Stenchikov, Georgiy, Kevin P Hamilton, Ronald J Stouffer, A Robock, V Ramaswamy, B D Santer, and Hans-F Graf, 2006: Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate models. Journal of Geophysical Research, 111, D07107, DOI:10.1029/2005JD006286. Abstract
Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860–1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.
Stenchikov, Georgiy, Kevin P Hamilton, A Robock, V Ramaswamy, and M Daniel Schwarzkopf, February 2004: Arctic oscillation response to the 1991 Pinatubo eruption in the SKYHI general circulation model with a realistic quasi-biennial oscillation. Journal of Geophysical Research, 109(D3), D03112, DOI:10.1029/2003JD003699. Abstract
Stratospheric aerosol clouds from large tropical volcanic eruptions can be expected to alter the atmospheric radiative balance for a period of up to several years. Observations following several previous major eruptions suggest that one effect of the radiative perturbations is to cause anomalies in the Northern Hemisphere extratropical winter tropospheric circulation that can be broadly characterized as positive excursions of the Arctic Oscillation (AO). We report on a modeling investigation of the radiative and dynamical mechanisms that may account for the observed AO anomalies following eruptions. We focus on the best observed and strongest 20th century eruption, that of Mt. Pinatubo on 15 June 1991. The impact of the Pinatubo eruption on the climate has been the focus of a number of earlier modeling studies, but all of these previous studies used models with no quasi-biennial oscillation (QBO) in the tropical stratosphere. The QBO is a very prominent feature of interannual variability of tropical stratospheric circulation and could have a profound effect on the global atmospheric response to volcanic radiative forcing. Thus a complete study of the atmospheric variability following volcanic eruptions should include a realistic representation of the tropical QBO. Here we address, for the first time, this important issue. We employed a version of the SKYHI troposphere-stratosphere-mesosphere model that effectively assimilates observed zonal mean winds in the tropical stratosphere to simulate a very realistic QBO. We performed an ensemble of 24 simulations for the period 1 June 1991 to 31 May 1993. These simulations included a realistic prescription of the stratospheric aerosol layer based on satellite observations. These integrations are compared to control integrations with no volcanic aerosol. The model produced a reasonably realistic representation of the positive AO response in boreal winter that is usually observed after major eruptions. Detailed analysis shows that the aerosol perturbations to the tropospheric winter circulation are affected significantly by the phase of the QBO, with a westerly QBO phase in the lower stratosphere resulting in an enhancement of the aerosol effect on the AO. Improved quantification of the QBO effect on climate sensitivity helps to better understand mechanisms of the stratospheric contribution to natural and externally forced climate variability.
Stenchikov, Georgiy, A Robock, V Ramaswamy, M Daniel Schwarzkopf, Kevin P Hamilton, and S Ramachandran, 2002: Arctic oscillation response to the 1991 Mount Pinatubo eruption: effects of volcanic aerosols and ozone depletion. Journal of Geophysical Research, 107(D24), 4803, DOI:10.1029/2002JD002090. Abstract
Observations show that strong equatorial volcanic eruptions have been followed by a pronounced positive phase of the Arctic Oscillation (AO) for one or two Northern Hemisphere winters. It has been previously assumed that this effect is forced by strengthening of the equator-to-pole temperature gradient in the lower stratosphere, caused by aerosol radiative heating in the tropics. To understand atmospheric processes that cause the AO response, we studied the impact of the 1991 Mount Pinatubo eruption, which produced the largest global volcanic aerosol cloud in the twentieth century. A series of control and perturbation experiments were conducted with the GFDL SKYHI general circulation model to examine the evolution of the circulation in the 2 years following the Pinatubo eruption. In one set of perturbation experiments, the full radiative effects of the observed Pinatubo aerosol cloud were included, while in another only the effects of the aerosols in reducing the solar flux in the troposphere were included, and the aerosol heating effects in the stratosphere were suppressed. A third set of perturbation experiments imposed the stratospheric ozone losses observed in the post-Pinatubo period. We conducted ensembles of four to eight realizations for each case. Forced by aerosols, SKYHI produces a statistically significant positive phase of the AO in winter, as observed. Ozone depletion causes a positive phase of the AO in late winter and early spring by cooling the lower stratosphere in high latitudes, strengthening the polar night jet, and delaying the final warming. A positive phase of the AO was also produced in the experiment with only the tropospheric effect of aerosols, showing that aerosol heating in the lower tropical stratosphere is not necessary to force positive AO response, as was previously assumed. Aerosol-induced tropospheric cooling in the subtropics decreases the meridional temperature gradient in the winter troposphere between 30°N and 60°N. The corresponding reduction of mean zonal energy and amplitudes of planetary waves in the troposphere decreases wave activity flux into the lower stratosphere. The resulting strengthening of the polar vortex forces a positive phase of the AO. We suggest that this mechanism can also contribute to the observed long-term AO trend being caused by greenhouse gas increases because they also weaken the tropospheric meridional temperature gradient due to polar amplification of warming.
Baldwin, M P., L J Gray, Timothy J Dunkerton, Kevin P Hamilton, P H Haynes, W J Randel, J R Holton, M J Alexander, I Hirota, T Horinouchi, D B A Jones, J S Kinnersley, C Marquardt, K Sato, and M Takahashi, 2001: The Quasi-biennial oscillation. Reviews of Geophysics, 39(2), 179-229. Abstract
The quasi-biennial oscillation (QBO) dominates the variability of the equatorial stratosphere (~16-50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven by propagating waves with periods unrelated to that of the resulting oscillation. Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropial waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the strength of Atlantic hurricanes. The effects of the QBO are not confined to atmospheric dynamics. Chemical constituents, such as ozone, water vapor, and methane, are affected by circulation changes induced by the QBO. There are also substantial QBO signals in many of the shorter-lived chemical constituents. Through modulation of extratropical wave propagation, the QBO has an effect on the breakdown of the wintertime stratospheric polar vortices and the severity of high-latitude ozone depletion. The polar vortex in the stratosphere affects surface weather patterns, providing a mechanism for the QBO to have an effect at the Earth's surface. As more data sources (e.g., wind and temperature measurements from both ground-based systems and satellites) become available, the effects of the QBO can be more precisely assessed. This review covers the current state of knowledge of the tropical QBO, its extratropical dynamical effects, chemical constituent transport, and effects of the QBO in the troposphere (~0-16 km) and mesosphere (~50-100 km). It is intended to provide a broad overview of the QBO and its effects to researchers outside the field, as well as a source of information and references for specialists. The history of research on the QBO is discussed only briefly, and the reader is referred to several historical review papers. The basic theory of the QBO is summarized, and tutorial references are provided.
Hamilton, Kevin P., R John Wilson, and Richard S Hemler, 2001: Spontaneous stratospheric QBO-like oscillations Simulated by the GFDL SKYHI general circulation model. Journal of the Atmospheric Sciences, 58(21), 3271-3292. Abstract
The tropical stratospheric mean flow behavior in a series of integrations with high vertical resolution versions of the Geophysical Fluid Dynamics Laboratory (GFDL) "SKYHI" model is examined. At sufficiently high vertical and horizontal model resolution, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed quasi-biennial oscillation (QBO) in many respects, although the simulated oscillation has a period less than half that of the real QBO. The same basic mean flow oscillation appears in both seasonally varying and perpetual equinox versions of the model, and most of the analysis in this paper is focused on the perpetual equinox cases. The mean flow oscillation is shown to be largely driven by eddy momentum fluxes associated with a broad spectrum of vertically propagating waves generated spontaneously in the tropical troposphere of the model. Several experiments are performed with the model parameters perturbed in various ways. The period of the simulated tropical stratospheric mean flow oscillation is found to change in response to large alterations in the sea surface temperatures (SSTs) employed. This is a fairly direct demonstration of the link between the stratospheric mean flow behavior and tropical convection that is inherent in current theories of the QBO. It is also shown in another series of experiments that the oscillation is affected by the coefficients used for the subgrid-scale diffusion parameterization. These experiments demonstrate that at least one key reason why reasonably fine horizontal resolution is needed for the model to simulate a mean flow oscillation is the smaller horizontal diffusion that can be used at high resolution.
