Waliser, D E., R-L Shia, John R Lanzante, and Abraham H Oort, 1999: The Hadley circulation: Assessing NCEP/NCAR reanalysis and sparse in-situ estimates. Climate Dynamics, 15(10), 719-735. Abstract PDF
We present a comparison of the zonal mean meridional circulations derived from monthly in-situ data (i.e., radiosondes and ship reports) and from the NCEP/NCAR reanalysis product. To facilitate the interpretation of the results, a third estimate of the mean meridional circulation is produced by subsampling the reanalysis at the locations where radiosonde and surface ship data are available for the in-situ calculation. This third estimate, known as the subsampled estimate, is compared to the complete reanalysis estimate to assess biases in conventional, in situ estimates of the Hadley circulation associated with the sparseness of the data sources (i.e., radiosonde network). The subsampled estimate is also compared to the in-situ estimate to assess the biases introduced into the reanalysis product by the numerical model, initialization process and/or indirect data sources such as satellite retrievals. The comparisons suggest that a number of qualitative differences between the in- situ and reanalysis estimates are mainly associated with the sparse sampling and simplified interpolation schemes associated with in-situ estimates. These differences include: (1) a southern Hadley cell that consistently extends up to 200 hPa in the reanalysis, whereas the bulk of the circulation for the in-situ and subsampled estimates tends to be confined to the lower half of the troposphere, (2) more well-defined and consistent poleward limits of the Hadley cells in the reanalysis compared to the in-situ and subsampled estimates, and (3) considerably less variability in magnitude and latitudinal extent of the Ferrel cells and southern polar cell exhibited in the reanalysis estimate compared to the in-situ and subsampled estimates. Quantitative comparison shows that the subsampled estimate, relative to the reanalysis estimate, produces a stronger northern Hadley cell (~20%), a weaker southern Hadley Cell (~20-60%), and weaker Ferrel cells in both hemispheres. These differences stem from poorly measured oceanic regions which necessitate significant interpolation over broad regions. Moreover, they help to pinpoint specific shortcomings in the present and previous in-situ estimates of the Hadley circulation. Comparions between the subsampled and in situ estimates suggest that the subsampled estimate produces a slightly stronger Hadley circulation in both hemispheres, with the relative differences in some seasons as large as 20-30%. These differences suggest that the mean meridional circulation associated with the NCEP/NCAR reanalysis is more energetic than observations suggest. Examination of ENSO-related changes to the Hadley circulation suggest that the in-situ and subsampled estimates significantly overestimate the effects of ENSO on the Hadley circulation due to the reliance on sparsely distributed data. While all three estimates capture the large-scale region of low-level equatorial convergence near the dateline that occurs during El Niño, the in-situ and subsampled estimates fail to effectively reproduce the large-scale areas of equatorial mass divergence to the west and east of this convergence area, leading to an overestimate of the effects of ENSO on the zonal mean circulation.
Gelman, M E., V Ramaswamy, and Abraham H Oort, et al., 1997: Stratospheric temperature trends derived from SPARC datasets 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, 153-156. Abstract
The SPARC Stratospheric Temperature Trends Assessment Group has collected 12 datasets of monthly mean, zonal mean stratospheric temperatures, for the purpose of deriving a best estimate of global stratospheric temperature trends. All but one of the datasets cover the years 1979 to 1994, and some extend further back in time. The pressure-altitude levels of the datasets vary, but overall they cover the range 100 to 0.4 hPa (approximately 16-55 km). The datasets represent compilations from ground-based instruments (e.g., radiosondes), satellite instruments (brightness temperatures), analyses (which use one or both of the two other data sources), and assimilation analyses (which use a numerical model in the procedure). Most datasets provide temperatures at specific pressure-levels, but some provide data as mean temperatures representative of various pressure-layers.
Temperature trends and their estimated errors have been calculated over the basic period 1979 to 1994 from each dataset, and for 1966 to 1994 for those datasets covering the extended period. This paper describes some attributes of the various datasets and plans of comparisons of temperature trend computations from the datasets.
Oort, Abraham H., and B Saltzman, 1997: José Pinto Peixoto. EOS, 78(47), 537, 542.
Oort, Abraham H., and J J Yienger, 1996: Observed interannual variability in the Hadley Circulation and its connection to ENSO. Journal of Climate, 9(11), 2751-2767. Abstract PDF
Based on a 26-yr set of daily global upper-air wind data for the period January 1964-December 1989, the interannual variability in the strength of the tropical Hadley cells is investigated. Although several measures of the intensity of the zonal-mean cells are discussed, the main focus is on the maximum in the streamfunction in the northern and southern Tropics. The streamfunction was computed from observed monthly mean latitude versus pressure cross sections of the zonal-mean meridional wind component. Significant seasonal variations are found in the strength, latitude, and height of the maximum streamfunction for both Hadley cells. Significant correlations are also observed between the Hadley cells and the El Niño-Southern Oscillation phenomenon. During the extreme seasons, only one "winter" Hadley cell dominates the Tropics, with the rising branch in the summer hemisphere and the sinking branch in the winter hemisphere. Superimposed on this "normal" one-cell winter Hadley circulation in the Tropics are two strengthened direct (i.e., energy releasing) Hadley cells found during episodes of warm sea surface temperature anomalies in the eastern equatorial Pacific (El Niño) and weakened Hadley cells during episodes of cold anomalies. The anomalies in the strength of the Hadley cells are strongly and inversely correlated with the anomalies in the strength of the Walker oscillation.
Overland, J E., P Turet, and Abraham H Oort, 1996: Regional variations of moist static energy flux in the Arctic. Journal of Climate, 9(1), 54-65. Abstract PDF
The authors investigate the climatological heating of the Arctic by the atmospheric moist static energy (MSE) flux from lower latitudes based on 25 year (November 1964-1989) of the GFDL dataset. During the five month winter period (NDJFM) the transport of sensible heat by transient eddies is the largest component (50%) at 70°N, followed by the transport of sensible heat by standing eddies (25%), and the moist static energy flux by the mean meridional circulation (25%). The mean meridional circulation (MMC) changes from a Ferrel cell to a thermally direct circulation near 60°N; maximum horizontal velocities in the thermally direct circulation peak near 70°N. North of the 60°N the sensible heat flux by the MMC is southward and opposes the greater northward transport of geopotential energy. The transport of energy is not uniform. Major pathways are the northward transport of positive anomalies through the Greenland and Barents Seas into the eastern Arctic and the southward transport of negative anomalies to the east of the Siberian high. The Atlantic pathway in winter relates to transport by transient eddies, while the western Siberian flux relates to the standing eddy pattern. Interannual variability of northward MSE is concentrated in these two regions. The western Arctic Ocean from about 30° to 60°W receives about 50 W m-2 less energy flux convergence than the eastern Arctic. This result compares well with the observed minimum January surface air temperatures in the Canadian Basin of the western Arctic and implies that the greater observed ice thickness in this region may have a thermodynamic as well as a dynamic origin.
Peixoto, J P., and Abraham H Oort, 1996: The climatology of relative humidity in the atmosphere. Journal of Climate, 9(12), 3443-3463. Abstract PDF
The present paper deals with the analysis of the time-average relative humidity fields in the atmosphere. Twice-daily estimates of relative humidity are used.
After some theoretical considerations on the relations between relative humidity, other moisture parameters, and temperature, a critical analysis of the various sources of data is made considering their possible limitations. Various methods of computing relative humidity are formulated and discussed.
