in the Gulf of Mexico, circumstantial evidence suggests that waves as large as 70 ft might have occurred in the storm path. Comparison with the operational analysis suggests that the current NCEP model system tends to underestimate the spatial extent of the serious wave impact.
Wu, C-R, Y-L Chang, Leo Oey, C-W J Chang, and Y-C Hsin, 2008: Air-sea interaction between tropical cyclone Nari and Kuroshio. Geophysical Research Letters, 35, L12605, DOI:10.1029/2008GL033942. Abstract
The air-sea interaction between tropical cyclone Nari (Sep/6–16/2001) and Kuroshio is studied using satellite observations and an ocean model. Nari crossed the Kuroshio several times, which caused variations in typhoon intensity. Nari weakened when it was over the shelf north of Kuroshio where cooling took place due to mixing of the shallow thermocline. The cyclonic circulation penetrated much deeper for the slowly-moving storm, regardless of Nari's intensity. Near-inertial oscillations are simulated by the model in terms of the vertical displacement of isotherms. The SST cooling caused by upwelling and vertical mixing is effective in cooling the upper ocean several days after the storm had passed. At certain locations, surface chlorophyll-a concentration increases significantly after Nari's departure. Upwelling and mixing bring nutrient-rich subsurface water to the sea surface, causing enhancement of phytoplankton bloom.
Hsu, H-M, Leo Oey, W Johnson, C Dorman, and R Hodur, May 2007: Model wind over the central and southern California coastal ocean. Monthly Weather Review, 135(5), DOI:10.1175/MWR3389.1. Abstract
Recent studies have shown the importance of high-resolution wind in coastal ocean modeling. This paper tests the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) at the 9-, 27-, and 81-km grid resolutions in simulating wind off the central and southern California coasts, including the Santa Barbara Channel (SBC). The test period is March–May (1999) when the wind changes from its characteristics more typical of winter, to spring when strong gradients exist in the SBC. The model results were checked against wind station time series, Special Sensor Microwave Imager wind speeds, and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis. The high-resolution (9-km grid) COAMPS wind shows expansion fans downwind of major capes where speed increases. The large-scale [O(100 km)] wind turns onshore in the Southern California Bight where both wind and wind stress curl weaken southward along the coast. The formation and evolution of the Catalina eddies are also simulated. These general features agree with observations. The turning appears to be the cumulative effect of synoptic cyclones shed downwind of Point Conception during periods of intense northerly wind. The turning and eddies are much weaker in the ECMWF reanalysis or the COAMPS field at the 81-km grid. Near the coast, observed small-scale (tens of kilometers) structures are reasonably reproduced by COAMPS at the 9-km grid. Results from the 9-km grid generally compare better with observations than the 27-km grid, suggesting that a more accurate model wind may be obtained at even higher resolution. However, in the SBC, simulated winds at both the 9- and 27-km grids show along-channel coherency during May, contrary to observations. The observed winds in the channel appear to be of small localized scales (~<10 km) and would require an improved model grid and perhaps also boundary layer physics to simulate.
Lin, X-H, Leo Oey, and D-P Wang, 2007: Altimetry and drifter data assimilations of loop current and eddies. Journal of Geophysical Research, 112, C05046, DOI:10.1029/2006JC003779. Abstract
The
goal is to quantify in an eddy-rich ocean environment the accuracy of
currents obtained from a multi-level primitive-equation ocean model that
assimilates altimetry sea-surface height anomaly (SSHA) and surface
drifters. The 1999–2000 period in the Gulf of Mexico is chosen for the
availability of drifters and Acoustic Doppler Current Profiler (ADCP)
measurements in the loop current and eddies during that period. Sequential
assimilations of SSHA and/or currents with statistical-interpolation schemes
are used. Experiments initialized with and without altimetry and/or drifter
assimilations, including a forecast case (without assimilation) for
comparison, are conducted. It is shown that the SSHA + Drifter analysis
consistently outperforms the analysis that assimilates only SSHA, especially
at smaller scales. Drifter-assimilation alone also constraints the pressure
field, such that the loop and eddies compare quite favorably with altimetry
SSH field. When compared against independent ADCP data, the analyses with
either SSHA or SSHA + Drifter assimilation yield amplitude and phase of
analysis-to-observed complex (velocity) correlation of 0.76~0.86 and
0.3°~7°, respectively. The mean speed and direction (absolute) errors are
0.4–5 cm s-1 (1~10% errors) and 10°~20°, respectively. The
correlations of the two dominant empirical orthogonal function (EOF) modes
with the corresponding observation modes at a yearlong mooring over the
northern slope are: mode-1
0.88~0.93 and mode-2
0.5~0.63. Both show
vertically coherent but strongly sheared modes 1 and 2 representing
propagation eddies and reversing mode-3 that intensifies for z < -300
m.
Oey, Leo, Tal Ezer, C Hu, and F E Muller-Karger, 2007: Baroclinic tidal flows and inundation processes in Cook Inlet, Alaska: numerical modeling and satellite observations. Ocean Dynamics, 57(3), DOI:10.1007/s10236-007-0103-8. Abstract
A
wetting and drying (WAD) algorithm is implemented in a baroclinic
three-dimensional ocean circulation model of Cook Inlet, Alaska, where large
tidal ranges 10 m) regularly expose extensive mudflats. The model includes
tides and wind- and buoyancy-induced flows. In the upper Inlet, the model
successfully simulates large amplification of tides and propagation of fast
(3~4 m s-1) tidal bores over shallow mudflats. The simulated
return flows during ebb expose large areas (~100 km2) of the
mudflats. Medium-resolution (250- and 500-m) images obtained from the
moderate resolution imaging spectroradiometer (MODIS) instruments aboard the
Terra and Aqua satellites were used to verify the model results by
identifying the location, extent, and temporal changes of the exposed
mudflat regions. The results demonstrate the value of operational,
medium-resolution remote sensing data in evaluating the WAD model. Sensitivity
tests show that WAD produces approximately 20% larger tidal amplitude and
10% slower phase than the corresponding experiment without WAD. In the deep
channel of the central Inlet, the confluence of saline water of the lower
Inlet with brackish water from rivers and melting ice from land around the
upper Inlet produces a salinity front. At the simulated front, strong
vertical circulation cells and surface convergence and currents develop,
especially during the flood. The characteristics resemble those of “rip
tides” often observed in this region.
Ocean forecasting with a General Circulation Model (GCM) commonly begins
from an initial analysis obtained by data assimilation. Instead of a single
initial state, bred-ensemble forecast [BEnF; which is used for weather
forecasting at the National Centers for Environmental Prediction] begins
from an ensemble of initial states obtained by using the GCM to breed
fast-growing modes into the analysis. Here we apply the technique to
forecast the locations and strengths of the Loop Current and rings from July
through September 2005. Model results are compared against satellite
observations, surface drifter trajectories, and moored currents. It is found
that BEnF gives closer agreements with observations than the conventional
single forecast. The bred-vectors (perturbed minus unperturbed
state-vectors) have growth rates
~0.04–0.08
day-1 and spatial (cyclone–anticyclone) scales
200–300 km
suggestive of baroclinic instability mode in the Loop Current and rings. As
in atmospheric applications, initializations with these growing vectors
contribute to the more accurate ensemble mean forecast.
An ocean general circulation model (OGCM) with wetting and drying (WAD) capabilities removes the vertical-wall coastal assumption and allows simultaneous modeling of open-ocean currents and water run-up (and run-down) across movable land–sea boundaries. This paper implements and tests such a WAD scheme for the Princeton Ocean Model (POM) in its most general three-dimensional setting with stratification, bathymetry and forcing. The scheme can be easily exported to other OGCM’s.