Koshyk, J N., and Kevin P Hamilton, 2001: The horizontal kinetic energy spectrum and spectral budget simulated by a high-resolution troposphere-stratosphere-mesosphere GCM. Journal of the Atmospheric Sciences, 58(4), 329-348. Abstract
Horizontal kinetic energy spectra simulated by high-resolution versions of the Geophysical Fluid Dynamics Laboratory SKYHI middle-atmosphere general circulation model are examined. The model versions considered resolve heights between the ground and ~80 km, and the horizontal grid spacing of the highest-resolution version is about 35 km. Tropospheric kinetic energy spectra show the familiar ~-3 power-law dependence on horizontal wavenumber for wavelengths between about 5000 and 500 km and have a slope of ~-5/3 at smaller wavelengths. Qualitatively similar behavior is seen in the stratosphere and mesosphere, but the wavelength marking the transition to the shallow regime increases with height, taking a value of ~2000 km in the stratosphere and ~4000 km in the mesosphere.
The global spectral kinetic energy budget for various height ranges is computed as a function of total horizontal wavenumber. Contributions to the kinetic energy tendency from nonlinear advective processes, from conversion of available potential energy, from mechanical fluxes through the horizontal boundaries of the region, and from parameterized subgrid-scale dissipation are all examined. In the troposphere, advective contributions are negative at large scales and positive over the rest of the spectrum. This is consistent with a predominantly downscale nonlinear cascade of kinetic energy into the mesoscale. The global kinetic energy budget in the middle atmosphere differs significantly from that in the troposphere, with the positive contributions at most scales coming predominantly from vertical energy fluxes.
The kinetic energy spectral calculated from two model versions with different horizontal resolution are compared. Differences between the spectra over the resolved range of the lower-resolution version are smallest in the troposphere and increase with height, owing mainly to large differences in the divergent components. The result suggests that the parameterization of dynamical subgrid-scale processes in middle-atmosphere general circulation models, as well as in high-resolution tropospheric general circulation models, may need to be critically reevaluated.
Hamilton, Kevin P., 2000: Free and forced interannual variability of the circulation in the extratropical Northern Hemisphere middle atmosphere In Atmospheric Science across the Stratopause, Geophysical Monograph 123, Washington, DC, American Geophysical Union, 227-239. Abstract
This paper is an informal review of research relating to the interannual variability of the stratospheric and mesospheric circulation in the northern extratropics. Observations and modelling studies concerning aspects of variability generated internally in the atmosphere are reviewed first, with a particular emphasis on the effects of the tropical quasi-biennial oscillation. Then studies relating to externally forced variability are reviewed, including those that deal with the effects of volcanic aerosols and solar irradiance.
Hamilton, Kevin P., and Songmiao Fan, 2000: Effects of the stratospheric quasi-biennial oscillation on long-lived greenhouse gases in the troposphere. Journal of Geophysical Research, 105(D16), 20,581-20,587. Abstract
An analysis is presented of results of an extended integration with a global general circulation model that includes treatment of a long-lived tropospheric trace constituent as well as a momentum source that forces a realistic stratospheric quasi-biennial oscillation (QBO). It is shown that the dynamical QBO may modulate stratosphere-troposphere exchange in such a manner as to produce a QBO in global-mean tropospheric tracer mixing ratio. For the growth rate of tropospheric methane this transport-induced QBO is expected to have a peak-to-peak amplitude of 1-2 ppbv yr-1, which, while modest compared with the full range of observed variability in growth rate, is still significant. The observed methane growth rate time series during 1983-1999 is shown to be consistent with the predicted QBO effect, although the record is definitely dominated by other interannual variations.
Pawson, S, K Kodera, Kevin P Hamilton, T G Shepherd, S R Beagley, B A Boville, J D Farrara, T D A Fairlie, A Kitoh, W A Lahoz, U Langematz, E Manzini, R John Wilson, and Jerry D Mahlman, et al., 2000: The GCM-reality intercomparison project for SPARC (GRIPS): scientific issues and initial results. Bulletin of the American Meteorological Society, 81(4), 781-796. Abstract
To investigate the effects of the middle atmosphere on climate, the World Climate Research Programme is supporting the project "Stratospheric Processes and their Role in Climate" (SPARC). A central theme of SPARC, to examine model simulations of the coupled troposphere-middle atmosphere system, is being performed through the initiative called GRIPS (GCM-Reality Intercomparison Project for SPARC). In this paper, an overview of the objectives of GRIPS is given. Initial activities include an assessment of the performance of middle atmosphere climate models, and preliminary results from this evaluation are presented here. It is shown that although all 13 models evaluated represent most major features of the mean atmospheric state, there are deficiencies in the magnitude and location of the features, which cannot easily be traced to the formulation (resolution or the parameterizations included) of the models. Most models show a cold bias in all locations, apart from the tropical tropopause region where they can be either too warm or too cold. The strengths and locations of the major jets are often misrepresented in the models. Looking at three-dimensional fields reveals, for some models, more severe deficiencies in the magnitude and positioning of the dominant structures (such as the Aleutian high in the stratosphere), although undersampling might explain some of these differences from observations. All the models have shortcomings in their simulations of the present-day climate, which might limit the accuracy of predictions of the climate response to ozone change and other anomalies forcing.
Bruhwiler, Lori, and Kevin P Hamilton, 1999: A numerical simulation of the stratospheric ozone quasi-biennial oscillation using a comprehensive general circulation model. Journal of Geophysical Research, 104(D23), 30,525-30,557. Abstract
The Geophysical Fluid Dynamics Laboratory's SKYHI general circulation model (GCM) including a new detailed stratospheric photochemistry module has been integrated for over 14 years with an imposed zonally symmetric momentum source designed to force a realistic quasi-biennial oscillation (QBO) in the tropical stratosphere. The GCM features an internally consistent calculation of the annual stratospheric circulation cycle and exhibits realistic extratropical stratospheric interannual variability, making it appropriate for the detailed investigation of QBO/annual cycle interactions. The simulated ozone QBO is generally realistic in the tropics and subtropics, and, in particular, the QBO in total column ozone agrees quite well with that derived from satellite observations. A detailed analysis of the QBO modulation of the zonal-mean ozone budget has been performed. The advective effects of the QBO-induced residual mean circulation are found to be strongly dependent on season, in accord with recent results from some two-dimensional model studies [Jones et al., 1998; Kinnersley and Tung, 1998]. In addition, the QBO modulation of explicitly resolved eddy transport in the GCM is found to make a significant contribution to the ozone budget, and this helps account for the strong seasonal synchronization of the ozone QBO.
Hamilton, Kevin P., 1999: Convective Excitation of Gravity Waves Experiment - CEGWE In Mesoscale Processes in the Stratosphere - Their Effect on Stratospheric Chemistry and Microphysics, Carlslaw, K.S., and G. T. Amanatidis, editors, European Commission, Air Pollution Research Report 69, 93-98.
Hamilton, Kevin P., 1999: Dynamical coupling of the lower and middle atmosphere: historical background to current research. Journal of Atmospheric & Solar-Terrestrial Physics, 61(1-2), 73-84. Abstract
The role of both gravity waves and planetary waves in coupling the circulation in the middle atmosphere with that in the troposphere is now well appreciated. The present article reviews the history of the study of middle atmospheric dynamics and its coupling with the lower atmosphere. The emphasis is on early developments, principally those before the mid-1970's.
Hamilton, Kevin P., 1999: The gravity wave parameterisation problem for global simulation models. SPARC Newsletter, 12, 7-14.