The global distribution of relative humidity at various levels shows that it is not zonally uniform with centers of various intensities at all latitudes. The global maps show maxima in the equatorial zone and minima in the dry subtropical belts around 30°N and 30°S. The land-sea contrast and variations related to the orographic relief are also apparent. The general pattern of relative humidity is similar at all levels but its magnitude decreases with altitude. The seasonal analyses show a similar pattern as the annual analyses but are slightly shifted toward the summer pole.
The saturation deficit is also evaluated. Cross sections of the saturation deficit show that the maxima are found in the middle to lower troposphere at subtropical latitudes, being most intense in the Northern Hemisphere during the summer season.
The temporal variability of the relative humidity due to transient eddies exhibits a bimodal structure with maxima in the midlatitudes of each hemisphere around 700 mb. The stationary eddy distributions are less pronounced than the transient ones and do not change substantially from one season to another.
To validate our results, several comparisons are made using independent sources of humidity data as well as cloud distributions at various levels. Thus, COADS data are used to obtain independent analyses of the surface relative humidity over the oceans, and satellite observations by SAGE are used at the 300-mb level. The rawinsonde-SAGE differences are on the order of 10% in the Tropics and 20% in the high latitudes, due in part to a clear-sky (dry) bias in the SAGE data. Our results are further compared with those obtained from operational analyses by the ECMWF. The differences do not exceed 5% in the Tropics but tend to be larger in the tropical upper troposphere and at all levels in the extratropics of the Southern Hemisphere, where the radiosonde network is quite sparse. In view of the obvious connections between the moisture distribution in the atmosphere and cloudiness, a cloud climatology is used to check its consistency with the present results. The latitudinal and interseasonal variations of cloudiness and relative humidity are similar, with maxima in the equatorial belt and at high latitudes and minima in the subtropics that shift poleward during summer and equatorward during winter.
Finally, some comments are made on the radiosonde-observing systems in the light of recent satellite studies of humidity. Mainly at the upper levels systematic localized differences are found between electrical hygristor and organic sensors, but the differences almost disappear in the middle and lower troposphere.
In spite of the shortcomings, limitations, and errors of the radiosonde network, the present analyses describe for the first time the large-scale, three-dimensional characteristics of the relative humidity in the global atmosphere.
The observed spatial patterns of temperature change in the free atmosphere from 1963 to 1987 are similar to those predicted by state-of-the-art climate models incorporating various combinations of changes in carbon dioxide, anthropogenic sulphate aerosol and stratospheric ozone concentrations. The degree of pattern similarity between models and observations increases through this period. It is likely that this trend is partially due to human activities, although many uncertainties remain, particularly relating to estimates of natural variability.
Vitart, F, Abraham H Oort, and K Mo, 1996: New results on the hydrology of the North African desert, based on the NMC reanalyses In Proceedings of the 20th Annual Climate Diagnostics Workshop, U.S. Dept. of Commerce/NOAA/NWS, 191-194.
Oort, Abraham H., and J J Yienger, 1995: Observed long-term variability in the Hadley circulation In Proceedings of the 19th Annual Climate Diagnostics Workshop, Springfield, VA, NTIS, 26-29.
Santer, B D., Abraham H Oort, V Ramaswamy, M Daniel Schwarzkopf, and Ronald J Stouffer, et al., 1995: A Search for Human Influences on the Thermal Structure of the Atmosphere, Program for Climate Model Diagnosis and Intercomparison, PCMDI Report No. 27, UCRL-ID-121956: Lawrence Livermore, CA, 26 pp. Abstract
Recent studies have shown that patterns of near-surface temperature change due to combined forcing by CO and anthropogenic sulfate aerosols are easier to identify in the observations than signals due to changes in CO alone (Santer et al., 1995; Mitchell et al., 1995a). Here we extend this work to the vertical structure of atmospheric temperature changes, and additionally consider the possible effects of stratospheric ozone reduction. We compare modelled and observed patterns over the lower troposphere to the lower stratosphere (850 to 50 hPa) and over the low- to mid-troposphere (850 to 500 hPa). In both regions there are strong similarities between observed changes and model-predicted signals. Over 850 to 50 hPa similarities are evident both in CO-only signals and in signals that incorporate the added effects of sulfate aerosols and stratospheric ozone reduction. These similarities are due largely to a common pattern of stratospheric cooling and tropospheric warming in the observations and model experiments. Including the effects of stratospheric ozone reduction results in a more realistic height for the transition between stratospheric cooling and results in a more realistic height for the transition between stratospheric cooling and tropospheric warming. In the low- to mid-troposphere the observations are in better agreement with the temperature-change patterns due to combined forcing than with the CO-only pattern. This is the result of hemispheric-scale temperature-change contrasts that are common to the observations and the combined forcing signal but absent in the CO-only case. The levels of model-versus-observed pattern similarity in both atmospheric regions increase over the period 1963 to 1987. If model estimates of natural internal variability are realistic, it is likely that these trends in pattern similarity are partially due to human activities.
Sun, D-Z, and Abraham H Oort, 1995: Humidity-temperature relationships in the tropical troposphere. Journal of Climate, 8(8), 1974-1987. Abstract PDF
Based on the observed interannual variations of water vapor and temperature over the past 26 years the authors have examined the relationship between the variations of water vapor and temperature in the tropical troposphere. The authors find that in both the lower and upper troposphere, tropical mean specific humidity increases with temperature. The rate of fractional increase of specific humidity with temperature at 500 mb is as large as that in the surface boundary layer. However, the rate of fractional increase of specific humidity with temperature is significantly smaller than that given by a model with a fixed relative humidity, particularly in the region immediately above the tropical convective boundary layer. The variations of tropical mean relative humidity show consistently a negative correlation with the temperature variations.
The authors have further compared the spatial structure of the specific humidity variations with that of the temperature variations. Though the vertical structure of tropical mean specific humidity has more variability than that of the tropical mean temperature, the leading EOF for the normalized specific humidity variations is almost exactly the same as the leading EOF for the normalized temperature variations. The characteristic horizontal structure of the specific humidity variations at levels in the free troposphere, however, is very different from that of the temperature variations. The leading EOF for the normalized specific humidity variations at levels in the free troposphere is characterized by regions with alternating positive and negative signs, while the leading EOF for the corresponding temperature variations has a single sign throughout the Tropics. When the variations are averaged zonally, the leading EOF for the normalized specific humidity variations still differs significantly from that of the normalized temperature variations, but the leading EOF has the same sign from the deep Tropics to the subtropics.
As an example of the technique of fingerprint detection of greenhouse climate change, a multi-variate signal or fingerprint of the enhanced greenhouse effect is defined using the zonal mean atmospheric temperature change as a function of height and latitude between equilibrium climate model simulations with control and doubled CO2 concentrations. This signal is compared with observed atmospheric temperature variations over the period 1963 to 1988 from radiosonde-based global analyses. There is a significant increase of this greenhouse signal in the observational data over this period. These results must be treated with caution. Upper air data are available for a short period only, possibly too short to be able to resolve any real greenhouse climate change. The greenhouse fingerprint used in this study may not be unique to the enhanced greenhouse effect and may be due to other forcing mechanisms. However, it is shown that the patterns of atmospheric temperature change associated with uniform global increases of sea surface temperature, with El Niño-Southern Oscillation events and with decreases of stratospheric ozone concentrations individually are different from the greenhouse fingerprint used here.
Oort, Abraham H., 1994: An investigation of the historical station records of upper air humidity In Proceedings of the 18th Annual Climate Diagnostics Workshop, U. S. Dept. of Commerce/NOAA/NWS, 346-349.