Oey, Leo, Tal Ezer, D-P Wang, S Fan, and X-Q Yin, 2006: Loop Current warming by Hurricane Wilma. Geophysical Research Letters, 33, L08613, DOI:10.1029/2006GL025873. Abstract
Hurricanes mix and cool the upper ocean, as shown here in observations and modeling of the Caribbean Sea and the Gulf of Mexico during the passage of hurricane Wilma. Curiously, the upper ocean around the Loop Current warmed prior to Wilma's entrance into the Gulf. The major cause was increased volume and heat transports through the Yucatan Channel produced by storm-induced convergences in the northwestern Caribbean Sea. Such oceanic variability may have important impacts on hurricane predictions.
Dong, C, and Leo Oey, 2005: Sensitivity of coastal currents near Point Conception to forcing by three different winds: ECMWF, COAMPS, and Blended SSM/I-ECMWF-Buoy Winds. Journal of Physical Oceanography, 35(7), DOI:10.1175/JPO2751.1. Abstract
Previous observational and modeling studies have indicated the importance of finescale winds in determining the circulation near Point Conception in the Santa Maria Basin (SMB) and the Santa Barbara Channel (SBC), California. There has not been a systematic attempt, however, to analyze and quantify the sensitivity of the near-surface circulation to different wind data. Here, a regional circulation model of the SMB and SBC is driven using three wind datasets: the European Centre for Medium-Range Weather Forecasts (ECMWF; 110 km 40%) is unidirectional (i.e., generally equatorward or poleward) and correlates well with the mode-1 wind (90%). The mode-2 current (20%) is cyclonic in the SBC and poleward inshore and equatorward offshore in the SMB; it correlates well with mode-1 sea level (70%), which suggests that mode-2 currents are driven by the pressure gradient. It is significant that neither mode-2 current nor mode-1 sea level correlates well with mode-1 wind stress curl (70%); rather, they correlate well with the time integral of the mode-1 wind stress curl. These conclusions support a previous theoretical idea that cyclonic circulation in the SBC and the inshore currents of the SMB are both driven by alongshore pressure setup induced by the time integral of the wind stress curl, rather than by the wind stress curl itself. This idea of a pressure setup is consistent with the differences found between the currents driven by COAMPS and SEB winds.
In shallow-water models, wetting and drying (WAD) are determined by the total water depth D=0 for'dry' and >0 for'wet'. Checks are applied to decide the fate of each cell during model integration. It is shown that with bottom friction values commonly used in coastal models, the shallow-water system may be cast into a Burger's type equation for D. For flows dominated by D (i.e. |D|>>| H|, where H(x,y) defines topography) a non-linear diffusion equation results, with an effective diffusivity that varies like D2, so that'dry' cells are regions where'diffusion' is very small. In this case, the system admits D=0 as part of its continuous solution and no checks are necessary. For general topography, and/or in the case of strong momentum advection,'wave-breaking' solution (i.e. hydraulic jumps and/or bores) can develop. A WAD scheme is proposed and applied to the Princeton Ocean Model (POM). The scheme defines'dry' cells as regions with a thin film of fluid O (cm). The primitive equations are solved in the thin film as well as in other regular wet cells. The scheme requires only flux-blocking conditions across cells' interfaces when wet cells become dry, while ‘dry' cells are temporarily dormant and are dynamically activated through mass and momentum conservation. The scheme is verified against the above-mentioned diffusion and Burger's type equations, and tested also for one and two-dimensional channel flows that contain hydraulic jumps, including a laboratory dam-break problem.
Oey, Leo, Tal Ezer, G Forristall, C Cooper, S DiMarco, and S Fan, 2005: An exercise in forecasting loop current and eddy frontal positions in the Gulf of Mexico. Geophysical Research Letters, 32, L12611, DOI:10.1029/2005GL023253. Abstract
As part of a model-evaluation exercise to forecast Loop Current and Loop Current eddy frontal positions in the Gulf of Mexico, the Princeton Regional Ocean Forecast System (PROFS) is tested to forecast 14 4-week periods Aug/25/99–Sep/20/00, during which a powerful eddy, Eddy Juggernaut (Eddy-J) separated from the Loop Current and propagated southwestward. To initialize each forecast, PROFS assimilates satellite sea surface height (SSH) anomaly and temperature (SST) by projecting them into subsurface density using a surface/subsurface correlation that is a function of the satellite SSH anomaly. The closest distances of the forecast fronts from seven fixed stations in the northern Gulf over a 4-week forecast horizon are then compared against frontal observations derived primarily from drifters. Model forecasts beat persistence and the major source of error is found to be due to the initial hindcast fields.
Oey, Leo, Tal Ezer, and Hyun-Chul Lee, 2005: Loop Current, rings and related circulation in the Gulf of Mexico: A review of numerical models and future challenges In Circulation in the Gulf of Mexico: Observations and Models, Washington, DC, American Geophysical Union, 31-56. Abstract
Progress in numerical models of the Loop Current, rings, and related circulation during the past three decades is critically reviewed with emphasis on physical phenomena and processes.
Schmitz, Jr, W J., R R Bidigare, A Lugo-Fernandez, Leo Oey, and W Sturges, 2005: A synopsis of the circulation in the Gulf of Mexico and on its Continental margins In Circulation in the Gulf of Mexico- Observations and Models, Washington, DC, American Geophysical Union, 11-29. Abstract
This article is a synopsis of the state-of-the-art knowledge and understanding of the circulation in the deep Gulf of Mexico as well as in the coastal flow regimes on its continental margins. The primary purpose is to review the ideas and results in this special new volume on the circulation in the Gulf of Mexico and integrate them with material from selected previous publications. This overview therefore contains more repetition and expository discussion than normally found in journal articles. An extensive reference list is provided for interested readers, a special feature for students. #This volume is also meant to appeal to an audience beyond the general population of physical oceanographers, to include a more diverse community of Marine Scientists and Engineers, along with social interests of a scientific/technical nature. The article in this volume by Biggs et al. is focused on a study of some specific biogeochemical implications of on- and off-margin flow (on- and off-continental shelf/slope regions) in the northern Gulf of Mexico. The articles by Chassignet et al. [this volume] and Morey et al. [this volume], for example, note particular applications. Potential influences by the currents in the Gulf of Mexico on processes of interest in other branches of Marine Science are occasionally considered in this synopsis (for example, there is a special section on the effects of current patterns on coral reefs). The reader may also refer to a comparatively recent review of some of the physical oceanographic influences that regulate the biology of the Gulf of Mexico by Wiseman and Sturges [1999]. Finally, this synopsis is meant to complement and expand upon the Introduction to this volume by Sturges et al., which includes consideration of some general societal interests motivating marine research in the Gulf of Mexico.
Fan, S, Leo Oey, and P Hamilton, 2004: Assimilation of drifter and satellite data in a model of the Northeastern Gulf of Mexico. Continental Shelf Research, 24(9), DOI:10.1016/j.csr.2004.02.013. Abstract
Drifter and satellite data are assimilated into a circulation model that hindcasts near-surface currents in the Northeastern Gulf of Mexico. Experiments without assimilation, and using assimilation of drifter, satellite sea-surface height (SSH) and sea-surface temperature (SST) data, in various combinations, were conducted. Currents derived from these experiments were used to compute drifter trajectories that were compared against observations. Surface geostropic current fields, calculated from satellite SSH, were also used to generate drifter paths. Assimilation that used a combination of drifter and satellite data reproduced the drifter trajectories with position errors ≈30–80 km over a 10-day period. Comparisons of the modeled currents with moored observations on the West Florida shelf show improvement when data assimilation is used, because of better simulation of deepwater processes (primarily the loop current).