Hamilton, Kevin P., R John Wilson, and Richard S Hemler, 1999: Middle atmosphere simulated with high vertical and horizontal resolution versions of a GCM: Improvements in the cold pole bias and generation of a QBO-like oscillation in the tropics. Journal of the Atmospheric Sciences, 56(22), 3829-3846. Abstract
The large-scale circulation in the Geophysical Fluid Dynamics Laboratory "SKYHI" troposphere-stratosphere-mesosphere finite-difference general circulation model is examined as a function of vertical and horizontal resolution. The experiments include one with horizontal grid spacing of ~35 km and another with ~100 km horizontal grid spacing but very high vertical resolution (160 levels between the ground and about 85 km). The simulation of the middle-atmospheric zonal-mean winds and temperatures in the extratropics is found to be very sensitive to horizontal resolution. For example, in the early Southern Hemisphere winter the South Pole near 1 mb in the model is colder than observed, but the bias is reduced with improved horizontal resolution (from ~70°C in a version with ~300 km grid spacing to less than 10°C in the ~35 km version). The extratropical simulation is found to be only slightly affected by enhancements of the vertical resolution. By contrast, the tropical middle atmospheric simulation is extremely dependent on the vertical resolution employed. With level spacing in the lower stratosphere ~1.5 km, the lower stratospheric zonal-mean zonal winds in the equatorial region are nearly constant in time. When the vertical resolution is doubled, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed QBO in many respects, although the simulated oscillation has a period less than half that of the real QBO.
Koshyk, J N., B A Boville, Kevin P Hamilton, E Manzini, and K Shibata, 1999: Kinetic energy spectrum of horizontal motions in middle-atmosphere models. Journal of Geophysical Research, 104(D22), 27,177-27,190.
Koshyk, J N., Kevin P Hamilton, and Jerry D Mahlman, 1999: Simulation of the k-5/3 mesoscale spectral regime in the GFDL SKYHI general circulation model. Geophysical Research Letters, 26(7), 843-846. Abstract
Data from very high horizontal resolution simulations with the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model are used to calculate the kinetic energy spectrum as a function of horizontal wavenumber, k, in the upper troposphere. The spectrum shows the familiar ~ -3 slope at scales longer than ~ 1000 km, in agreement with previous general circulation model and observational studies. At shorter scales, the spectrum becomes shallower with a slope ~ -5/3, also in agreement with available observations. The -5/3 slope spans about a decade of the resolved scales and this result represents the first successful simulation of such a broad range of the mesoscale regime by a global model. Partitioning of the flow between rotational and divergent components shows that the rotational part dominates at large scales and that there is approximate equipartition between rotational and divergent parts at mesoscales. Analysis of the parameterized kinetic energy dissipation shows that vertical diffusion dominates horizontal diffusion for a wide range of wavenumbers extending well into the k-5/3 regime.
Hamilton, Kevin P., 1998: Dynamics of the tropical middle atmosphere: A tutorial review. Atmosphere-Ocean, 36(4), 319-354. Abstract
The general circulation of the tropical stratosphere, mesosphere and lowermost thermosphere is discussed at a tutorial level. Observations of the quasi-biennial and semiannual oscillations by both in situ and satellite techniques are first reviewed. The basic dynamics controlling the zonal-mean component of the circulation are then discussed. The role of radiative diabatic cooling in constraining the zonal-mean circulation in the middle atmosphere is emphasized. It is shown that the effectiveness of this radiative constraint is reduced at low latitudes, allowing for the sustained mean flow accelerations over long periods of time characteristic of the quasi-biennial and semiannual oscillations in the tropics.
The current view is that the dominant driving for the equatorial mean flow accelerations seen in the middle atmosphere derives from vertically-propagating waves. This process is illustrated here in its simplest context, i.e., the Plumb (1977) model of the interaction of monochromatic internal gravity waves with the mean flow (based on earlier work of Lindzen and Holton, 1968; Holton and Lindzen, (1972). It is shown that the dynamics illustrated by this simple model can serve as the basis for an explanation of the quasi-biennial oscillation.
The paper then describes some of recent developments in the theory of the quasi-biennial and semiannual oscillations, including aspects related to the interaction between tropics and midlatitudes in the middle atmosphere. The paper concludes with a discussion of the effects of the long period dynamical variations in the tropical circulation on the chemical composition of the stratosphere.
Hamilton, Kevin P., 1998: Dynamics of the tropical middle atmosphere: A tutorial review In Summer School on the Middle Atmosphere, Ontario, Canada, Canadian NAM Project, 212-256.
Hamilton, Kevin P., 1998: Effects of an imposed quasi-biennial oscillation in a comprehensive troposphere-stratosphere-mesosphere general circulation model. Journal of the Atmospheric Sciences, 55(14), 2393-2418. Abstract
A 48-yr. integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI troposphere-stratosphere-mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly two-thirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonal-mean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model.
The QBO was also found to have a profound influence on stratospheric stationary waves at low latitudes. Near and above 10 mb the NH stationary waves were found to penetrate across the equator during the westerly QBO phase, but to be restricted to latitudes poleward of ~10°N during the easterly phase. This means that the equatorial QBO in prevailing wind near and above 10 mb has a significant zonally asymmetric component. If this is also true in the real atmosphere, there are important implications for the adequacy of the current observational rawindsonde network near the equator.
Analysis of the zonal-mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the QBO cycle. The exception appears to be the easterly acceleration phase below about 20 mb. The implications of this for the generation of a self-consistent QBO by GCMs will be considered.
The effects of the imposed QBO on the troposphere were found to be very modest. There does appear to be a statistically significant weakening (by ~1 m s-1) of the high-latitude winter vortex in the middle and upper troposphere. Given the very high predictability of the stratospheric QBO itself, this effect could possibly be used to enhance the skill of seasonal weather forecasts. No significant QBO influence was found in the model precipitation field.
Hamilton, Kevin P., 1998: Book review: "Gravity Currents in the Environment and the Laboratory" by J. E. Simpson. EOS, 79(6), 71.
Hamilton, Kevin P., 1998: Observations of tropical stratospheric winds before World War II. Bulletin of the American Meteorological Society, 79(7), 1367-1371. Abstract
This paper discusses observations of the winds in the tropical stratosphere taken before the advent of regular operational balloon soundings in this region. These observations are at least broadly consistent with modern measurements, in the sense that they show that the winds in the tropical stratosphere have been undergoing some strong interannual variations over the last century. However, the available data appear to be too sparse to construct a detailed chronology of the quasi-biennial oscillation before about 1950.
Forbes, J M., M E Hagan, X Zhang, and Kevin P Hamilton, 1997: Upper atmosphere tidal oscillations due to latent heat release in the tropical troposphere. Annales Geophysicae, 15(9), 1165-1175. Abstract
Latent heat release associated with tropical deep convective activity is investigated as a source for migrating (sun-synchronous) diurnal and semidiurnal tidal oscillations in the 80-150 km height region. Satellite-based cloud brightness temperature measurements made between 1988 and 1994 and averaged into 3-h bins are used to determine the annual- and longitude-average local-time distribution of rainfall rate, and hence latent heating, between ± 40° latitude. Regional average rainfall rates are shown to be in good agreement with climatological values derived from surface rain gauge data. A global linearized wave model is used to estimate the corresponding atmospheric perturbations in the mesosphere/lower thermosphere (80-150 km) resulting from upward-propagating tidal components excited by the latent heating. The annual-average migrating diurnal and semidiurnal components achieve velocity and temperature amplitudes of order 10-20 m s -1 and 5-10 K, respectively, which represent substantial contributions to the dynamics of the region. The latent heat forcing also shifts the phase (local solar time of maximum) of the semidiurnal surface pressure oscillation from 0912 to 0936 h, much closer to the observed value of 0944 h.