Oort, Abraham H., L A Anderson, and J P Peixoto, 1994: Estimates of the energy cycle of the oceans. Journal of Geophysical Research, 99(C4), 7665-7688. Abstract
A new formulation of the general problem of the large-scale energetics for the global oceans is presented and analyzed. Using a variety of ocean surface observations, some of the terms in the energy balance equations, such as the time rates of change, the generation rates of available gravitational potential energy G(P) and kinetic energy G(K), and the conversion rate from available gravitational potential energy into kinetic energy C(P,K), are estimated for annual and seasonal mean conditions. An attempt is also made to measure the uncertainty of these terms in order to assess the realiability of the results. The interseasonal variability is analyzed showing that the winter hemisphere represents the most active region of the globe. Using the data and some reasonable assumptions, pictures of the energy cycle in the global oceans for yearly and seasonal conditions are constructed. The hemispheric and global dissipation rates of available gravitational potential energy D(P) and of kinetic energy D(K) are estimated as residuals, assuming that the contribution from the internal energy C(I,K) is only a minor factor. After analyzing the general consistency of the picture of the energy cycle for the oceans, the conclusion is reached that the G(P) and G(K) terms are about equally important terms needed to describe and understand the structure and dynamics of the global ocean circulation.
Raval, A, Abraham H Oort, and V Ramaswamy, 1994: Observed dependence of outgoing longwave radiation on sea surface temperature and moisture. Journal of Climate, 7(5), 807-821. Abstract PDF
The authors have empirically examined the dependence of the outgoing longwave radiation (OLR) on sea surface temperature (Ts), precipitable water (W), and height-mean relative humidity (RH). The OLR is obtained from 4 yr of data from the Earth Radiation Budget Experiment (ERBE), while Ts, W, and RH are obtained from objective analyses of rawinsonde and ship data. It is found that in the midlatitudes, the surface temperature explains over 80% of the variability in the clear-sky OLR (Fcs) and almost half of the variability in the total OLR (Ftot). It fails badly in the tropics and subtropics, however, where Ts explains only about 20% of the variability in Fcs and is largely decoupled from Ftot. The two-dimensional contour plot of the OLR binned with respect to Ts and RH is marked by distinct changes in the gradient that are consistent with inferences from earlier investigations. For low values of Ts(<10°C), the OLR depends mainly on Ts. For values of Ts above 10°C, the OLR depends increasingly on RH. Specifically, in the tropics (Ts ~ 25°C), the total and clear-sky OLR depend significantly on both Ts and RH. The well-known drop in OLR in the tropics with increasing Ts correlates directly to an increase in RH, and not to changes in Ts. The authors suggest that the observed dependence of the OLR on Ts and RH be a minimum performance standard for climate models. This approach is illustrated by comparing the observed dependence with the results of a radiative transfer model and an R15 general circulation model, and by discussing the strengths and limitations of using RH to parameterize the OLR.
Sun, D-Z, and Abraham H Oort, 1994: Interannual variations of water vapor in the tropics In Proceedings of the 18th Annual Climate Diagnostics Workshop, U. S. Dept. of Commerce/NOAA/NWS, 198-201.
van den Dool, H, S Saha, Abraham H Oort, and W Ebisuzaki, 1994: On the role of atmospheric water in the continuity equation In Proceedings of the 18th Annual Climate Diagnostics Workshop, U. S. Dept. of Commerce/NOAA/NWS, 244-247.
Oort, Abraham H., 1993: Observed humidity trends in the atmosphere In Proceedings of the 17th Annual Climate Diagnostics Workshop, Springfield, VA, NTIS, 24-30.
Oort, Abraham H., and H Liu, 1993: Upper-air temperature trends over the globe, 1958-1989. Journal of Climate, 6(2), 292-307. Abstract PDF
New time series of the hemispheric and global mean temperature anomalies in the troposphere and lower stratosphere are presented for the period May 1958 through December 1989. The statistics are based on objective monthly analyses of all available daily soundings from the global rawinsonde network (~700-800 stations). The results are compared with Angell's earlier statistics based on a subset of 63 stations. Excellent agreement is found with these earlier results as well as with an 11-year set of satellite-derived microwave sounding unit data. These detailed comparisons support the conclusion that the rawinsonde network can provide reliable estimates of the actual interseasonal hemispheric-scale temperature changes that have occurred between the earth's surface and about 20 km (50 mb) height since the 1950s.
Peixoto, J P., and Abraham H Oort, 1993: O forçamento do estado médio zonal da atmosfera. (The forcing of the zonal mean state of the atmosphere). Memórias Da Academia Das Ciências De Lisboa, (Classe De Ciências), 32, 149-174. Abstract
The zonal mean state of the atmosphere can be looked upon as a forced regime by the eddies, friction and diabatic heating, as was discussed in a previous communication (Peixoto and Oort, 1984). The interaction between the eddies and the mean zonal state is also relevant in the energetics of the atmosphere, namely in the generation rates of kinetic energy and eddy available potential energy. As mentioned before (Peixoto and Oort, 1984), the global interaction between the eddies and the zonal mean state can be studied combining the eddy transports of enthalpy and momentum. Eliassen and Palm (1961) studied these interactions for the first time. Their work led to a fictitious vector in the meridional plane, the so-called E-P vector, where the eddy flux of momentum represents the meridional component and the eddy flux of heat the vertical component.
In order to study this interaction we used two data sets, namely the European Center for Medium-Range Weather Forecasts (ECMWF) data for the globe for the year 1985 and the Geophysical Fluid Dynamics Laboratory (GFDL) data for the period 1963-1973 for the Northern Hemisphere. In the present communication, meridional cross sections are presented with the E-P fluxes in the form of arrows and their divergence fields by contour lines. The direction of the E-P flux in this kind of display indicates the relative contribution of the eddy fluxes of heat snd momentum, whereas the divergence gives a measure of the total forcing for the zonal state by the eddies.
The eddy contributions were split into two components, the transient and stationary components. The results for the ECMWF and GFDL data are presented in various panels for yearly, winter and summer conditions. In most cases, the analyses were done separately for the dry atmosphere and for the moist atmosphere by including water vapor. Besides the analyses of the mean fields, the mean residual currents in the atmosphere were also evaluated.
In general, the corresponding results for both sets of data agree well, showing a maximum divergence in low levels and at high latitudes. The stationary E-P flux contributions are, as a rule, smaller in magnitude than the corresponding transient eddy contributions. In the Northern Hemisphere, the interannual differences are more pronounced than in the Southern Hemisphere. It is also important to stress that the transient perturbations are the most significant factor for the atmospheric dynamics in the Southern Hemisphere where the stationary eddies are mostly irrelevant. The inclusion of water vapor does not substantially change the patterns, but reinforces the intensities mainly in the lowest levels. In summer, the residual current is less intense than in winter, shifting into the summer hemisphere as expected.
Peixoto, J P., and Abraham H Oort, 1992: Physics of Climate, New York, NY: American Institute of Physics, 520 pp.