Oey, Leo, 2004: Vorticity flux through the Yucatan Channel and Loop Current variability in the Gulf of Mexico. Journal of Geophysical Research, 109, C10004, DOI:10.1029/2004JC002400. Abstract
Recent observations (the CANEK Program [ Candela et al., 2002 ]) suggest that potential vorticity (PV) flux anomaly (VFA) at Yucatan Channel may serve as a useful indicator of Loop Current variability, including Loop Current extension, retraction, and eddy shedding. Intuitively, anticyclonic VFA extends the Loop Current into the Gulf of Mexico and cyclonic VFA causes retraction or even shedding. However, this intuition is inconsistent with PV conservation. The problem is reexamined here by careful analyses of the relation between VFA and Loop Current variability using (1) the results of a 15-year numerical simulation of shedding specified with simple forcing, and (2) CANEK and satellite observations. Both model and observations indicate that Loop Current eddy shedding or retraction tends to occur shortly (1∼2 months) after the influx of VFA at Yucatan has turned anticyclonic, and that these events are sometimes preceded by a more prolonged period of influx of cyclonic VFA. These findings suggest that contrary to intuition, influx of cyclonic VFA tends to extend the Loop Current into the Gulf, thus making the Loop Current more susceptible to retract or shed an eddy, and influx of anticyclonic VFA may then “trigger” retraction or eddy shedding. However, the Loop Current's behaviors are much more complex than can be prescribed by these simple rules. A much longer observational data set, coupled with more refined model experiments and sophisticated analyses, is required to further quantify the phenomenon.
Oey, Leo, Tal Ezer, and W Sturges, 2004: Modeled and observed empirical orthogonal functions of currents in the Yucatan Channel, Gulf of Mexico. Journal of Geophysical Research, 109, C08011, DOI:10.1029/2004JC002345. Abstract
Candela et al. [2003] have reported empirical orthogonal function (EOF) analyses based on 23-month current-meter and acoustic Doppler current profiler measurements in the Yucatan Channel. Those authors noted the difference between EOFs obtained from observations and their z-level models and EOFs calculated by Ezer et al. [2003] from the results of a terrain-following model. Here a new analysis is reported that explains this difference, and that also suggests the importance of shelf-edge meander mode of the core Loop Current in the channel. We show that the terrain-following model gives EOFs with characteristics similar to those observed when data from the upper slope and shelf in the western portion of the model channel are omitted. Modes 1 and 2 have tripole and dipole structures with energies (35%, 26%), respectively, of total energy, and correlate with "slow" vacillation of the core-current for periods >50 days. Exclusion of upper-slope and shelf data eliminates a short-period and energetic component inherent in Ezer et al.'s original mode 1 EOF. This mode correlates with frontal meanders of the core current over the shelf edge in the western portion of the channel. The short-period mode may be missing or underestimated in observational and z-level models' analyses, since there were only a few moorings over the upper slope and shelf, and z-level models have step-like topography with generally lower resolution in shallower seas.
Oey, Leo, C Winant, E Dever, W Johnson, and D-P Wang, 2004: A model of the near-surface circulation of the Santa Barbara channel: comparison with observations and dynamical interpretations. Journal of Physical Oceanography, 34(1), 23-43. Abstract
Previous studies indicate the importance of wind, wind curl, and density differences in driving the near-surface circulation in the Santa Barbara Channel (SBC). Here model sensitivity experiments and dynamical analyses of the near-surface currents in the SBC are presented. Various approximations of the wind—from coarse-resolution European Centre for Medium-Range Weather Forecasts (ECMWF) archives to a high-resolution dataset that incorporates buoy, oil-platform, and land-based wind stations—are used. In some experiments, observed temperatures at 10 moorings are also assimilated into the model. Model solutions are sensitive to channel-scale [O(10 km)] wind distribution. Modeled currents forced by the ECMWF wind yield poor results when compared with observations. The simulation using the high-resolution wind (without assimilation) captures the observed spatial and seasonal patterns of the circulation, though the intensity is underestimated. With assimilation, the intensity is increased. In particular, the western-channel cyclone is reproduced well. Momentum analyses suggest that the cyclone is maintained by oppositely directed, time-dependent pressure gradients (PG) along the northern and southern coasts of the channel. These PGs are, in turn, caused by warming episodes probably related to wind relaxations. Momentum analysis also identifies along-channel PG (APG) as a dynamic index of the seasonal circulation. APG is strongly poleward in summer and autumn and becomes weak in winter. The poleward APG is eroded by equatorward wind bursts in late winter through spring during which period it changes sign to weakly equatorward. The APG becomes poleward again in early summer with the arrival of a large-scale warming signal from the Southern California Bight. The model does poorly in the eastern portion of the channel, in which region remote forcing at long periods (10–30 days) has been identified in previous observational studies. The model fails to reproduce the intense springtime (April) equatorward current -0.2 m s-1) at the eastern channel entrance. The corresponding variance is also underestimated. The remote forcing is not accounted for in the model because climatological conditions are specified at the open boundary in the Southern California Bight.
Oey, Leo, and H C Zhang, 2004: The generation of subsurface cyclones and jets through eddy-slope interaction. Continental Shelf Research, 24(18), DOI:10.1016/j.csr.2004.07.007. Abstract
A mechanism for the generation of subsurface cyclones and jets when a warm ring smashes onto a continental slope and shelf is proposed based on the results of a primitive-equation three-dimensional numerical model. The warm ring initially ‘sits’ over a slope with an adjoining shelf in a periodic channel, and its subsequent evolution is examined. The ‘inviscid’ response is cyclonic ‘peeling-off’ of the on-slope portion of the warm ring. The cyclone propagates away (to the left looking on-slope) from the warm ring, and is bottom-intensified as well as slope-trapped (cross-slope scale ≈ Rossby radius). The near-surface flow ‘leaks’ further onto the shelf while subsurface currents are blocked by the slope. The ‘viscous’ response consists of the formation of a bottom boundary layer (BBL) with a temporally and spatially dependent displacement thickness. The BBL ‘lifts’ the strong along-slope (leftward) current or jet (speed >0.5 m s-1) away from the bottom. The jet, coupled with weak stratification within the BBL and convergence due to downwelling across the slope, becomes supercritical. Super-inertial disturbance in the form of a hydraulic jump or front, with strong upwelling and downwelling cell, and the jet, propagate along the slope as well as off-slope and upward into the water column. The upward propagation is halted at z≈ztrap when mixing smoothes out the ‘jump’ to an along-slope scale λtrap that allows the ambient jet to bend the propagation path horizontal. At this ‘matured’ stage, ztrap≈−250 m, λtrap≈50 km, and the jet's cross-slope and vertical scales are ≈30 km and 50 m, respectively. An example that illustrates the process under a more realistic setting in the Gulf of Mexico when the Loop Current impinges upon the west Florida slope is given. The phenomenon may be relevant to the recent oil industry's measurements in the Gulf, which at times indicate jets at z≈−150 m through −400 m over the slope.
.