Hamilton, Kevin P., 1997: Meteorological measurements on ozonesonde ascents: A valuable resource for stratospheric climatology. SPARC Newsletter, 9, 23.
Hamilton, Kevin P., 1997: Observation of an ultraslow large-scale wave near the tropical tropopause. Journal of Geophysical Research, 102(D12), 13,457-13,464. Abstract
An analysis is made of very high resolution radiosonde data taken at seven western Pacific island stations during TOGA-COARE (Tropical Ocean-Global Atmosphere and Coupled Ocean-Atmosphere Response Experiment). Evidence is found for a wave near the tropopause with periods ~30-40 days and with very slow (~3 m s-1) eastward propagation. This oscillation appears quite coherent across the region considered (a 37 degrees longitude spread). The oscillation has a short vertical wavelength (~3 km) and is strongly attenuated above the tropopause and thus would be very hard to observe in either satellite data or conventional meteorological analyses.
Hamilton, Kevin P., 1997: Progress towards gravity wave parameterization for global climate models In Stratospheric Processes and Their Role in Climate (SPARC), Proceedings of the First SPARC General Assembly, WMO/TD-No. 814, WCRP-99, Geneva, Switzerland, World Meteorological Organization, 295-298.
Hamilton, Kevin P., 1997: The role of parameterized drag in a troposphere-stratosphere-mesosphere general circulation model In Gravity Wave Processes: Their Parameterization in Global Climate Models, New York, NY, Springer-Verlag, 337-350. Abstract
This paper discusses simulations of the global circulation up to 0.01 mb obtained using a general circulation model with an imposed zonal wind relaxation that forces the zonal-mean state close to observations. This enables estimates to be made of the zonal drag required to bring general circulation model simulations of the middle atmospheric circulation into line with observations. The diagnosed drag is then compared with results from the Lindzen (1981) parameterization of wave drag from unresolved gravity waves. With reasonable values of the wave input parameters, the parameterization can capture many of the features of the diagnosed drag distribution, but only if some modification of the Lindzen procedure is adopted to allow the region of parameterized wave breaking to extend to lower heights.
Hamilton, Kevin P., and Richard S Hemler, 1997: Appearance of a supertyphoon in a global climate model simulation. Bulletin of the American Meteorological Society, 78(12), 2874-2876.
Jones, P, Kevin P Hamilton, and R John Wilson, 1997: A very high resolution general circulation model simulation of the global circulation in Austral Winter. Journal of the Atmospheric Sciences, 54(8), 1107-1116. Abstract
This paper discusses a simulation obtained with the Geophysical Fluid Dynamics Laboratory "SKYHI" troposphere-stratosphere-mesosphere general circulation model run at very high horizontal resolution (~60-km grid spacing) and without any parameterization of subgrid-scale gravity wave drag. The results are for a period around the austral winter solstice, and the emphasis is on the simulated Southern Hemisphere (SH) winter circulation. Comparisons are made with results obtained from lower horizontal resolution versions of the same model.
The focus in this paper is on two particularly striking features of the high-resolution simulation: the extratropical surface winds and the winter polar middle atmospheric vortex. In the extratropical SH, the simulated surface westerlies and meridional surface pressure gradients in the high-resolution model are considerably stronger than observed and are stronger than those simulated at lower horizontal resolution. In the middle atmosphere, the high-resolution model produces a simulation of the zonal mean winter polar vortex that is considerably improved over that found with lower resolution models (although it is still significantly affected by the usual cold pole bias). Neither the improvement of the middle atmospheric polar vortex simulation nor the deterioration of the simulation of surface winds with increased model resolution shows a clear convergence, even at the ~60-km grid spacing employed here .
Randel, W J., and Kevin P Hamilton, et al., 1997: Stratospheric climatology studies for SPARC In Stratospheric Processes and Their Role in Climate (SPARC), Proceedings of the First SPARC General Assembly, WMO/TD-No. 814, WCRP-99, Geneva, Switzerland, World Meteorological Organization, 75-78.
Hamilton, Kevin P., 1996: Comprehensive meteorological modelling of the middle atmosphere: A tutorial review. Journal of Atmospheric and Terrestrial Physics, 58(14), 1591-1628. Abstract
This paper reviews the current state of comprehensive, three-dimensional, time-dependent modelling of the circulation in the middle and upper atmosphere from a meteorologist's perspective. The paper begins with a consideration of the various components of a comprehensive model (or general circulation model, GCM), including treatments of processes that can be explicitly resolved and those that occur on scales too small to resolve (and that must be parameterized). The typical performance of GCMs in simulating the tropospheric climate is discussed. Then some important background on current ideas concerning the general circulation of the stratosphere and mesosphere is presented. In particular, the transformed-Eulerian mean flow formalism, the role of vertically-propagating internal gravity waves in driving the large-scale circulation, and the notion of a stratospheric surf zone are all briefly reviewed. Using this background as a guide, some middle atmosphere GCM results are discussed, with a focus on simulations made recently with the GFDL 'SKYHI' troposphere-stratosphere- mesosphere GCM. The presentation attempts to emphasize the interaction between theory and comprehensive modelling. Many theoretical notions cannot be confirmed in detail from observations of the real atmosphere due to the various limitations in the observational methods, but can be very completely examined in GCMs in which every atmospheric variable is known perfectly (within the limits of the numerical methods). It will be shown that our understanding of both the role of gravity waves in the general circulation and the nature of the stratospheric surf zonehas benefited from analysis of GCM results.
From the point of view of the upper atmosphere, one of the most interesting aspects of GCMs is their ability to generate a self-consistent field of upward-propagating gravity waves. This paper concludes with a discussion of the gravity wave field in the middle atmosphere of GCMs. Comparisons of the explicitly-resolved gravity wave field in the SKYHI model with observations are quite encouraging, and it seems that the model is capable of producing a gravity wave field with many realistic features. However, the simulated horizontal spectrum of the eddy momentum fluxes associated with the waves is quite shallow, suggesting that much of the spectrum that is important for maintaining the mean circulation is not explicitly resolvable in current GCMs. A brief discussion of current efforts at parameterizing the mean flow effects of the unresolvable gravity waves is presented.
Hamilton, Kevin P., 1996: Tides In Encyclopedia of Climate and Weather, Vol. 2, New York, Oxford University Press, 761-764.
Wilson, R J., and Kevin P Hamilton, 1996: Comprehensive model simulation of thermal tides in the Martian atmosphere. Journal of the Atmospheric Sciences, 53(9), 1290-1326. Abstract
This paper discusses the thermotidal oscillations in simulations performed with a newly developed comprehensive general circulation model of the Martian atmosphere. With reasonable assumptions about the effective thermal inertia of the planetary surface and about the distribution of radiatively active atmospheric aerosol, the model produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude. With any reasonable aerosol distribution, the simulated diurnal pressure oscillation has a very strong zonal variation, in particular a very pronounced zonal wavenumber-2 modulation. This results from a combination of the prominent wave-2 component in the important boundary forcings (topography and surface thermal inertia) and from the fact that the eastward-propagating zonal wave-1 Kelvin normal mode has a period near 1 sol (a Martian mean solar day of 88 775 s). The importance of global resonance is explicitly demonstrated with a series of calculations in which the global mean temperature is arbitrarily altered. The resonant enhancement of the diurnal wave-1 Kelvin mode is predicted to be strongest in the northern summer season. In the model simulations there is also a strong contribution to the semidiurnal tide from a near-resonant eastward-propagating wave-2 Kelvin mode. It is shown that this is significantly forced by a nonlinear steepening of the diurnal Kelvin wave. The daily variations of near-surface winds in the model are also examined. The results show that the daily march of wind at any location depends strongly on the topography, even on the smallest horizontal scales resolved in the model (~ few hundred km). The global tides also play an important role in determining the near-surface winds, especially so in very dusty atmospheric conditions.