Peixoto, J P., Abraham H Oort, M de Almeida, and A Tomé, 1991: Entropy budget of the atmosphere. Journal of Geophysical Research, 96(D6), 10,981-10,988. Abstract PDF
The balance equations for the entropy in the atmosphere are presented and discussed. Using observed energy fluxes and atmospheric temperatures, we present estimates of the various terms in the global mean entropy budget. The largest boundary fluxes of entropy are associated with the emission of longwave radiation. The fluxes of entropy associated with turbulent and molecular diffusion are found to be much smaller. On the planetary scale the mean outgoing flux of entropy at the top of the atmosphere is found to be about 22 times larger than the mean incoming flux of entropy through solar radiation. The rates of entropy production and destruction by the various irreversible processes that occur in the atmosphere are also computed. The entropy production terms involved in the release of latent heat and the absorption of solar radiation are, by far, the largest sources of entropy for the atmosphere, whereas the production of entropy associated with the absorption of longwave radiation is an order of magnitude smaller. The destruction is mainly accomplished through cooling by outgoing longwave radiation. The contributions of the sensible heat fluxes and friction are relatively small. Regional contributions to the total entropy generation in the atmosphere are studied by considering an equatorial region bounded by two latitudinal walls at 15 degrees N and 15 degrees S and a polar region poleward of 70 degrees N. The rates of entropy generation by the various diabatic processes are highest in the equatorial region; part of the generated entropy is exported to higher latitudes.
Pan, Y-H, and Abraham H Oort, 1990: Correlation analyses between sea surface temperature anomalies in the eastern equatorial Pacific and the world ocean. Climate Dynamics, 4, 191-205. Abstract
Based on data from the Comprehensive Ocean-Atmosphere Data Set (COADS), objective analyses of the monthly mean sea surface temperature (SST) were prepared at GFDL for each month of the 110-year period 1870-1979. Time series of various indices characterizing the SST anomalies averaged over the eastern equatorial Pacific (EEP), the tropical oceans and the world ocean are presented for monthly, yearly and decadal time-averaging periods. Global correlations maps are given for each decade of the 1870-1979 period. They show the spatial connections between the monthly SST anomalies in the EEP and in other parts of the world ocean and how these connections vary for the different decades. On the intermonthly time scale the SST anomalies in the EEP and in other parts of the world ocean and how these connections vary for the different decades. On the intermonthly time scale the SST anomalies in the EEP and those in the tropical and world oceans are found to be highly correlated, with maximum correlations values of 0.91 at zero lag for the tropical oceans during the 1950-1959 decade and 0.81 for the world ocean during the 1970-1979 decade. Positive correlation values of r greater than or equal to 0.36 persist on average from about 4 months before to about 8 months after the EEP anomalies occur. There is a clear tendency for the tropical and world ocean anomalies to lag behind the EEP anomalies. Comparing different oceans, we find the tendency for the tropical SST anomalies in the Indian and Atlantic Oceans to lag behind those in the EEP region by about 1 and 3 months, respectively. On the interannual time scale the EEP anomalies are also well correlated with those in the other regions, having an average correlation of 0.84 for the tropical oceans and of about 0.7 for the world ocean.
Pan, Y-H, Abraham H Oort, and W Richardson, 1990: The variations of the SST in the Eastern and Western Tropical Pacific and their relationship with those in the world ocean In International Conference on East Asia and Western Pacific Meteorology and Climate, Hong Kong, World Scientific Publishing Co, 412-419. Abstract
The seasonal change of the connections between the SST in the EEP with those in the world ocean are given by 12 correlation maps of the 12 calendar months for the period of 1950-1979. The standard deviation map calculated from these 12 correlation maps has shown that the areas with larger values are also located in the western Pacific but mainly in the tropical regions of south- western Pacific and south-eastern Indian oceans. Discussions about the seasonal change of the SST in the EEP as well as those in the world ocean will be given.
Pan, Y-H, W Richardson, and Abraham H Oort, 1990: Some different charactersitics of SST between the western and eastern tropical Pacific Ocean In Air-Sea Interaction in Tropical Western Pacific, Beijing, China, China Ocean Press, 189-194. Abstract
In a previous paper [Pan and Oort 1990], the high correlation between the sea surface temperature (SST) in the eastern equatorial Pacific (EEP) with those in the world ocean has been shown by the similarities of the spatial distributions between the 11 correlation maps for the 11 decades of 1870- 1979. Then, the characteristics of the SST in the western tropical Pacific can be discussed by the standard deviation distribution from these 11 correlation maps. The area with the largest magnitude of standard deviation appears from the south-east Pacific moving north-westward to cross the western equatorial Pacific (WEP) and turning north-eastward. It implicates that the variation of SST over the western Pacific seems more independent from those in the EEP, particularly, in the Australian and east Asian monsoon region. The annual cycle of the thirty-year (1950-1979) climatology of the SST also presents the differences between the EEP and WEP. Probably, this is just to show the importance of monsoon activities in the processes of air-sea interaction over those regions.
Oort, Abraham H., S C Ascher, S Levitus, and J P Peixoto, 1989: New estimates of the available potential energy in the world ocean. Journal of Geophysical Research, 94(C3), 3187-3200. Abstract PDF
On the basis of extensive global data sets the total amounts of internal, potential, latent, and kinetic energy in the world oceans and atmosphere are estimated and compared for annual mean and seasonal conditions. Next, an expression for the available gravitational potential energy in the oceans is derived. The computed amounts of zonal mean and transient eddy available potential energy and the amount of kinetic energy present in the oceans are found to be much smaller (at least 1 order of magnitude) than those in the atmosphere. Because of the sparseness of directly measured subsurface values, the estimates of the kinetic energy and the transient eddy available potential energy were obtained by extrapolation from the corresponding values measured at the ocean surface. The fact that relatively large density variations are found only in the upper few hundred meters of the oceans and the greater stability of the oceans are the basic reasons for the generally small values of the potential and kinetic energy in the oceans compared with those in the atmosphere.
Oort, Abraham H., 1989: Angular momentum cycle in the atmosphere-ocean-solid earth system. Bulletin of the American Meteorological Society, 70(10), 1231-1242. Abstract PDF
Some of the contributions of Victor Paul Starr (1909-1976) as a scholar and teacher at the Massachusetts Institute of Technology are described. His work on the atmospheric branch of the earth's angular momentum cycle is emphasized. Certain recent efforts to include the oceanic and solid earth branches of the cycle are discussed.
Nakamura, N, and Abraham H Oort, 1988: Atmospheric heat budgets of the polar regions. Journal of Geophysical Research, 93(D8), 9510-9524. Abstract
The energy budget and its annual variation are studied for the polar regions of the atmosphere. Composite satellite and rawinsonde data are used to compute the rate of storage of energy in the atmosphere, the net radiation flux at the top of the atmosphere, and the energy fluxes across 70°N and 70°S, while the energy flux across the Earth's surface is estimated as a residual. Data produced by a general circulation model are used to obtain plausible estimates of the energy flux across 70°S, where direct estimates tend to be poor. We find that the heat exchange between the atmosphere and the ocean-cryosphere system plays a much larger role in the Arctic than in the Antarctic polar cap. When the polar caps are extended from 70° to 60° latitude, both the land-sea distributions and the different terms balancing the energy budgets become more comparable between the two regions.
Oort, Abraham H., 1988: Climate observations and diagnostics In Physically-based Modelling and Simulation of Climate and Climatic Change - Part II, Kluwer Academic Publishers, 813-840. Abstract
An observational-statistical approach to the study of the Earth's climate is described. The approach is based mainly on in situ atmospheric and oceanic observations which are analyzed using the conservation equations of total mass, angular momentum, energy and water. Through such analyses one can diagnose the role of the various components in the climatic system. The different observational networks and some frequently used methods of objective analysis are also presented. Finally, some examples are given to show the possibilities of the approach to the case of global and regional budgets of energy and water.