Ezer, Tal, Leo Oey, Hyun-Chul Lee, and W Sturges, January 2003: The variability of currents in the Yucatan Channel: Analysis of results from a numerical ocean model. Journal of Geophysical Research, 108(C1), 3012, DOI:10.1029/2002JC001509. Abstract
The flow through the Yucatan Channel and into the Gulf of Mexico is a major component of the Gulf Stream and the subtropical gyre circulation. Surprisingly, however, little is known about the forcing and physical parameters that affect the current structures in the Channel. This paper attempts to improve our understanding of the flow through the Channel with a detailed analysis of the currents obtained from a primitive-equation model that includes the Gulf and the entire Caribbean Sea and forced by 6-hourly wind from ECMWF. The analysis includes two parts: First, the overall statistics of the model results, including the Loop Current (LC) variability, the frequency of LC eddy-shedding, and the means and standard deviations (SD) of transports and currents, are compared with observations. Secondly, an Empirical Orthogonal Function (EOF) analysis attempts to identify the physical parameters responsible for the dominant modal fluctuations in the Channel. The model LC sheds seven eddies in 4 years at irregular time intervals (6.6, 7.1, 5.3, 11.9, 4.2, 10.9 months). The model's upper (thickness ~800 m) inflow into the Gulf of Mexico occupies two-thirds of the Channel on the western side, with a near-surface maximum (4-year) mean of around 1.5 m s-1 and SD approximately 0.4 m s-1 . Three (return) outflow regions are identified, one in the upper layer (thickness ~600 m) on the eastern third of the Channel, with mean near the surface of about 0.2 m s -1 and SD approximately 0.14 m s -1 , and two deep outflow cores, along the western and eastern slopes of the Channel, with (Mean, SD) approximately (0.17, 0.05) and (0.09, 0.07) ms-1 , respectively. The total modeled Channel transport varies from 16 to 34 Sv (1 Sverdrup = 106 m3s-1) with a mean around 25 Sv. The above velocity and transport values agree quite well with observations by Maul et al. [1985], Ochoa et al. [2001], and Sheinbaum et al. [2002]. The deep return transport below 800 m was found to correlate with changes in the Loop Current extension area, in agreement with the observational analysis by Bunge et al. [2002]. The EOF mode#1 of the along-channel currents contains 50% of the total energy. It is surface-trapped, is 180° out of phase across the channel, and correlates well with the cross-channel vacillations of the LC frontal position. The EOF mode#2 contains 18% of the energy, and its structure mimics that of the mean flow: dominated by two vertically more coherent regions that are180° out of phase across the Channel. The mode is dominated by two periods, approximately 11 months and 2 months respectively, and correlates with the upper-channel inflow transport. The third and fourth modes, together, account for 18% of the total energy. Their combined time series correlates with the deep current over the sill, and is dominated by fluctuations with a period approximately 205 days coincident with the dominant low-frequency fluctuations inherent in Maul et al.'s [1985] sill measurement. Thus the dominant mode of flow fluctuations in the Yucatan Channel is caused by LC cross-frontal movements which may not be directly related to LC eddy-sheddings, while higher modes correspond to transport fluctuations that affect eddy-sheddings, and to bottom-trapped current fluctuations, the cause of which has yet to be fully uncovered.
Oey, Leo, Hyun-Chul Lee, and W J Schmitz, Jr, October 2003: Effects of winds and Caribbean eddies on the frequency of Loop Current eddy shedding: A numerical model study. Journal of Geophysical Research, 108(C10), 3324, DOI:10.1029/2002JC001698. Abstract
The Loop Current (LC) is known to shed eddies at irregular intervals from 3 to 17 months. The causes of this irregularity have not, however, been adequately identified previously. We examine the effects of various types of external forcing on shedding with a model of the western North Atlantic Ocean (96°-55°W, 6°-50°N). We force the model with steady transport at 55°W, with winds, and include eddies in the Caribbean Sea. We examine their separate effects. With steady transport only, the model sheds rings at a dominant period of 9-10 months. Wind-induced transport fluctuations through the Greater Antilles Passages cause shedding at shorter intervals (≈3-7 months). Caribbean eddies (anticyclones) cause shedding at longer periods (≈14-16 months). Potential vorticity conservation indicates that Caribbean eddies tend to deter northward extension of the LC into the Gulf, which can lead to longer periods between eddy shedding. Fluctuating inflow at the Yucatan Channel that is associated with winds and/or Caribbean eddies can cause an LC eddy to temporarily (~1 month) detach from and then reattach back to the LC, a phenomenon often observed. Model results also suggest that southwest of Hispaniola, warm eddies are spun up by the local wind stress curl. This type of eddy drifts southwestward, then westward after merging with the Caribbean Current, and then northward as it progresses toward the Yucatan Channel; these eddies significantly affect the shedding behavior of warm-core rings. The timescale for spin up and drift from Hispaniola is about 100 days. Satellite data indicate the existence of these eddies in the real ocean.
Wang, D-P, Leo Oey, Tal Ezer, and P Hamilton, 2003: Near-surface currents in DeSoto Canyon (1997–99): Comparison of current meters, satellite observation, and model simulation. Journal of Physical Oceanography, 33(1), 313-326. Abstract
This study evaluates a data-assimilated model simulation of near-surface circulation in DeSoto Canyon (DSC), Gulf of Mexico, with emphasis on analyzing moored current-meter observations and comparing them with satellite data and model results. The study period is for two years from April 1997 to April 1999. The model results are from a high-resolution Gulf of Mexico model forced by analyzed wind and surface heat flux. Two types of data are used to deduce near-surface circulation: moored current meters at 13 locations in the DSC, and satellite sea level anomaly. The moored currents are mapped through multivariate objective analysis to produce surface currents and surface geopotentials, against which satellite- and model-derived sea surface heights and geostrophic currents are compared. Coupled patterns between the observations, model results, and satellite data are obtained using the singular value decomposition (SVD) analysis. There are two dominant modes: a “single-eddy” mode, in which currents are concentrated at the foot of the canyon, and an “eddy-pair” mode, in which one eddy is at the foot of the canyon and the other, a counterrotating eddy, is over the head of canyon. Mode 1 appears to be associated with the mesoscale eddy traveling around the Loop Current crest and trough, and mode 2 is associated with the intrusion of Loop Current crest and trough over the west Florida shelf. The observed and model currents are in good agreement about the means and variances. The model currents also appear to be well constrained by the steep topography. However, the model velocity field contains only the first mode. The satellite-derived velocity field, on the other hand, contains both the first and second modes; though, the satellite field does not adequately resolve the velocity structures over the slope.
Ezer, Tal, Leo Oey, and Hyun-Chul Lee, 2002: Simulation of velocities in the Yucatan Channel In Oceans, Columbia, MD, MIS/IEEE Publ., Marine Techn. Society, 1467-1471. Abstract
As part of the analysis of results from high resolution numerical simulations of the Gulf of Mexico and the Caribbean Sea, the structure and variability of the flow across the Yucatan Channel are described and compared with observations. The main model inflow into the Gulf is found near the surface in the western part of the Channel, while return flows back into the Caribbean Sea are found near the surface on the eastern side of the Channel and along the eastern and western slopes around 1500 m depth, in agreement with recent observations. Variations in the upper inflow and deep outflow transports seem to correlate with variations in the extension of the Loop Current, as suggested by previous analyses of observations and models. Such correlations are especially high near the time when Loop Current eddies are shed into the Gulf of Mexico.
Oey, Leo, and Hyun-Chul Lee, 2002: Deep eddy energy and topographic Rossby waves in the Gulf of Mexico. Journal of Physical Oceanography, 32(12), 3499-3527. Abstract
Observations suggest the hypothesis that deep eddy kinetic energy (EKE) in the Gulf of Mexico can be accounted for by topographic Rossby waves (TRWs). It is presumed that the TRWs are forced by Loop Current (LC) pulsation, Loop Current eddy (LCE) shedding, and perhaps also by LCE itself. Although the hypothesis is supported by model results, such as those presented in Oey, the existence of TRWs in the model and how they can be forced by larger-scale LC and LCEs with longer-period vacillations have not been clarified. In this paper, results from a 10-yr simulation of LC and LCEs, with double the resolution of that used by Oey, are analyzed to isolate the TRWs. It is shown that along an east-to-west band across the gulf, approximately over the 3000-m isobath, significant EKE that accounts for over one-half of the total spectrum is contained in the 20–100-day periods. Bottom energy intensification exists in this east–west band with vertical decay scales of about 600–300 m decreasing westward. The decrease agrees with theTRW solution. The band is also located within the region where TRWs can be supported by the topographic slope and stratification used in the model and where wavenumber and frequency estimates are consistent with the TRW dispersion relation. The analysis indicates significant correlation between pairs of east–west stations, over distances of approximately 400 km. Contours of lag times suggest offshore (i.e.,downslope) phase propagation, and thus the east–west band indicates nearly parabathic and upslope energy propagation. Ray tracing utilizing the TRW dispersion relation and with and without (for periods >43 days) ambient deep currents shows that TRW energy paths coincide with the above east–west high-energy band. It also explains that the band is a result of TRW refraction by an escarpment (with increased topographic gradient) across the central gulf north of the 3000-m isobath, and also by deep current and its cyclonic shear, and that ray convergence results in localized EKE maxima near 91°W and 94°–95°W. Escarpment and cyclonic current shear also shorten TRW wavelengths. Westward deep currents increase TRW group speeds, by about 2–3 km day1 according to the model, and this and ray confinement by current shear may impose sufficient constraints to aid in inferring deep flows. Model results and ray paths suggest that the deep EKE east of about the 91°W originates from under the LC while farther west the EKE also originates from southwestward propagating LCEs. The near-bottom current fluctuations at these source regions derive their energy from short-period (<100 days) and short-wavelength (<200 km) near-surface fluctuations that propagate around the LC during its northward extrusion phase and also around LCEs as they migrate southwestward in the model.