The results for the diurnal and semidiurnal surface pressure oscillations in seasonal integrations of the model are compared in detail with the observations at the two Viking Lander sites (22°N and 48°N). The observations over much of the year can be reasonably reproduced in simulations with a globally uniform aerosol mixing ratio (and assuming more total aerosol in the northern winter season, when the largest dust storms are generally observed). There are features of the Viking observations that do not seem to be explainable in this way, however. In particular, in early northern summer, the model predicts amplitudes for the diurnal pressure oscillation at both lander sites that are at least a factor of 2 larger than observed. Results are presented showing that the low amplitudes observed could be explained if the dust distribution tended to be concentrated over the highlands, rather than being uniformly mixed. Annual cycle simulations with a version of the model with an interactive dust transport do in fact reveal the tendency of the circulation to organize so that larger dust mixing ratios occur over highlands, particularly near subsolar latitudes. When the model includes globally uniform surface dust injection and parameterized dust sedimentation, the annual cycle of the diurnal and semidiurnal tides at both lander sites can be rather well reproduced, except for the periods of global dust storms. The attempts to simulate the observed rapid evolution of the tidal pressure oscillations during the onset of a global dust storm also demonstrate the importance of a nonuniform dust concentration. Simulations with the version of the model incorporating interactive dust are able to roughly reproduce the Viking observations when a strong zonally uniform dust injection is prescribed in the Southern Hemisphere Tropics and subtropics.
Hamilton, Kevin P., 1995: Aspects of mesospheric simulation in a comprehensive general circulation model In The Upper Mesosphere and Lower Thermosphere: A Review of Experiment and Theory, Geophysical Monograph, 87. Washington, DC, American Geophysical Union, 255-264. Abstract
The simulation of mesospheric circulation in the 40 level GFDL "SKYHI" troposphere-stratosphere-mesosphere general circulation model is examined. The model is shown to produce a "reversed" equator-pole temperature gradient near the upper summer mesosphere with consequent closing off of the summertime easterly jet. This results from the mean flow driving associated with explicitly resolved internal gravity waves. The mean easterlies in the summer upper mesosphere become weaker as the horizontal resolution in the model is improved, presumably reflecting a more complete representation of the gravity wave spectrum. the rate of loss of the eddy kinetic energy in the model due to subgrid scale dissipation is computed. The model results in the mesosphere compare reasonably well with the limited data available concerning the turbulent dissipation rates obtained from in situ rocket experiments. Also discussed here for the first time are simulations with a diurnally- varying version of the SKYHI general circulation model. The diurnal tidal signal appears to be of realistic amplitude in the model, but it is striking that the other eddy components (gravity waves, Rossby normal modes, quasi- stationary planetary waves) are strong enough to dominate the tide in any instantaneous map of the global mesospheric wind field. The mean flow driving (Eliassen-Palm flux divergence) associated with the diurnal tide is also computed. The present results for this quantity differ considerably in detail from earlier calculations which were based on more idealized tidal theories. The tidal driving of the mean flow in the SKYHI model is an important, though not dominant, effect in the summer mesosphere region.
Hamilton, Kevin P., 1995: Comment on. Journal of the Atmospheric Sciences, 52(10), 1834-1838.
Hamilton, Kevin P., 1995: Comprehensive simulation of the middle atmospheric climate: Some recent results. Climate Dynamics, 11, 223-241. Abstract
This study discusses the results of comprehensive time-dependent, three-dimensional numerical modelling of the circulation in the middle atmosphere obtained with the GFDL "SKYHI" troposphere-stratosphere-mesosphere general circulation model (GCM). The climate in a long control simulation with an intermediate resolution version (~ 3° in horizontal) is briefly reviewed. While many aspects of the simulation are quite realistic, the focus in this study is on remaining first-order problems with the modelled middle atmospheric general circulation, notably the very cold high latitude temperatures in the Southern Hemisphere (SH) winter/spring, and the virtual absence of a quasi-biennial oscillation (QBO) in the tropical stratosphere. These problems are shared by other extant GCMs. It was noted that the SH cold pole problem is somewhat ameliorated with increasing horizontal resolution in the model. This suggests that improved resolution increases the vertical momentum fluxes from the explicitly resolved gravity waves in the model, a point confirmed by detailed analysis of the spectrum of vertical eddy momentum flux in the winter SH extratropics. This result inspired a series of experiments with the 3° SKYHI model modified by adding a prescribed zonally-symmetric zonal drag on the SH winter westerlies. The form of the imposed momentum source was based on the simple assumption that the mean flow drag produced by unresolved waves has a spatial distribution similar to that of the Eliassen-Palm flux divergence associated with explicitly resolved gravity waves. It was found that an appropriately-chosen drag confined to the top six model levels (above 0.35 mb) can lead to quite realistic simulations of the SH winter flow (including even the stationary wave fields) through August, but that problems still remain in the late-winter/springtime simulation. While the imposed momentum source was largely confined to the extratropics, it produced considerable improvement in the simulation of the equatorial semiannual oscillation, with both the easterly and westerly phases being somewhat more intense than in the control simulation. A separate experiment was conducted in which the SKYHI model was simplified so that it had no topography and so that the seasonal cycle was frozen in perpetual equinox conditions. These changes result in a model that has much reduced interhemispheric asymmetry. This model spontaneously produces a long period mean flow oscillation of considerable amplitude in the tropical upper stratopause. The implication of this result for the general issue of obtaining a QBO in comprehensive GCMs is discussed.
Hamilton, Kevin P., 1995: Do historical "zero depth" hydrographic data contain useful information on climate trends?Canadian Meteorological and Oceanographic Society (CMOS) Bulletin, 23(6), 6-8.
Hamilton, Kevin P., and R A Vincent, 1995: High-resolution radiosonde data offer new prospects for research. EOS, 76(49), 497, 506-507.
Hamilton, Kevin P., R John Wilson, Jerry D Mahlman, and L Umscheid, 1995: Climatology of the SKYHI troposphere-stratosphere-mesosphere general circulation model. Journal of the Atmospheric Sciences, 52(1), 5-43. Abstract
The long-term mean climatology obtained from integrations conducted with different resolutions of the GFDL "SKYHI" finite-difference general circulation model is examined. A number of improvements that have been made recently in the model are also described. The versions considered have <3°C x <3.6°C, <2° x <2.4°C, and <1° x <1.2°C latitude-longitude resolution, and in each case the model is run with 40 levels from the ground to 0.0096 mb. The integrations all employ a fixed climatological cycle of sea surface temperature. Over 25 years of integration with the <3°C model and shorter integrations with the higher-resolution versions are analyzed. Attention is focused on the December-February and June-August periods.
The model does a reasonable job of representing the atmospheric flow in the troposphere and lower stratosphere. The simulated tropospheric climatology has an interesting sensitivity to horizontal resolution. In common with several spectral GCMs that have been examined earlier, the surface zonal-mean westerlies in the SKYHI extratropics become stronger with increasing horizontal resolution. However, this "zonalization" of the flow with resolution is not as prominent in the upper troposphere of SKYHI as it is in some spectral models. It is noteworthy that--without parameterized gravity wave drag--the SKYHI model at all three resolutions can simulate a realistic separation of the subtropical and polar night jet streams and a fairly realistic strength of the lower-stratospheric winter polar vortex.
The geographical distribution of the annual-mean and seasonal precipitation are reasonably well simulated. When compared against observations in an objective manner, the SKYHI global precipitation simulation is found to be as good or better than that obtained by other state-of-the-art general circulation models. However, some significant shortcomings remain, most notably in the summer extratropical land areas and in the tropical summer monsoon regions. The time-mean precipitation simulation is remarkably insensitive to the horizontal model resolution employed. The other tropospheric feature examined in detail is the tropopause temperature. The whole troposphere suffers from a cold bias of the order of a few degrees Celcius, but in the <3°C SKYHI model this grows to about <6°C at 100 mb. Interestingly, the upper-tropospheric bias is reduced with increasing horizontal resolution, despite that the cloud parameters in the radiation code are specified identically in each version.