Karoly, D J., and Abraham H Oort, 1987: A comparison of Southern Hemisphere circulation statistics based on GFDL and Australian analyses. Monthly Weather Review, 115(9), 2033-2059. Abstract PDF
Two sets of observed atmospheric circulation statistics for the Southern Hemisphere (SH) are compared. The first set was compiled at the Geophysical Fluid Dynamics Laboratory (GFDL) and consists of global objective analyses of circulation statistics accumulated at individual rawinsonde stations for the period May 1963-April 1973. The second set was obtained from daily hemispheric numerical analyses prepared operationally at the World Meteorological Centre, Melbourne, Australia for the period September 1972-August 1982. This study extends the earlier comparison of circulation statistics from station-based and from numerical analysis-based methods by Lau and Oort for the Northern Hemisphere to the Southern Hemisphere.
The domain used for the comparison is a 5 degree x 5 degree latitude-longitude grid from 10 degrees to 90 degrees S and seven pressure levels from 1000 to 100 mb. The circulation statistics examined include (i) ten-year averages of the monthly mean fields (measures of the mean circulation), (ii) ten-year averages of the standard deviations and covariances of daily values (measures of the daily transient eddy variability) and (iii) year-to-year standard deviations of the monthly mean fields (measures of the interannual variability). The statistics are presented using horizontal maps on pressure surfaces and latitude-pressure sections of zonal averages.
The two sets of circulation statistics were derived using very different analysis methods and they apply for different time periods. The similarities and differences between the statistics from the two datasets indicate the reliability of the statistics and can be used to define a better composite set of circulation statistics for the SH.
The relatively large differences in the statistics can generally be attributed to the sparse conventional observation network in the SH, particularly over the large ocean regions, and deficiencies in the analysis methods. The two sets agree reasonably well from 850 to 500 mb over the land masses, where the observation network is less sparse. In the upper troposphere, the magnitudes of the daily transient eddy statistics from the Australian dataset are smaller due to the analysis method and the inclusion of satellite data. Over the data-sparse regions, the use of the zonal average as the first guess for the GFDL dataset has led to reduced spatial variability, smoother fields and underestimation of extreme values.
Oort, Abraham H., 1987: The search for unity in the climatic system In L'institut d'astronomie et de géophysique, Georges Lemaître, Belgium, Université Catholique de Louvain, 1-28. Abstract
Studies of the earth's climate system clearly show that the atmosphere, oceans, land surfaces, cryosphere and biosphere are intimately connected with each other. Some of the complexities of the climate system are due to the very different time scales found in the subsystems and due to the operation of a variety of interactions and feedback processes between them. The role of the oceans and atmosphere in storing heat seasonally and in transporting energy poleward to maintain the climatic balance is discussed in detail.
Oort, Abraham H., Y-H Pan, R W Reynolds, and C F Ropelewski, 1987: Historical trends in the surface temperature over the oceans based on the COADS. Climate Dynamics, 2, 29-38. Abstract
Based on a new and unique compilation of surface ship data over the world ocean, the Comprehensive Ocean-Atmosphere Data Set (COADS), the temperature records since 1870 were thoroughly analyzed. Results for the air and sea surface temperature are presented for the two hemispheres and for individual 10°-latitude wide belts. The results support most earlier results of a relatively warm period over the oceans in the late 19th century, a cool period between about 1905 and 1930, and a warm period from the 1940's onward. Of considerable interest and perhaps unique in the historical record are the diverging trends in the midlatitude sea and air temperatures of the two hemispheres during the 1960's and early 1970's with relative cooling in the Northern Hemisphere and relative heating in the Southern Hemisphere.
Oort, Abraham H., and Y-H Pan, 1986: Diagnosis of historical ENSO events In World Meteorological Organization Programme on Long-Range Forecasting, Vol. 1. Geneva, World Meteorological Organization, 249-258.
Oort, Abraham H., and Y-H Pan, 1986: Interannual Variability of Surface Marine Fields In NOAA Technical Memorandum (NOAA TM ERL ESG-23), 149-155. Abstract
Possible climatic trends in the zonal mean SST anomalies have been studied in five 10 degrees -lat.-wide belts at 50-60 degrees N, 20-30 degrees N, 10 degrees S-0 degrees. 30-20 degrees S, and 60-50 degrees S, where 41, 62, 76, 77, and 99% of the total area is covered by ocean. The COADS (Comprehensive Ocean Atmosphere Data Set) was used. The investigation procedure is described. The results, represented on graphs, show the following: an early warm period in the SST from 1870 to <> 1940 at all latitudes: a strong cooling between 1900 and 1910; a cold period between 1910 and 1930; and a warming period between 1930 and 1941. There is a trend toward cooling between the 1960s and the 1970s in the Northern Hemisphere middle and high latitudes in the Southern Hemisphere.
Carissimo, B C., Abraham H Oort, and T H Vonder Haar, 1985: Estimating the meridional energy transports in the atmosphere and ocean. Journal of Physical Oceanography, 15(1), 82-91. Abstract PDF
The poleward energy transports in the atmosphere-ocean system are estimated for the annual mean and the four seasons based on satellite measurements of the net radiation balance at the top of the atmosphere, atmospheric transport data, and atmospheric and oceanic storage data. To eliminate the occurrence of spurious nonzero transports of energy at the north or south poles, various types of corrections had to be made so that the global balances are maintained. This also enabled us to estimate the uncertainties in the procedures used. The uncertainties found are similar to those reported by Hastenrath based on different satellite data sets but using the same correction method. Finally, oceanic heat transports are computed for the annual mean and seasons. One of the crucial terms in the heat budget, the interseasonal storage of energy in the oceans, is estimated for three difference layers, 0-112, 0-275 and 0-550 m, enabling a further error estimate in the inferred oceanic heat transports. The present results confirm the presence of a strong annual cycle in the transport of energy by the oceans.
Lau, Ngar-Cheung, and Abraham H Oort, 1985: Response of a GFDL general circulation model to SST fluctuations observed in the tropical Pacific Ocean during the period 1962-1976 In Coupled Ocean-Atmosphere Models, Amsterdam; The Netherlands, Elsevier Science Publishers, 289-302. Abstract
The results of a special 15-year integration of an atmospheric general circulation model are compared with observations. In the 30 degrees S - 30 degrees N strip over the Pacific, the lower boundary of the model is forced by sea-surface temperatures which vary continuously according to actual observations during the period January 1962 - December, 1976. Everywhere else, the sea-surface temperatures follow a normal annual cycle without year-to-year variations. Using global teleconnection maps and time series of certain tropical circulation indices, such as the low-level and upper-level zonal winds, the surface pressure and the 200-mb height, it is shown that the dominant spatial modes of atmospheric variability in the tropics are very well simulated. Some of these modes are conspicuously absent from an earlier integration in which the surface temperatures everywhere were prescribed to follow the normal seasonal cycle.
Oort, Abraham H., 1985: Balance conditions in the earth's climate system. Advances in Geophysics, 28A, 75-98.
Oort, Abraham H., and M C Maher, 1985: Observed long-term variability in the global surface temperatures of the atmosphere and oceans In Coupled Ocean-Atmosphere Models, Amsterdam; The Netherlands, Elsevier Science Publishers, 183-198. Abstract
Based on historical surface observations available at the U.S. National Climatic Center at Asheville, N.C., the monthly global fields of air-surface temperature, sea-surface temperature and sea-air temperature difference were analyzed for a 45-year period since May 1920. All available data were used. A clear long-term trend was found in the monthly time series of sea-air temperature difference for the tropics, being about 0.5 degrees C less during the 1920's and 1930's than during the 1960's and 1970's and with a gradual change in the years between. Probably this trend is not real, but due to the gradual change in observing the sea-surface temperature from using buckets to engine-intake data. Our analyses suggest a reasonable way to correct the historical sea-surface temperatures for changes in observing techniques, making them more useful for climate research. Clear evidence of El Niño/Southern Oscillation episodes are found throughout the 45-year period, although they may have been less frequent during the 1920's and 1930's than during the later years.