Oey, Leo, D-P Wang, T Hayward, C Winant, and M Hendershott, 2001: . Journal of Geophysical Research, 106(C5), 9213-9222. Abstract
The observed near-surface circulation in the Santa Barbara Channel indicates in particular two patterns: a dominant cyclonic circulation mode and a less frequent upwelling flow mode. To explain the dynamics that may govern these two flow regimes, momentum balance from a hindcast model of currents in the channel, forced by observed hourly winds and hydrographic data, was calculated. The along-channel balance was found to be between wind, which was eastward (i.e., equatorward), sea level tilt, which was westward (i.e., poleward), and Coriolis, which was westward if the wind was (1) intense west and east of the channel and was eastward if the wind was (2) weaker in the east. Wind condition 1 produced southward cross-channel flow in the midchannel, connected by eastward currents upstream (downstream) along the northern (southern) coast of the channel, while wind condition 2 produced northward cross-channel flow connected by cyclonic recirculation in the west and westward inflow in the east. It is suggested that the former corresponds to the dynamical balance that may occur in the upwelling flow mode, while the latter corresponds to the cyclonic circulation mode.
Oey, Leo, 1999: A forcing mechanism for the poleward flow off the southern California coast. Journal of Geophysical Research, 104(C6), 13,529-13,539. Abstract
It is shown that when the wind distribution along a coast is anisotropic, such that its cross-shore scale is smaller than its alongshore scale, the coastal sea level (or the upper layer anomaly) "A" of an ocean forced by both wind and wind curl is governed by a modified (nondimensionalized) Kelvin wave equation: (equation omitted) where k0 and k1 are wind stress and wind stress curl at the coast, respectively, y is the alongshore distance, and t is the time. Numerical experiments, from a simple reduced-gravity type with idealized forcing and coastline to a three-dimensional primitive equation model with a realistic coastline, bottom topography of the Southern California Bight and the Santa Barbara Channel, and observed wind stresses, were carried out to show that the observed near-coast near-surface poleward flow in the region is primarily forced by the equatorward weakening of the wind curl, in the bight. Beta provides natural damping by weakening and widening the current through westward propagating Rossby waves and causes the current to lead the coastal pressure field by 1-2 months, which improves the agreement with observations of the phasing of the modeled currents but is otherwise not required in forcing the poleward flow.
Mellor, George L., Leo Oey, and Tal Ezer, 1998: Sigma coordinate pressure gradient errors and the seamount problem. Journal of Atmospheric and Oceanic Technology, 15(5), 1122-1131. Abstract
In a recent paper by Mellor, et al., it was found that, in two-dimensional (x, z) applications with finite horizontal viscosity and zero diffusivity, the velocity error, associated with the evaluation of horizontal density or pressure gradients on a sigma coordinate grid, prognostically disappeared, leaving behind a small and physically insignificant distortion in the density field. The initial error is numerically consistent in that it decreases as the square of the grid increment size. In this paper, we label this error as a sigma error of the first kind.
In three-dimensional applications, the authors have encountered an error that did not disappear and that has not been understood by us or, apparently, others. This is a vorticity error that is labeled a sigma error of the second kind and is a subject of this paper. Althogh it does not prognostically disappear, it seems to be tolerably small. To evaluate these numerical errors, the authors have adopted the seamount problem initiated by Beckman and Haidvogel. It represents a stringent test case, as evidenced by their paper, wherein the model is initialized with horizontal isopycnals, zero velocity, and no forcing; then, any velocities that develop must be considered errors.
Two appendices are important adjuncts to the paper, the first providing theoretical confirmation and understanding of the numerical results, and the second delving into additional errors related to horizontal or isosigma diffusion. It is, however, shown that satisfactory numerical solutions are obtained with zero diffusivity.
Oey, Leo, 1998: Subtidal energetics in the Faroe-Shetland Channel: Coarse-grid model experiments. Journal of Geophysical Research, 103(C6), 12,689-12,708. Abstract
This paper compares various forcings that contribute to the regional subtidal energetics of the ocean passageway northwest of Scotland, in the Faroe-Shetland Channel (FSC), where inflow of warm and saline North Atlantic water mixes with outflow of cold and less-saline Nordic Sea waters. On the scales resolvable by the (coarse) grid sizes of delta x = delta y = 20 km and 20 vertical sigma layers, changes in currents' energetics caused by wind, Atlantic inflow (doubled), decreased horizontal mixing, surface heat flux, and open-boundary density specifications were 28, 43, 40, 16, and 5%, respectively, of the kinetic energy of a background quasi-steady slope current. The subtidal currents were not sensitive to the atmospheric pressure forcing and the surface relaxation to monthly climatology nor to the inclusion of additional (apart from M2) tidal constituents K1, O1, and S2. Wind-induced motions resulted in transport fluctuations of about 1.5 Sv in the FSC, maximum in winter and minimum in summer, and alongshore and cross-shore current variances of 0.1 and 0.05 m s-1, respectively, in fair agreement with observations. Spectral peaks at periods of 23-30 hours were found and were shown to correspond to resonant continental shelf waves in the channel.
Cummins, P F., and Leo Oey, 1997: Simulation of barotropic and baroclinic tides off northern British Columbia. Journal of Physical Oceanography, 27(5), 762-781. Abstract
The tidal response of northern British Columbia coastal waters is studied through simulations with a three-dimensional, prognostic, primitive equation model. The model is forced at the boundaries with the leading semidiurnal and diurnal constituents and experiments with stratified and homogeneous fluid are compared. The barotropic response shows good agreement with previously published studies of tides in the region. A comparison with tide gauge measurements indicates that average relative rms differences between observations and the model surface elevation field are less than 5% for the largest constituents.
An internal tide is generated in cases where the model is initialized with a vertical stratification. Diagnostic calculations of the baroclinic energy flux are used to identify regions of generation and propagation of internal tidal energy. With a representative summer stratification, the integrated offshore flux is about 0.5 gigawatts, higher than previously estimated from theoretical models. Comparisons between observed and modeled M2 current ellipses are discussed for several moorings and demonstrate the significant influence of the internal tide.
Kourafalou, V H., Leo Oey, J Wang, and Tong Lee, 1996: The fate of river discharge on the continental shelf. 1. Modeling the river plume and the innershelf coastal current. Journal of Geophysical Research, 101(C2), 3415-3434. Abstract
We study the development and evolution of buoyant river plumes on the continental shelf. Our calculations are based on three-dimensional numerical simulations, where the river runoff is introduced as a volume of zero salinity water in the continuity equation and mixing is provided by the model's turbulence closure scheme and wind forcing. In the absence of wind forcing the modeled river plumes typically consist of an offshore bulge and a coastal current in the direction of Kelvin wave propagation. We propose a plume classification scheme based on a bulk Richardson number, which expresses the relative magnitude of the buoyancy-induced stratification versus the available mixing. When the ratio of the discharge and shear velocities is greater (less) than 1, the plume is categorized as supercritical (subcritical); that is, the width of the bulge is greater (less) than the width of the coastal current. Supercritical plumes are often characterized by a meandering pattern along the coastal current, caused by a baroclinic instability process. For a given discharge, subcritical plumes are produced by large mixing and/or shallow water depths. In the presence of wind forcing the favorable conditions for offshore removal of coastal low-salinity waters include high river runoff and strong upwelling-favorable wind stress. When the rivers are treated as individual sources of freshwater ("point source" behavior), the wind-driven flow may exhibit substantial spatial variability. Under the above removal conditions, strong offshore transport takes place in "jetlike" flow regions within the river plume, in contrast to the downwind acceleration of adjacent waters. When the rivers are treated as a long "line source" of freshwater, the plume region resembles a coastal low-salinity band and the above removal conditions trigger offshore transport that is most pronounced at the "head" of the source.