The simulated polar vortex in the Northern Hemisphere winter in the upper stratosphere is unrealistically confined to high latitudes, although the maximum zonal-mean zonal wind is close to observed values. Near the strato- pause the June-August mean temperatures at the South Pole are colder than observations by ~<65°C, < 50°C, and <30°C in the < 3°C, <2°C, and < 1°C simulations, respectively. The corresponding zonal-mean zonal wind patterns display an unrealistically strong polar vortex. The extratropical stratospheric stationary wave field in the Northern Hemisphere winter is examined in some detail using the multi- year averages available from the <3°C SKYHI integration. Comparison with comparable long-term mean observations suggests that the model captures the amplitude and phase of the stationary waves rather well.
The SKYHI model simulates the reversed equator-pole temperature gradient near the summer mesopause. The simulated summer polar mesopause temperatures decrease with increasing horizontal resolution, although even at <1 degree C resolution the predicted temperatures are still warmer than observed. The increasing resolution is accompanied by increased westerly driving of the mean flow in the summer mesosphere by dissipating gravity waves. The present results suggest that the SKYHI model does explicitly resolve a significant component of the gravity waves required to produce the observed summer mesopause structure. The seminannual oscillation near the tropical stratopause is reasonably well simulated in the < 3°C version. The main deficiency is in the westerly phase, which is not as strong as observed. There is also a second peak in the amplitude of the semiannual wind oscillation at the top model level (0.0096 mb) corresponding to the observed mesopause semiannual oscillation. This simulated mesopause oscillation is weaker (by a factor of ~3) than that observed. The simulation in the tropical stratopause and mesosphere changes quite significantly with increasing resolution, however, in the tropical lower stratosphere of the <3°C model the zonal-mean zonal wind displays a very weak (~3 m s-1 peak to peak) interannual variation, which--while rather irregular--does display a roughly biennial period and the downward phase propagation that is characteristic of the observed quasibiennial oscillation.
Hamilton, Kevin P., 1995: Interannual variability in the Northern Hemisphere winter middle atmosphere in control and perturbed experiments with the GFDL SKYHI general circulation model. Journal of the Atmospheric Sciences, 52(1), 44-66.
Hamilton, Kevin P., 1994: Interdecadal climate variations over the high-latitude North Atlantic as seen in 235 years of surface air temperature data. Canadian Meteorological and Oceanographic Society (CMOS) Bulletin, 22(2), 11-14.
Hamilton, Kevin P., 1994: Meteorology and Climatology In Encyclopedia of Applied Physics, VCH Publishers, 215-237.
Hamilton, Kevin P., 1994: Modelling middle atmosphere interannual variability In Proceedings of the Fifth COSPAR Colloquium, Pergamon Press, Inc., 751-757. Abstract
The GFDL "SKYHI" general circulation model is designed to perform comprehensive simulations of the dynamics of the global troposphere, stratosphere and mesosphere. In the version considered here the primitive equations are discretized on a grid with 3° horizontal resolution and 40 levels between the ground and the mesopause. A control simulation of over twenty years duration was performed using a climatological annual cycle of sea surface temperature. The interannual variability of the stratospheric circulation in this model has some impressively realistic features. In particular the simulated variance of monthly mean zonal mean temperature and wind in the extratropical Northern Hemisphere agrees fairly well with observations. The major deficiency is the absence of a realistic quasi-biennial oscillation (QBO) in the simulated winds in the tropical lower stratosphere.
The model has also been run through a large number of boreal winter simulations with the mean flow in the tropical lower stratosphere arbitrarily modified to be appropriate for different extremes of the QBO. The effects of these modifications on the simulated circulation in both the tropical and extratropical middle atmosphere was then examined. The model results are consistent with the observed tendency for the polar vortex to be weaker (stronger) when the QBO is in its easterly (westerly) phase.
Hamilton, Kevin P., 1994: Simulation of vertically-propagating waves in comprehensive general circulation models: opportunities for comparison with observations In UARS/SNES Workshop on Wind Observations in the Middle Atmosphere, 15-18 November 1994, Paris, CNES-HQ, 4.3-4.15.
Hamilton, Kevin P., R John Wilson, and Hans Vahlenkamp, 1994: Three-dimensional visualization of the polar stratospheric vortex. Canadian Meteorological and Oceanographic Society (CMOS) Bulletin, 22(4), 4-6.
Yuan, L, and Kevin P Hamilton, 1994: Equilibrium dynamics in a forced-dissipative f-plane shallow-water system. Journal of Fluid Mechanics, 280, 369-394. Abstract
The equilibrium dynamics in a homogeneous forced-dissipative f-plane shallow-water system is investigated through numerical simulations. In addition to classical two-dimensional turbulence, inertio-gravity waves also exist in this system. The dynamics is examined by decomposing the full flow field into a dynamically balanced potential-vortical component and a residual `free' component. Here the potential-vortical component is defined as part of the flow that satisfies the gradient-wind balance equation and that contains all the linear potential vorticity of the system. The residual component is found to behave very nearly as linear inertio-gravity waves. The forcing employed is a mass and momentum source balanced so that only the large-scale potential-vortical component modes are directly excited. The dissipation is provided by a linear relaxation applied to the large scales and by an eighth-order linear hyperdiffusion. The statistical properties of the potential-vortical component in the fully developed flow were found to be very similar to those of classical two-dimensional turbulence. In particular, the energy spectrum of the potential-vortical component at scales smaller than the forcing is close to the ~k-3expected for a purely two-dimensional system. Detailed analysis shows that the downscale enstrophy cascade into any wavenumber is dominated by very elongated triads involving interactions with large scales. Although not directly forced, a substantial amount of energy is found in the inertio-gravity modes and interactions among inertiogravity modes are principally responsible for transferring energy to the small scales. The contribution of the inertio-gravity modes to the flow leads to a shallow tail at the high-wavenumber end of the total energy spectrum. For parameters roughly appropriate for the midlatitude atmosphere (notably Rossby number ~0.5), the break between the roughly ~k-3 regime and this shallower regime occurs at scales of a few hundred km. This is similar to the observed mesoscale regime in the atmosphere. The nonlinear interactions among the inertio-gravity modes are extremely broadband in spectral space. The implications of this result for the subgrid-scdale closure in the shallow-water model are discussed.
Hamilton, Kevin P., 1993: A general circulation model simulation of El Niño effects in the extratropical northern hemisphere stratosphere. Geophysical Research Letters, 20(17), 1803-1806. Abstract
This paper examines the effects of imposed tropical Pacific sea surface temperature perturbations on the December-February mean circulation in the Northern Hemisphere stratosphere as simulated by the GFDL "SKYHI" troposphere-stratosphere-mesosphere GCM. The results are in impressive agreement with observational studies that have shown that, on average, ENSO events are associated with a mixed zonal wave 1 and 2 perturbation to the stratospheric stationary wave field (phased so that the Aleutian High at midstratospheric levels is intensified). This is a useful test of the sensitivity of the model stratospheric climate to changes in tropospheric forcing. These results for the effects of the Pacific SST anomalies are used in a very rough estimate of the impact of year-to-year changes in SST on the interannual variability of the extratropical stratospheric circulation. The results of this calculation suggest that interannual SST variation accounts for only a small fraction of stratospheric variability at high latitudes.