Rosen, R D., D A Salstein, J P Peixoto, Abraham H Oort, and Ngar-Cheung Lau, 1985: Circulation statistics derived from level III-b and station-based analyses during FGGE. Monthly Weather Review, 113(1), 65-88. Abstract PDF
A number of Northern Hemisphere circulation fields and statistics are derived for the months of January and June 1979 from level III-b analyses produced by GFDL using a 4-dimensional data assimilation scheme which incorporates measurements from a wide variety of sources. In particular, hemispheric maps and zonal cross sections of the wind, specific humidity, and the eddy fluxes of momentum, heat and moisture are examined. Certain quantities related to the atmosphere's energy cycle are also considered. These fields and statistics are compared with those derived from analyses that rely solely on the conventional rawinsonde station data taken during the same months. In the case of the monthly mean zonal and meridional winds, we also present results based on the level III-b analyses of the ECMWF. The station-based analyses yield zonal mean statistics and hemispheric integrals that are generally comparable to those from the level III-b analyses. For example, the intensity of the Northern Hemisphere Hadley cell in January produced by the station analyses lies between those of the III-b analyses, which differ by as much as 35%. On regional scales, however, there are some large differences in the circulation fields between the station-based and level III-b analyses over areas of sparse station coverage. For example, the station-based analysis of the 200 mb field of transient eddy momentum flux in January does not include a significant region of northward flux over the northeast Pacific that is contained in the GFDL analysis. It is not yet clear, though, to what extent model biases may be affecting the GFDL analysis in this or in other station-sparse areas. In the case of the subtropical Pacific jet in January, the station-based analysis appears to underestimate its extent, but there are also considerable differences between the two level III-b analyses in this region. In addition, the GFDL analyses often appear to be noisy. Improvements in the level III-b analyses need to be made before full confidence can be placed in results based on modern data assimilation techniques.
Bryan, F O., and Abraham H Oort, 1984: Seasonal variation of the global water balance based on aerological data. Journal of Geophysical Research, 89(D7), 11,717-11,730. Abstract PDF
The distribution of evaporation minus precipitation over the globe and its seasonal variations are estimated from global atmospheric circulation statistics for the period May 1963 to April 1973. Meridional profiles of evaporation minus precipitation over the Atlantic, Pacific, and Indian oceans, over all oceans combined, and over all continents combined, as well as the total evaporation minus precipitation over each oceanic and continental region, are shown. The Pacific Ocean is found to have an excess of precipitation and the Atlantic an excess of evaporation throughout the year. Over the Indian Ocean, precipitation exceeds evaporation during December-February, while evaporation exceeds precipitation during the rest of the year and in the annual mean. The results are generally in qualitative agreement with previous estimates of the annual mean world water balance based on surface observations. There are large quantitative discrepancies, however, particularly in the subtropics. A comparison with the analysis of 2 months of data from the FGGE period suggests that the primary source of error in our results is associated with spatial sampling deficiencies in the general circulation statistics. It appears that in many regions the current operational rawinsonde network is inadequate to give reliable quantitative estimates of differentiated quantities such as are required in computing the atmospheric water balance.
Oort, Abraham H., and Y-H Pan, 1984: The observed effects of tropical heating over the east pacific on global climate variations. Acta Oceanologica Sinica, 3(4), 488-498. Abstract
Based on a statistical analysis of monthly data for 1958-73, the East Equatorial Pacific centered at 130 degrees W is considered as a key region for global climate variation. When the region is warm, the latent heat from the sea surface increases and the semipermanent systems in the global atmosphere become more active.
Wahr, J, and Abraham H Oort, 1984: Friction- and mountain-torque estimates from global atmospheric data. Journal of the Atmospheric Sciences, 41(2), 190-204. Abstract PDF
Seasonal, zonal surface torques between the atmosphere and the Earth are estimated and compared by using data from a number of independent sources. The mountain torque is computed both from surface pressure data and from isobaric height data. The friction torque is estimated from the oceanic stress data of Hellerman and Rosenstein. Results for the total torque are inferred from atmospheric angular momentum data. The globally integrated total torque is compared with astronomical observations of the Earth's rotation rate. These comparisons help to assess the quality of the different results.
Zonal torques are also computed by using results from a GFDL general circulation model of the atmosphere. A comparison with the corresponding results inferred from real data is presented and interpreted in terms of model accuracy.
Oort, Abraham H., 1983: Global Atmospheric Circulation Statistics, 1958-1973, NOAA Professional Paper 14, Rockville, MD: NOAA, 180 pp. Abstract
An extensive description of the three-dimensional, global structure of the general circulation of the atmosphere is presented in the form of a large number of global maps, cross sections, meridional profiles and hemispheric integrals. The parameters analyzed include the horizontal wind components, temperature, geopotential height and specific humidity, as well as their variances and covariances at 11 levels between the Earth's surface and the 50- mb level. The basic analyses were performed for each month of the 180-month period, May 1958 through April 1973, covering only the Northern Hemisphere during the first five years, but the entire globe during the last ten years.
A full description is given of the 1963-1973 mean climate conditions, while, for selected parameters, year-to-year variations are shown during the entire 15-year period. The basic results are presented on 47 microfiches (each containing 170 frames) inserted in envelopes at the end of the publication. This unique presentation makes it possible to provide a wealth of information in an easily accessible form. A selection of the most important horizontal maps and zonal-mean cross sections for the mean December-February and June- August seasons is given in Appendix A.
Following a description of the data sources, data distributions, data reduction and analyses procedures used, important questions of reliability and representativeness of the final statistics are addressed through extensive comparisons with earlier results. The final chapter highlights some of the possible applications of the statistics, such as interhemispheric comparisons, global energy, water and angular momentum budget calculations, and the determination of global climate trends.
Oort, Abraham H., and J P Peixoto, 1983: Global angular momentum and energy balance requirements from observations. Advances in Geophysics, 25, 355-490.
Oort, Abraham H., and J P Peixoto, 1983: Interhemispheric comparisons based on a 15-year atmospheric data set In First International Conference on Southern Hemisphere Meteorology, Boston. MA, American Meteorological Society, 12-16.
Pan, Y-H, and Abraham H Oort, 1983: Global climate variations connected with sea surface temperature anomalies in the eastern equatorial Pacific Ocean for the 1958-73 period. Monthly Weather Review, 111(6), 1244-1258. Abstract PDF
The sea surface temperature anomalies in the eastern equatorial Pacific Ocean are shown to demarcate a "key region" near 130 degrees W for observed variations in the global general circulation. Various techniques are used to describe global conditions during warm and cold sea surface conditions in the key region based on a global 15-year set of surface and upper air analyses.
Earlier work is confirmed that during warm episodes 1) westerly jets in both hemispheres are strengthened, and 2) some of the semi-permanent circulation features, such as the intertropical convergence zone, the subtropical high over the western and central Pacific, and the Aleutian low, increase in strength, and vice versa for cold episodes.
The time series of monthly-mean atmospheric temperature averaged over the entire mass of the Northern Hemisphere is found to be highly correlated with the sea surface temperature anomalies in the key region.The highest correlation of r = 0.65 is found when the atmosphere lags the ocean by 6 months. The maximum temperature response seems to occur in the upper troposphere in the tropics probably associated with variations in convective activity.