Kourafalou, V H., Tong Lee, Leo Oey, and J Wang, 1996: The fate of river discharge on the continental shelf. 2. Transport of coastal low-salinity waters under realistic wind and tidal forcing. Journal of Geophysical Research, 101(C2), 3435-3455. Abstract
A three-dimensional numerical simulation of shelf circulation is presented. We employ realistic forcing for the Southest U.S. Continental Shelf during the spring season. We show that the strongest offshore tranpsort of river-induced, coastal, low-salinity waters and associated materials occurs near the surface. The preferred mean pathway is in the northeastward direction, and it takes about 2 months to cross the entire shelf. Owing to the mean direction of surface transport and the topography of the South Atlantic Bight shelf, the preferred location for springtime removal is off Charleston, South Carolina, and presumably in the vicinity of the Charleston Bump. The transport and fate of the river-induced, coastal, low-salinity waters during the spring season are determined by (1) the stratification of nearshore waters, which is due to the high river runoff and causes the decoupling between "near-surface" and "near-bottom" layers; (2) the prevailing northeastward winds, which cause significant offshore transport within the shallow near-surface Ekman layer; and (3) the tidally induced bottom stirring (M2 tides). Comparison of model and data time series of currents shows very good agreement. Standard deviations of the model and data-computed empirical orthogonal functions are almost identical, while the respective variance-conserving spectra agree both in amplitude and phase.
Oey, Leo, 1996: Flow around a coastal bend: A model of the Santa Barbara Channel eddy. Journal of Geophysical Research, 101(C7), 16,667-16,682. Abstract
A steady, equatorward wind stress is applied over a two-layer ocean (infinitely deep lower layer) west of an otherwise straight meridional coast with a right-angle bend. Initial (t ~ 10 days) response consists of an equatorward current (Kelvin wave) that triggers a cyclone around the bend through viscous production and advection of vorticity, a process akin to eddy shedding in flows without rotation. The response at large times is governed by a Kelvin wave forced by the equatorward weakening of the (assumed positive) wind stress curl, which produces a poleward current near the coast. Application to the Santa Barbara Channel cyclone is discussed, and the cyclone-formation process is further demonstrated with a three-dimensional model with topography and stratification.
Oey, Leo, 1996: Simulation of mesoscale variability in the Gulf of Mexico: Sensitivity studies, comparison with observations, and trapped wave propagation. Journal of Physical Oceanography, 26(2), 145-175. Abstract
A primitive equation Gulf of Mexico model was used to examine variability of the Loop Current (LC) and Loop Current eddies (LCE). Realistic results were obtained for a certain range of values of the horizontal mixing coefficient: eddy paths were west and southwestward; eddy propagation speeds from 3 to 5 km day-1; the ratio of minor to major eddy axes about 0.8; eddy shedding periods from 200 to 500 days; eddy lifetimes from 100 to 200 days; eddy sizes from 200 to 400 km; and eddy swirl transports, as fractions of the specified inflow of 30 Sv. were from 0.55 to 0.85. On the other hand, the maximum vertical deepening of the 20°C isotherm was 15% to 50% less than that observed, resulting in weaker near-surface currents of about 0.65 m s-1, in comparison to observed values of 0.88 to 1.7 m s-1. A strong correlation between eddy shedding and decreasing or reversing lower-layer (below 750 m) transport in the Yucatan Channel is found. In the western Gulf, current variability is produced by eddy arrivals, as well as by forcing due to bottom-intensified topographic Rossby waves, which propagate along the slope from the east with a group velocity of about 12 km day-1 and periods of about 30 - 100 days. These waves are generally preceded by faster coastally trapped wave propagation, and all are produced by LC pulsation, eddy shedding, and westward propagation.
Oey, Leo, 1995: Eddy- and wind-forced shelf circulation. Journal of Geophysical Research, 100(C5), 8621-8637. Abstract
Cochrane and Kelly (1986) proposed a cyclonic gyre as the large-scale mean circulation on the Louisiana-Texas (LATEX) shelf, produced by a convergence of coastal currents in the west and a divergence in the east. While currents near the coast are presumably wind and buoyancy driven, the origin of the eastward flow on the outer shelf and shelf break, which forms the seaward limb of the gyre, as well as the nearshore convergence and divergence, are not well understood. A numerical model is used to show that the western convergence and shelf break current are driven by collision and stalling of westward propagating Loop Current eddies in the northwest Gulf of Mexico and the divergence in the east is caused by shoreward intrusion in the Mississippi Canyon. The western convergence and shelf break current are modulated by the wind curl, strongest in summer and weakest in winter. On the shelf, westward transport is comparable to that observed (~ 0.15 Sv, 1 Sv = 106 m3 s-1) only when the westward wind stress is significant (> 0.3 dyn cm-2). A peak transport of 0.21 to 0.25 Sv occurs in autumn, of which 0.1 Sv is due to wind, 0.07 Sv is due to river buoyancy, and 0.04 to 0.08 Sv is due to eddies. Without the mean westward wind, buoyant waters from the Mississippi do not spread onto the LATEX shelf.
Ahsan, A K M., M S Bruno, Leo Oey, and R I Hires, 1994: Wind-driven dispersion in New Jersey coastal waters. Journal of Hydraulic Engineering, 120(11), 1264-1273.
Manning, J P., and Leo Oey, et al., 1994: Observations of bottom currents and estimates of resuspended sediment transport at the New York Bight 12-mile dumpsite. Journal of Geophysical Research, 99(C5), 10,221-10,239. Abstract
To document storm events that may induce a redistribution of sediment in the vicinity of the New York Bight 12-mile sewage sludge dumpsite, current meter moorings were deployed in water depths from 20 m (near the mouth of New York Harbor) to 53 m (within the Hudson Shelf Valley) from July 1986 through June 1989. Ten usable instrument records ranging from one month to one year in duration were obtained; eight of them near-bottom records. Seasonal and geographic variability of wind-induced flow were examined. The wind is most efficient in driving the subtidal currents in the 2-10 day frequency band during winter when the water column is well mixed and when the eastward component of the wind often induces and sustains an up-valley (northward) bottom flow. Maximum efficiency occurs for wind from 300 degrees (WNW) and at sites located within the Hudson Shelf Valley. A continental shelf bottom boundary layer model (Glenn and Grant, 1987) was used to estimate resuspended sediment transport. Model inputs include bottom currents (observed), orbital wave velocities (estimated), and sediment grain size (from the literature). Model output indicates that sediment resuspension at the current meter sites occurs approximately 5% of the time, primarily during winter months. The difference in along-valley flux between two moorings provides a rough estimate (6-month time series) of deposition and erosion. The net deposition (+.02 mm) was no greater than the deposition and erosion resulting from individual storms. A three-dimensional circulation model (You et al., 1991) is applied to increase the spatial resolution of the near-bottom current field (4 km grid) for a storm event in May of 1987. Given these velocities that vary in space and time, the redistribution of sediment was modeled for different surface wave conditions. Areas of deposition aligned with the Hudson Shelf Valley due to less wave-induced resuspension in deeper waters. Given all the uncertainties in the input variables (grain size, surface waves) and the simplistic assumptions made in modeling the deposition and erosion, it is still uncertain how much sludge is permanently removed from the area, but episodic redistribution of surficial sediment evidently occurs throughout the Inner New York Bight.
Mellor, George L., Tal Ezer, and Leo Oey, 1994: The pressure gradient conundrum of sigma coordinate ocean models. Journal of Atmospheric and Oceanic Technology, 11(4), 1126-1134. Abstract
Much has been written of the error in computing the horizontal pressure gradient associated with sigma coordinates in ocean or atmospheric numerical models. These also exists the concept of "hydrostatic inconsistency" whereby, for a given horizontal resolution, increasing the vertical resolution may not be numerically convergent.