Hamilton, Kevin P., 1993: An examination of observed southern oscillation effects in the northern hemisphere stratosphere. Journal of the Atmospheric Sciences, 50(20), 3468-3473. Abstract
The effects of the Southern Oscillation on the December-February mean circulation in the Northern Hemisphere stratosphere were investigated using 34 years of data. No evidence for a significant relation between the Southern Oscillation (SO) and the zonally averaged flow is found for any region poleward of 20°N. The effects of the tropical quasi-biennial oscillation (QBO) on the zonal mean flow are much stronger, and this complicates the detection of SO effects. Some more suggestive results are evident when hemispheric maps of height anomalies at 50 or 30 mb are composited for the warm extremes of the SO. The present findings are broadly consistent with earlier suggestions that, on average, the Aleutian high is intensified during the warm extremes of the Southern Oscillation. Even using the 34 years of data now available, however, the statistical significance of this relationship cannot be demonstrated unequivocally. Once again the separation of SO effects from QBO influences in the limited data available is a serious problem.
Hamilton, Kevin P., 1993: Model simulation of the stratospheric penetration of the southern oscillation. TOGA Notes, 13, 7-11.
Hamilton, Kevin P., 1993: What we can learn from general circulation models about the spectrum of middle atmospheric motions In Coupling Processes in the Lower and Middle Atmosphere, Kluwer Academic Publishers, 161-174. Abstract
This article presents an informal survey of some recent results from the GFDL "SKYHI" general circulation model that relate to the spectrum of stratospheric and mesospheric gravity waves. Attention is focussed on the extratropical summer hemisphere where upward propagating gravity waves should dominate the eddy component of the circulation. After a brief description of the SKYHI model, an examination is made of the large scale simulation and its dependence on model resolution. Then a comparison is made of statistics relating to the gravity wave field derived from rocketsonde observations at a number of extratropical stations with comparable model results. The spatial spectra of both kinetic energy and vertical eddy momentum transport in the simulated summer middle atmosphere are then discussed. The results of these investigations show that the SKYHI model gravity wave field has some impressively realistic features. On the other hand the present results also strongly suggest that much of the gravity wave vertical momentum flux resides at scales that are not practically resolvable in current global models.
Manzini, E, and Kevin P Hamilton, 1993: Middle atmospheric traveling waves forced by latent and convective heating. Journal of the Atmospheric Sciences, 50(14), 2180-2200. Abstract
The excitation and propagation of equatorial planetary waves and inertia-gravity waves were studied by comparing simulations from the comprehensive GFDL troposphere-stratosphere-mesosphere SKYHI general circulation model (GCM) and from a linear primitive equation model with the same domain and numerical resolution. The basic state of the linear model is time dependent and is derived from the mean zonal wind and temperature obtained from the full SKYHI model. The latent and convective heating fields of this SKYHI integration are used as the forcing for the linear model in a parallel simulation.
The wavelength and frequency characteristics of the prominent vertically propagating equatorial Kelvin and Rossby-gravity waves are remarkably similar in the linear model and in SKYHI. Amplitudes are also similar in the lower stratosphere, indicating that the latent and convective heating is the dominant mechanism producing equatorial wave activity in the GCM. The amplitude of these waves in the upper stratosphere and mesosphere is larger in the linear model than in SKYHI. Given that the linear and SKYHI models have comparable radiative damping and horizontal subgrid scale diffusion, it appears that the wave amplitudes in SKYHI are limited by some nonlinear saturation, possibly involving the subgrid-scale vertical mixing.
At low latitudes the linear model reproduces the flux of upward-propagating inertia-gravity waves seen in the full model. The results also show that a significant fraction of the inertia-gravity wave activity found in the midlatitude mesosphere of the SKYHI model can be accounted for by tropical convective heating.
The global-scale Rossby normal modes seen in observations were also identified in the analyses of westward-propagating planetary waves in both models. They are of realistic amplitude in the SKYHI simulation but are much weaker in the linear model. Thus, it appears that latent and convective heating is not the main source of excitation for the Rossby normal modes.
Hamilton, Kevin P., and L Yuan, 1992: Experiments on tropical stratospheric mean-wind variations in a spectral general circulation model. Journal of the Atmospheric Sciences, 49(24), 2464-2483. Abstract
A 30-level version of the rhomboidal-15 GFDL spectral climate model was constructed with roughly 2-km vertical resolution. In common with other comprehensive general circulation models, this model fails to produce a realistic quasi-biennial oscillation (QBO) in the tropical stratosphere.
A number of simulations were conducted in which the zonal-mean winds and temperatures in the equatorial lower and middle stratosphere were instantaneously perturbed and the model was integrated while the mean state relaxed toward its equlibrium. The time scale for the mean wind relaxation varied from somewhat over one month at 40 km to a few months in the lower stratosphere. This is similar to the time scales of observed QBO wind reversals. The wind relaxations in the model also displayed the downward phase propagation characteristic of QBO wind reversals, and mean wind anomalies of opposite sign to the imposed perturbation appear at higher levels. In the GCM, however, the downward propagation is clear only above about 20 mb.
Detailed investigations were made of the zonal-mean zonal momentum budget in the equatorial stratosphere in these experiments. The mean flow relaxations above 20 mb were mostly driven by the vertical Eliassen-Palm flux convergence. The anomalies in the horizontal Eliassen-Palm fluxes from extratropical planetary waves, however, were found to be the dominant effect forcing the mean flow back to its equilibrium at altitudes below 20 mb. The vertical eddy momentum fluxes near the equator in the model were decomposed using space-time Fourier analysis. While total fluxes associated with easterly and westerly waves are comparable to those used in simple mechanistic models of the QBO, the GCM has its flux spread over a very broad range of wavenumbers and phase speeds.
The effects of vertical resolution were studied directly by repeating part of the control integration with a 69-level version of the model with greatly enhanced vertical resolution in the lower and middle stratosphere. The results showed that there is almost no sensitivity of the simulation in the tropical stratosphere to the increased vertical resolution.
Hamilton, Kevin P., 1991: Climatological statistics of stratospheric inertia-gravity waves deduced from historical rocketsonde wind and temperature data. Journal of Geophysical Research, 96(D11), 20,831-20,839. Abstract
Ten years of rocketsonde observations of wind and temperature in the 28-57 km height range at twelve stations (spanning 8°S to 76°N) were analyzed. The aim was to determine the geographical and seasonal variation of statistics relating to the propagation of inertia-gravity waves. As noted by earlier investigators, there is a clear tendency for the rocketsonde hodographs to display clockwise rotation with height in the extratropical Northern Hemisphere. This is consistent with the notion that the variations are dominated by inertia-gravity waves with upward energy propagation. By using the wind and temperature data simultaneously it was possible to determine a dominant direction of horizontal wave propagation for each profile. This quantity has an impressive seasonal variation in midlatitudes, with strong eastward propagation apparent in summer and westward propagation in winter. This seasonal cycle is consistent with theoretical notions of how the mean flow ought to affect wave propagation.
Two stations within 10° of the equator are included in this analysis. The results at these near-equatorial stations contrast strongly with those at higher latitudes. There is a clear tendency for the hodographs at these stations to display quite linear polarization (rather than the systematic rotation generally seen at higher latitudes). The wind variations also show a remarkable tendency to align themselves in the zonal direction. When the wind and temperature variations are used together it can be shown that the variations seen are consistent with a clear dominance of eastward propagation. This could indicate that the transient disturbances in the tropical upper stratosphere consist largely of equatorial Kelvin waves.
Hamilton, Kevin P., 1990: A look at the recently proposed solar-QBO-weather relationship. Journal of Climate, 3(4), 497-503. Abstract
Surface meteorological data at several stations over the period 1875-1936 are examined in relation to solar activity. In particular an attempt is made to see if these historical data can be reconciled with the sun-QBO-weather relationship recently found in modern (post-1950) data by van Loon and Labitzke (vLL). The basic problem in extending vLL's analysis in earlier periods is ignorance of the phase of the QBO. In the present study, vLL's computations are repeated for the historical data using several million possible sequences for the phase of the QBO. The results reveal problems in reproducing vLL's results in the earlier data. This indicates either that the QBO behaved differently in the past, or that vLL's results for a solar-weather relationship are not stable over the long term.