Peixoto, J P., and Abraham H Oort, 1983: The atmospheric branch of the hydrological cycle and climate In Variations in the Global Water Budget, Reidel Publishing Co, 5-65. Abstract
Based on daily observations from about 1000 rawinsonde stations, tables and global distributions of the various water vapour fields are presented for mean annual, winter and summer conditions covering the 10-year period, May 1963 through April 1973. The fields include horizontal maps of precipitable water, of total zonal, meridional and vertical transports by eddy and mean circulations, as well as meridional profiles and zonal mean cross-sections of these quantities. The connections between the atmospheric branch and the hydrology of the Earth's surface were studied with the aid of the divergence fields of mean total water vapour flux and through horizontal and vertical streamfunction analyses. The divergence maps agree quite well with maps of evaporation minus precipitation (E-P) obtained from classic, climatological surface data. Over the oceans, the divergence fields show a good correlation with the evaporation and surface salinity maps.
From the zonal mean streamfunction analyses, the total vertical transports of water substance were inferred, and compared with the contributions by standing eddies and mean meridional circulations. The resulting vertical fluxes by transient eddies show the great importance of cumulus convection in the tropics for the atmospheric circulation. The main sources and sinks of atmospheric water vapour, as well as the dominant mean trajectories of water, are identified. In the light of various climatological and oceanographic considerations, the results show that the study of the atmospheric branch is essential to improve understanding of the Earth's water balance on both regional and global scales.
Lau, Ngar-Cheung, and Abraham H Oort, 1982: A comparative study of observed northern hemisphere circulation statistics based on GFDL and NMC analyses. Part II: Eddy statistics and the energy cycle. Monthly Weather Review, 110(8), 889-906. Abstract PDF
The comparison between two sets of observed circulation statistics undertaken by Lau and Oort (1981) is continued in this study by examining the temporal variance and covariance statistics in these sets. The first (GFDL) set is compiled by interpolating monthly averaged station statistics. The second set is based on twice-daily operational NMC analyses. The statistics for six winter and six summer seasons within the 1966-73 period are compared. The hemispheric fields examined include transient eddy kinetic energy at 300 mb, root-mean-squares of geopotential height and temperature at 300 and 850 mb, respectively, the horizontal transport by transient eddies of westerly momentum and geopotential height at 300 mb, and of heat at 850 mb. The patterns of horizontal eddy transports are presented in a vectorial format to delineate local relationships with the time-mean flow and the centers of eddy activity. Latitude-height distributions for zonally-averaged patterns of the above statistics are also presented.
The transient eddy statistics in the two sets are in good agreement over regions with adequate data coverage. The NMC set generally gives relatively higher eddy amplitudes and stronger eddy transports over the data-sparse oceans. The maximum deviations between the two sets in these regions are about 20-30%.
The two sets of analyses are further used to calculate the spatial integrals for the energy reservoirs and various energy conversion rates in the atmosphere. The transient and stationary eddies are treated separately in the formulation of the energy cycle. The largest differences are found in the transfer rate of kinetic energy from the stationary waves to the transient disturbances, and for the terms associated with the conversion of available potential energy into kinetic energy. The GFDL and NMC estimates of the other components of the energy cycle do not differ from each other by > 20%. The results from both sets of analyses imply that the transient eddies are very efficient in depleting the available potential energy of the stationary waves through their ability to transport heat down the local temperature gradient. The dissipative time scale associated with this mechanism is several days.
Holopainen, E O., and Abraham H Oort, 1981: Mean surface stress curl over the oceans as determined from the vorticity budget of the atmosphere. Journal of the Atmospheric Sciences, 38(2), 262-269. Abstract PDF
The vertically integrated atmospheric vorticity budget over the oceans offers, in principle, a possibility of determining the surface stress curl from upper wind data without the need to specify a relationship between the surface stress and surface wind. Results for the wind stress curl obtained by this vorticity method, using upper wind data for the period 1968-73, are compared with the recent stress-curl calculations by Hellerman from surface data.
The two completely independent methods give basically similar mean latitudinal distribution of the stress curl. In the midlatitudes of the Southern Hemisphere, where the transient eddies are the main mechanism of vorticity transfer, the two estimates of the basin-wide longitudinal averages of the stress curl do not deviate from each other by more than approximately 20%. However, in the Northern Hemisphere the agreement is less. This seemingly strange result appears to be due to the sensitivity of the vorticity method to errors in the estimates of vorticity advection by the standing waves.
It is concluded that for the time being the geographical pattern of the mean surface stress curl can, at least in the Northern Hemisphere, be estimated from surface data (using a drag formulation) more accurately than from upper wind data (using the vorticity method). Together the two methods offer a useful quality check for the upper air data.
Holopainen, E O., and Abraham H Oort, 1981: On the role of large-scale transient eddies in the maintenance of the vorticity and enstrophy of the time-mean atmospheric flow. Journal of the Atmospheric Sciences, 38(2), 270-280. Abstract PDF
The global distribution of the forcing of time-mean flow due to large-scale, horizontal Reynolds stresses is determined from upper wind statistics for the period 1968-73. The role of this forcing in the maintenance of the vorticity and enstrophy of the time-mean flow is discussed.
The most striking effect of transient eddy stresses is the tendency to shift the subtropical maxima in the time-mean flow and the associated vorticity patterns poleward. However, significant longitudinal variations in forcing occur, also. Calculations of the dominant terms in vorticity budgets of the North Pacific Low, the North Atlantic Low, and the Siberian High, which may be called the centers of action of wintertime circulation at sea level in the Northern Hemisphere, are presented. In all three cases, transient eddies are found to be important in maintaining the centers against the dissipative action of surface friction.
In terms of the enstrophy budget, the hemispheric and global-mean effects of transient eddies on the mean flow are small, both in December-February and June-August. In the Northern Hemisphere, where the results are most reliable, the eddies are weakly dissipative with a time scale on the order of several months.
When separating the time-mean flow into the contributions from the axisymmetric component and from the stationary disturbances, it is found that the transient eddy stresses tend to maintain the axisymmetric mean flow, but to weaken the stationary disturbances. There are significant latitudinal variations in the enstrophy forcing of the stationary disturbances. Thus eddy forcing is an important factor in maintaining the enstrophy of stationary disturbances in the extratropics, while it tends to destroy their enstrophy in the tropics.
Lau, Ngar-Cheung, and Abraham H Oort, 1981: A comparative study of observed northern hemisphere circulation statistics based on GFDL and NMC analyses. Part I: The time-mean fields. Monthly Weather Review, 109(7), 1380-1403. Abstract PDF
Two sets of monthly mean analyses based essentially on the same observational data, but employing different analysis procedures, are compared. The first set was compiled at the Geophysical Fluid Dynamics Laboratory and consists of horizontal interpolations of monthly averaged circulation statistics accumulated at individual rawinsonde stations. The second set was derived from twice-daily gridded analyses produced by the National Meteorological Center on an operational basis. The data used cover nine winters and nine summers within the 1963-73 period. The spatial domain extends in latitude from 20 degrees N to 90 degrees N, and in the vertical from 850 to 100 mb. The circulation statistics examined include 1) hemispheric distributions of 9-year averages as well as month-to-month standard deviations for the horizonal wind components and geopotential heights at 850, 500 and 200 mb, and the temperature at 850 mb; and 2) latitude-height sections for the zonally averaged wind and temperature, the standing eddy variances of zonal and meridional wind components, geopotential height and temperature, and the meridional transports of westerly momentum, geopotential energy and heat by standing waves.
Over certain data-sparse regions, the two analyses are further compared with actual values reported in Monthly Climatic Data for the World by the few rawinsonde stations located in those regions.