In this paper, it is shown that the differencing scheme cited here, though conventional, is not hydrostatically inconsistent; the sigma coordinate, pressure gradient error decreases with the square of the vertical and horizontal grid size. Furthermore, it is shown that the pressure gradient error is advectively eliminated after a long time integration. At the other extreme, it is shown that diagnostic calculations of the North Atlantic Ocean using rather coarse resolution, and where the temperature and salinity and the pressure gradient error are held constant, do not exhibit significant differences when compared to a calculation where horizontal pressure gradients are cmputed on z-level coordinates. Finally, a way of canceling the error ab initio is suggested.
Oey, Leo, and George L Mellor, 1993: Subtidal variability of estuarine outflow, plume, and coastal current: A model study. Journal of Physical Oceanography, 23(1), 164-171. Abstract
The time evolution of an estuary plume and its coastal front over a continental shelf is numerically calculated here using a three-dimensional model with eddy mixing based on the turbulence kinetic energy closure. The plume and front system is found to be unsteady with a natural period of about 5-10 days, during which the plume pulsates and intermittent coastal currents propagate down the coast.
Oey, Leo, and P Chen, 1992: A model simulation of circulation in the Northeast Atlantic shelves and seas. Journal of Geophysical Research, 97(C12), 20,087-20,115. Abstract
A three-dimensional, primitive-equation simulation of the circulation in the northeast Atlantic shelves and seas, defined by 51° - 76°N latitudes and 20°W - 22°E longitudes, has been conducted for the period February-March 1988. The simulation was initialized from a 585-day quasi-equilibrium calculation and included realistic meteorological forcing, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and relaxation to wintertime climatology for model depths > 500 m. The calculation is the first part of an overall effort to nest a high-resolution region for simulation of eddies and fronts in the Norwegian Coastal Current (NCC). This paper presents detailed simulation strategies and discusses results from the coarse-grid region to study the larger-scale model response induced by atmospheric forcing, so that its effects on flow dynamics in the nested grid can be better understood. The mean and variability of the simulated flow field are compared, whenever possible, with published observations. In particular, we examine in detail the simulated wind-induced response in the Skagerrak transport, which produces blocking and outbreak of the Skagerrak and North Sea waters. These transport variabilities can be expected to be important in the development of the NCC meanders and eddies further north.
Oey, Leo, and P Chen, 1992: A nested-grid ocean model: with application to the simulation of meanders and eddies in the Norwegian Coastal Current. Journal of Geophysical Research, 97(C12), 20,063-20,086. Abstract
Oceanic flow phenomena vary in such large ranges of time and spatial scales that even with the fastest and largest computer to-date, one cannot, at reasonable costs, compute large-scale circulation and at the same time resolve mesoscale features like fronts and/or eddies; yet these features are dynamically important, and their inclusion may determine how well we can model the mean flow, including the deep-ocean circulation. A two-way interactive, nested-grid, primitive-equation model is developed here for coastal ocean applications. Notable features of the model are (1) nesting can be specified on any subregion of a coarse-grid, large domain, and there can be more than one nest if required; (2) the nested region can be "hot-started" from earlier calculation results of the coarse-grid region, that is, the code automatically (by a change of an input flag) generates topography, wind forcing, climatology, currents, density and other prognostic variables in the nest and steps forward in time; (3) a time-splitting integration, with small timesteps in the nest and large timesteps in the coarse-grid domain, is used; and (4) nested variables are driven by coarse-grid solutions around the nest's boundaries, where a flow relaxation scheme may be used, and at the same time drive the coarse-grid evolution through its averaged action in the overlapped region. To demonstrate its robustness, the model is applied in a February/March 1988 real-time simulation of meanders and eddies in the Norwegian Coastal Current, initialized from a 585 days' quasi-equilibrium calculation. The simulation includes meteorological forcings, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and wintertime hydrography for depths > 500 m. From March 20 to 31, the development of a meander between the Froya and the Halten Banks is simulated. The timing and location of the meander agree well with observed hydrography.
Oey, Leo, Tal Ezer, George L Mellor, and P Chen, 1992: A model study of "bump" induced western boundary current variabilities. Journal of Marine Systems, 3, 321-342. Abstract
A time-dependent, three-dimensional numerical model is used to study the effects of a bottom irregularity or "bump" on western boundary current (WBC) variabilities along a simplified shelf and slope. Numerical experiments with (i) no bottom bump, (ii) a small bump and (iii) a large bump have been conducted. Case (i) produces low variabilities and cases (ii) and (iii) show significant increase in slope and shelf energetics both downstream and upstream of the bump. Disturbances generated at the bump are well correlated with flow variabilities upstream. Downstream variabilities are caused by meander development following the WBC deflection by the bump, while topographic waves excite upstream variabilities. The model also indicates two modes of deflection paths, small- and large-amplitude paths, downstream of the bump. These findings are further supported by results obtained from a Gulf Stream simulation which incorporates the bathymetry of the U.S. South Atlantic Bight, and which has a more realistic boundary forcing. The simulated eddy kinetic energy distribution shows three regions of variability which are of interest: one inshore (and slightly downstream) and one offshore of the Charleston Bump, and a third region over the shelfbreak some 150-200 km upstream of the Bump. The inshore and offshore maxima are due to the small and large amplitude deflection paths of the model Gulf Stream, respectively, while the upstream maximum is presumably due to topographic wave activity.
Oey, Leo, Y Zhang, and P Chen, 1992: Simulation of the Norwegian coastal current in the vicinity of the Halten Bank: comparison with observations and process study of bank-induced meanders. Journal of Marine Systems, 3, 391-416. Abstract
Results from a high-resolution, three-dimensional, real-time simulation of the Norwegian Coastal Current are compared at six current meter moorings deployed during March 1988 in the vicinity of the Halten Bank. The simulation was initialized from a set of fairly arbitrary velocity and density fields. The objectives are to examine (i) how accurately the results reproduce the observed means and variabilities, and (ii) how the dominant flow dynamics may be explained in term of the circulation produced by unstable baroclinic meanders over topography.
The simulation reproduces the means fairly well; the rms errors are less than 34% in five of the six moorings for the east/west velocity component, and are less than 32% in three moorings for the north/south component. The agreements in current variabilities are good only at three moorings, for which an averaged rms error of about 22% was obtained. At the other three moorings, the largest errors for the variabilities occur in the subsurface, where energetics are underestimated by as much as 60% or more in the simulation. The discrepancies are most likely due to insufficient vertical resolution, which results in a poor representation of the baroclinic structure, and also due to the smoothed topography used in the simulation. On the other hand, a meander upstream of the Halten Bank on March 26, 1988, is reproduced well by the model. The simulation sugests that the meander is a result of amplification of waves and eddies shed from a smaller bank upstream of the Halten Bank through dynamic instability. A process-oriented simulation has been conducted to support this hypothesis.
Oey, Leo, and P Chen, 1991: Frontal waves upstream of a diabathic blocking: A model study. Journal of Physical Oceanography, 21(11), 1643-1663. Abstract
A time-dependent, three-dimensional primitive-equation model is used here to study meanders that develop upstream of a western boundary current along a continental slope blocked by a diabathic topographic feature. It is found that episodic, large-amplitude meanders and shelfward intrusions occur at upstream distances, which coincide approximately with the topographic standing wavelength. In addition to its potential applications to other problems of front-topography interaction, the phenomenon may be relevant to branching and eddy intrusions of the Kuroshio southwest of Kyushu, Japan, and to the initiation and amplification of Gulf Stream meanders upstream of the "Charleston Bump," a topographic hump on the continental slope of the U.S. South Atlantic Bight.
Blumberg, A, and Leo Oey, 1985: Modeling circulation and mixing in estuaries and coastal oceans. Advances in Geophysics, 28A, 525-547.