Hamilton, Kevin P., 1989: Interhemispheric asymmetry and annual synchronization of the ozone quasi-biennial oscillation. Journal of the Atmospheric Sciences, 46(7), 1019-1025. Abstract
The quasi-biennial oscillation (QBO) in total column ozone has been examined at several tropical stations. The ozone QBO at Mauna Loa (19.5 degrees N) was found to have a remarkable annual synchronization. Both positive and negative extremes in the deseasonalized ozone time series almost always occur between December and March. The annual cycle-QBO phase locking is much more pronounced in this ozone record than it is for the familiar QBO in the prevailing tropical stratospheric winds. This result is taken as evidence that the dynamical QBO acts to modulate a strong seasonal ozone transport from midlatitudes to the tropics. If this transport is connected with quasi-stationary planetary waves, then this interpretation offers an obvious explanation for the interhemispheric asymmetry in the ozone QBO that has been noted in many earlier studies.
Hamilton, Kevin P., 1988: A detailed examination of the extratropical response to tropical El Niño/Southern Oscillation events. Journal of Climatology, 8, 67-86. Abstract
It is now clear that conditions in the tropical Pacific Ocean and the overlying atmosphere can have significant associations with the seasonal-mean circulation of the mid-latitude atmosphere, particularly in winter. Notable is a tendency for an intensification of the climatological standing wave pattern over the North Pacific and North America during the warm tropical conditions associated with El Niño/Southern Oscillation (ENSO) events. However, the actual mid-latitude circulation anomalies do show a great deal of variability among individual ENSO years. This paper addresses the question of whether the variability of the mid-latitude response to ENSO events can be attributed to differences in the details of the tropical anomalies from event to event. The investigation embraced the period 1899-1982 and employed data on tropical sea surface temperatures (SST's), tropical rainfall, and mid-latitude atmospheric surface pressures and air temperatures.
The results suggest that a major factor in determining the strength of the Northern Hemisphere extratropical response is the SST anomaly in the far western Pacific/Indonesian region. In particular, it appears that the strong extratropical teleconnections with ENSO events occur more frequently when the SST in the far western Pacific is anomalously warm (or at least not overly cold). This finding appears to be consistent with the results of at least some recent numerical experiments conducted with comprehensive general circulation models.
Hamilton, Kevin P., 1988: Equatorial Atlantic sea surface temperature variations. Atmosphere-Ocean, 26(4), 668-678. Abstract
Monthly mean sea surface temperature (SST) anomalies were computed for six 10° boxes stretching across the equatorial Atlantic Ocean for the period 1890-1979. These values were used to produce a time-longitude section of the interannual SST variability along the equator. This section shows cycles of basin-wide warming and cooling occurring with irregular periods that typically range between two and four years. The warming and cooling events in these cycles normally display some westward phase propagation. The peak magnitudes of the interannual SST anomalies are generally of the order of 1°C or less, except in the Gulf of Guinea where they can be somewhat larger.
An estimate was made of the basin-wide equatorial SST anomaly in each month (excluding the Guld of Guinea). This was composited around the times of the warm and cold extremes of the Pacific Southern Oscillation. This analysis revealed a detectable, but rather weak, tendency for phase locking of the interannual SST variations in the equatorial Pacific and Atlantic Oceans.
Hamilton, Kevin P., 1988: Evaluation of the gravity wave field in the middle atmosphere of the GFDL "SKYHI" general circulation model In Workshop on Systematic Errors in Models of the Atmosphere, CAS/JSC Working Group on Numerical Experimentation, Report No. 12, WMO/TD No. 273, World Meteorological Organization, 264-271.
Hamilton, Kevin P., and Jerry D Mahlman, 1988: General circulation model simulation of the semiannual oscillation of the tropical middle atmosphere. Journal of the Atmospheric Sciences, 45(21), 3212-3235. Abstract
A study has been made of the evolution of the zonal-mean zonal wind and temperature in a multiyear integration of the 40-level, 3° x 3.6° resolution "SKYHI" general circulation model (GCM) that has been developed at GFDL. In the tropical upper stratosphere the mean wind variation is dominated by a strong semiannual oscillation (SAO). The peak SAO amplitude in the model is almost 25 m s-1 and occurs near the 1 mb level. The phase of the SAO near the stratopause is such that maximum westerlies occur shortly after the equinoxes. These features are in good agreement with the available observations. In addition the meridional width of the stratopause SAO in the GCM compares well with observations.
A diagnostic analysis of the zonal-mean momentum balance near the tropical stratopause was performed using the detailed fields archived during the GCM integration. It appears that the easterly accelerations in the model SAO are provided by a combination of (i) divergence of the meridional component of the Eliassen-Palm flux associated with quasi-stationary planetary waves and (ii) mean angular momentum advection by the residual meridional circulation. The effects of the residual circulation dominate in the summer hemisphere, while the eddy contributions are more important in the winter hemisphere. The westerly accelerations in the model SAO result from the convergence of the vertical momentum transport associated with gravity waves that have a broad distribution of space and time scales. Thus, in contrast to some simple theoretical models, large-scale equatorial Kelvin waves appear to play only a very minor role in the dynamics of the SAO in the SKYHI GCM.
A second equatorial SAO amplitude maximum was found in the tropical upper mesosphere of the GCM. this apparently corresponds to the mesopause SAO that has been identified in earlier observational studies. While the observed phase of this oscillation is reproduced in the model, the simulated amplitude is unrealistically small.
The model integration included the computation of the concentration of N2O. The results show a fairly realistic simulation of the semiannual variation of tropical stratospheric N2O mixing ratio seen in satellite observations.
Hamilton, Kevin P., 1981: Effects of atmospheric tides on the general circulation of the stratosphere, mesosphere and lower thermosphere. Handbook for MAP, 2, 246-255.
Hamilton, Kevin P., 1981: Latent heat release as a possible forcing mechanism for atmospheric tides. Monthly Weather Review, 109(1), 3-17. Abstract
The consequences of the hypothesis of Lindzen (1978) that latent heat release may be a significant excitation mechanism for the semidiurnal atmospheric tide are examined in some detail. Harmonic analysis of hourly rainfall data from 79 tropical stations shows that the semidiurnal variation of rainfall in the tropics is ~ 1 mm day-1 and has a phase near 0300 LST, just as Lindzen's theory requires. Analysis of data at 85 midlatitude stations shows that the semidiurnal rainfall oscillation there has its phase rather later (about 0600). The results of simple classical tidal theory calculations which indicate that the geographical distribution of the surface pressure response to latent heat forcing largely follows that of the forcing itself are presented. This result is then used to suggest a plausible explanation for the observed seasonal cycle of the semidiurnal pressure oscillation in midlatitudes. Further calculations show that the magnitude of the non-migrating components of the semidiurnal barometric oscillation produced by latent heat excitation is not likely to be unrealistically large. These calculations also suggest that Lindzen's hypothesis might be verified by observing the phase of the semidiurnal pressure oscillation in particularly arid regions.
The rainfall observations also show a strong diurnal (24 h) component in the rainfall both in the tropics and in midlatitudes. The effects of latent heat release on the 24 h tide are briefly discussed.
Hamilton, Kevin P., 1980: The geographical distribution of the solar semidiurnal surface pressure oscillation. Journal of Geophysical Research, 85(C4), 1945-1949. Abstract
New observations of the solar semidiurnal surface pressure oscillations (S2(P)) at 35 stations throughout the world are presented. These observation are used to supplement those in the literature to produce a large compilation of S2(P) data (403 stations). The geographical distribution of S2(P) is then examined by fitting a series of Hough functions to the station data.
Hamilton, Kevin P., 1980: Observations of the solar diurnal and semidiurnal surface pressure oscillations in Canada. Atmosphere-Ocean, 18(2), 89-97. Abstract
New observations of the solar diurnal and semidiurnal barometric oscillations at sixty Canadian stations are presented.