Mellor, George L., Leo Oey, R I Hires, and B Galperin, 1985: Three-dimensional numerical models for hindcasting or forecasting estuarine tides, currents and salinities In Applications of Real-time Oceanographic Circulation Modeling: Symposium Proceedings, Washington, DC, Marine Technology Society, 211-225.
Oey, Leo, George L Mellor, and R I Hires, 1985: A three-dimensional simulation of the Hudson-Raritan estuary. Part I: Description of the model and model simulations. Journal of Physical Oceanography, 15(12), 1676-1692. Abstract
A time-dependent, three-dimensional, finite difference simulation of the Hudson-Raritan estuary is presented. The calculation covers July-September 1980. The model estuary is forced by time-dependent observed winds, tidal elevation at open boundaries, and river and sewage discharges. Turbulence mixing coefficients in the estuary are calculated according to a second-moment, turbulence-closure submodel. Horizontal diffusivities are zero in the simulation and small-scale eddies produced by the interaction of unsteady, three-dimensional velocity and salinity fields with coastline and bottom bathymetry were resolved by the model. These eddies are important physical elements in shear dispersion processes in an estuary.
Model results show unstably stratified water columns produced by advection of waters of different densities. These instabilities produce intense mixing with verical eddy diffusivities reaching 2-3 times their neutral values. They occur most frequently at slack currents, during initial stages of flooding currents and also during up-estuary wind events. These three-dimensional, time-dependent solutions extend previous analytical model results and are consistent with observations in partially mixed and well mixed estuaries.
Model results show large subtidal response of velocity and salinity fields to wind forcing. Wind forcing modifies the density-induced flows in deep channels in the estuary and also the horizontal circulation in Raritan Bay where the average water depth is less than 5 m and tidal currents are weak.
Oey, Leo, George L Mellor, and R I Hires, 1985: A three-dimensional simulation of the Hudson-Raritan estuary. Part II: Comparison with observation. Journal of Physical Oceanography, 15(12), 1693-1709. Abstract
Results from time-dependent, three-dimensional numerical simulation of the Hudson-Raritan estuary are compared with observations. The comparison includes: 1) instantaneous salinity contours across a transect in the estuary; 2) amplitudes and phases of tidal constituents at four tide gauge and five current meter stations; 3) mean currents at nine meter locations, and mean salinity in the Hudson River; 4) kinetic energy spectra; and 5) response to wind forcing of subtidal current at an observational station near the mouth of the estuary.
Observations confirm the model's prediction of existence of density advection instabilities induced by differential advection of the three-dimensional density field. These instabilities produce intense vertical mixing and should significantly modify dispersion processes in the estuary. Effects of neap-spring tides on vertical stratifications are also simulated by the model. Simulated M2 phases at three tide gauge stations show improvement over the M2 phases obtained from a two-dimensional, vertically integrated tidal model. The improvement is presumably due to bottom boundary layer resolution and, therefore, improved representation of bottom friction in the three-dimensional model. Simulated (instantaneous and mean) currents compare reasonably well with observations, except at narrow channel regions where the model's resolution is inadequate. Simulated "density-induced" mean currents are weaker than those observed, a discrepancy attributed to neglect of temperature variations in the model. Horizontal diffusion coefficients are null in this model. The burden of horizontal dispersion is generally handled well by the model's adequate resolution of small-scale advective processes, as suggested by the model's correct simulation of the k-3 transfer spectrum law at high wavenumber k. In narrow rivers that are modeled two-dimensionally (x, z), the estimate of the horizontal dispersion due to vertical variabilities in velocity and salinity appears to be correct; however, mixing by lateral variability is absent so that the saline intrusion is somewhat underpredicted. At the mouth of the estuary, simulated subtidal current responses to wind forcing generally agree with observed responses. The response is partly barotropic, which is a result of balance between bottom friction, sea level setup from the adjacent continental shelf and wind stress, modified by local vertical velocity shears and baroclinic responses.
Oey, Leo, George L Mellor, and R I Hires, 1985: A three-dimensional simulation of the Hudson-Raritan estuary. Part III: Salt flux analyses. Journal of Physical Oceanography, 15(12), 1711-1720. Abstract
Salt fluxes and volume transports in an estuary vary considerably over subtidal time scales of a few days to weeks in response to wind and neap-spring tidal forcings. Results from a numerical simulation of the Hudson-Raritan estuary are used to study subtidal variations of salt fluxes and the physical mechanisms for salt balance in the estuary. Simulated salt fluxes are compared with available observations. Observations support the model's finding that analysis of volume and salt fluxes based on short-length data records (<30 days) can lead to misleading conclusions.
"Tidal trapping" effects due to coastline irregularities contribute most to the salt balance at the Sandy Hook-Rockaway Point transect and at the Narrows. A two-week observational record is analyzed to support this finding. Simulated subtidal variation of the tidal trapping term at the Sandy Hook-Rockaway Point transect compares well with that observed. In the Raritan Bay, where tidal currents are weak and effects of winds are significant, contributions to salt balance from vertical velocity and salinity gradients are comparable to transverse contributions. This occurs despite the fact that surface-to-bottom salinity differences during the simulation period-a period of low freshwater flow-never exceed 0.5% throughout most regions of the bay. A two-dimensional, depth-integrated xy-t model, in which the horizontal dispersion coefficients are modeled empirically, may not perform well in this case.
Oey, Leo, George L Mellor, and R I Hires, 1985: Tidal modeling of the Hudson-Raritan Estuary. Estuarine, Coastal and Shelf Science, 20, 511-527. Abstract
Tidal flow characteristics in the Hudson-Raritan Estuary are studied with a two-dimensional, depth-averaged finite difference model. Rivers are modeled as one-dimensional channels with variable width and depth and are calculated as part of the two-dimensional calculations at no extra computational cost. An extensive comparison of numerical, tidal calculations with observational data than has previously appeared in the literature is presented. Computed velocity and tidal elevation fields compare well with observations. Comparison with observations at the Sandy Hook-Rockaway Point transect indicates that the barotropic tidal residual current contributes significantly to the overall steady circulation in the harbor. The residual current is mainly induced by the coastal geometry and bottom topography through the nonlinear inertia effects.
Oey, Leo, 1984: On steady salinity distribution and circulation in partially mixed and well mixed estuaries. Journal of Physical Oceanography, 14(3), 629-645. Abstract
Perturbation analysis based on small a = Ra0.23 Fm0.9, where Ra is the Rayleigh number and Fm is the Froude number, is used to study steady-state circulation and salinity distribution in estuaries. The classical Hansen and Rattray's similarity solution is obtained for the special case of linear variation of longitudinal dispersion coefficient KH, in a channel of constant width B and depth D. It is argued that KH, B and D must vary in real estuaries in such a way that the general solution is regular throughout the length of the estuary and shows a salinity structure which resembles that observed in a real estuary.
It is shown that Hansen and Rattray's theory for predicting the importance of upstream salt transport due to the vertical gravitational circulation in estuaries is valid to a good degree of approximation for arbitrary longitudinal variations in width, depth, fresh water discharge, wind stress and various dispersion and mixing coefficients. This finding is checked against available observations in the Mersey estuary, in the channel of Rio Guayas and in the Hudson River. It is also checked against a real-time three-dimensional numerical model's results of New York Harbor.
Finally, Pritchard's classification of estuaries in terms of their principal tidally-averaged advective and diffusive processes is translated on the Hansen-Rattray circulation-stratification diagram. The diagram shows the relative importance of various terms in the salt balance equation.
Oey, Leo, and George L Mellor, 1983: Real time, 3-D simulation of the New York Harbor. EOS, 64(45), 744.
Oey, Leo, 1982: Implicit schemes for differential equations. Journal of Computational Physics, 45, 443-468. Abstract
Implicit high-order accurate temporal-integration schemes are developed to reduce partial differential equations of evolution type to a sequence of ordinary differential equations involving the spatial directions only. The ordinary differential equations are solved using various accurate integration methods. The schemes are free from extraneous boundary and/or initial conditions and are stable for large Courant number.