The time-mean fields in the two data sets are found to be generally in excellent agreement over the North American and Eurasian continents, where a dense observing network exists. The deviations between the data sets are large over the oceans and northern Africa, where the GFDL analyses give relatively weaker zonal wind speeds in the jet exit regions, stronger ageostrophic motions in the meridional direction, lower temperatures in the subtropical lower troposphere, and higher temperatures above the subtropical tropopause. The maximum local deviations are on the order of 10-15 m s-1 for zonal wind, 6-8 m s-1 for meridional wind, 50-70 m for geopotential height, and 2-4 degrees C for temperature. These discrepancies are associated with much weaker standing eddy kinetic energy, and much stronger equatorward transports of geopotential energy by the stationary waves in the GFDL analyses. The inter-monthly variability of the monthly mean fields in the GFDL set is generally weaker over the oceans.
The spatial correlation coefficients for the monthly mean fields in the two data sets do not exhibit any discernible trends during the 9-year period. This suggest that the procedural changes in the NMC analysis system during this period did not result in serious inhomogeneities in the time series of the NMC fields.
Holopainen, E O., Ngar-Cheung Lau, and Abraham H Oort, 1980: A diagnostic study of the time-averaged budget of atmospheric zonal momentum over North America. Journal of the Atmospheric Sciences, 37(10), 2234-2242. Abstract PDF
The terms in the time-mean zone momentum equation which depend only on the large-scale motions are evaluated for the North American continent during the winter season. The computations are based on two different sets of upper air atmospheric circulation statistics between 850 and 200 mb. The first (GFDL) data set consists of global objective horizontal analyses of monthly circulation statistics evaluated at individual stations; the second (NMC) data set was compiled by processing twice-daily synoptic analyses on a hemispheric grid. The mean residual forces needed for balance are discussed assuming that they represent the effects of horizontal and vertical subgrid-scale processes.
The results derived from the two largely independent data sets are similar. Zonal momentum is produced in the free atmosphere over North America by a local, thermally direct meridional circulation, with mean poleward ageostrophic flow above 700 mb. This production is partially counterbalanced by a net export of zonal momentum from the region. The flux divergence associated with large-scale vertical transports appears to be insignificant.
A residual force of considerable magnitude is needed for balance. In the lower troposphere, this force has an accelerating effect on the zonal flow almost everywhere over the eastern portion of the North American continent, and a decelerating effect over the western mountainous region where, however, the results are less certain. In the upper troposphere, the residual force over the entire continent has a net decelerating effect, but significant geographical differences appear to occur.
A tentative interpretation of the results for the residual force is offered.
Oort, Abraham H., 1979: Some personal views on the use of GCM's In Report of the JOC Study Conference on Climate Models: Performance, Intercomparison and Sensitivity Studies, Vol. 2, Global Atmospheric Research Programme, Joint Organizing Committee, GARP Publications No. 22, World Meteorological Organization, 1030-1036.
Stefanick, M, and Abraham H Oort, 1979: Geographical distribution of interannual variability of mean monthly temperatures, 1963-1973. EOS, 60(18), 264.
Ellis, J, T H Vonder Haar, S Levitus, and Abraham H Oort, 1978: The annual variation in the global heat balance of the earth. Journal of Geophysical Research, 83(C4), 1958-1962. Abstract
An annual variation with a range of 31 W m-2 is found in the global net radiation balance of the earth. The net radiation flux values measured from satellites and the changes in total heat content computed from independent sets of atmospheric and oceanic data show annual variations which are consistent with each other in both phase and magnitude. The net energy gain and loss by the planet within a year is stored and released within the system primarily by the oceans.
Oort, Abraham H., 1978: Adequacy of the rawinsonde network for global circulation studies tested through numerical model output. Monthly Weather Review, 106(2), 174-195. Abstract PDF
Numerical output from a GFDL global climate model was used to determine whether the present distribution of rawinsonde stations is adequate to deduce the atmospheric structure and its variability in space and time over the globe. Spatial data gaps were found to cause typical rms wind errors averaged over a hemisphere of 2 to 3 m s-1, increasing for the zonal wind component to 5 or 6 m s-1 at jet stream levels. In temperature the spatial data gaps led to rms errors on the order of 0.5 to 1 degree C in the free atmosphere, in geopotential height between 20 and 30 gpm in the upper troposphere, and in specidic humidity between 1 and 2 g kg-1 near the surface and about 0.3 g kg-1 at 500 mb.
Errors due to instrumental deficiencies, unrepresentativeness of the local soundings, deficiencies in the analysis technique and gaps in the time series were found to be less important than those due to the spatial gaps, even in the Northern Hemisphere.
In the Northern Hemisphere, the rawinsonde network was found to be generally adequate to measure large-scale circulation statistics. However, in the Southern Hemisphere the incorporation of additional data sources (rawinsonde, satellite or otherwise) is necessary, especially for defining the fluxes by the mean meridional and stationary eddy circulations.
Levitus, S, and Abraham H Oort, 1977: Global analysis of oceanographic data. Bulletin of the American Meteorological Society, 58(12), 1270-1284. Abstract PDF
A project to objectively analyze a large quantity of oceanographic data for the world ocean is described. Preliminary results are encouraging within the limits of data available. Results are being used in a variety of ways but at present primarily for studies of the ocean's role in the global heat balance. A brief discussion of the data used, the method of analysis, and some preliminary results is presented.
Oort, Abraham H., 1977: The Interannual Variability of Atmospheric Circulation Statistics, NOAA Professional Paper 8, Rockville, MD: NOAA, 76 pp. Abstract
The year-to-year variability of various zonal mean general circulation parameters is described, based on daily data from about 600 Northern Hemisphere rawinsonde stations for the two 5-yr periods May 1958 through April 1963, and May 1968 through April 1973. Most of the statistics presented are based on the 1958-1963 sample. The parameters studied include the wind components, temperature, geopotential height, and specific humidity, as well as their variances in time and space and their meridional and vertical transports. The main body of the report consists of tables and graphs of interannual standard deviations computed for the different calendar months as a function of latitude and pressure. A comparison between the results for the first and second 5-yr periods shows that the statistics do not depend generally on the specific 5-yr period chosen. Together with the results for the 1958-1963 mean conditions given in an earlier paper by Oort and Rasmusson, the present compilation should give a reasonably complete and representative picture of recent climate conditions in the atmosphere of the Northern Hemisphere.
Oort, Abraham H., and P H Chan, 1977: On the role of the Asian monsoon in the angular momentum and kinetic energy balances of the tropics. Pure and Applied Geophysics, 115(5/6), 1167-1186. Abstract
The balance conditions of relative angular momentum and time-mean kinetic energy and their annual variations are studied for the Northern Hemisphere tropical belt. The belt is divided into two roughly equal size parts, the monsoon and the extramonsoon regions. The data used consist of all available daily rawinsonde reports from the world aerological network for the two 5-year periods 1958-63 and 1968-73. In winter, the trade winds in the monsoon and extramonsoon regions are both sources of westerly relative angular momentum for the middle latitude circulation. However, it is found that the angular momentum gained in the extramonsoon region of the Tropics is mostly destroyed by a net southward flow of mass in that region, and becomes regenerated in the monsoon region by a net northward flow of mass there. This excess of angular momentum together with the angular momentum picked up locally in the monsoon region is almost all exported across its northern boundary. It is further found that in winter the Tropics are also an important source of mean kinetic energy for middle latitudes. Again almost all export of kinetic energy was found to take place across the northern boundary of the monsoon sector. Most of this energy must be generated through the pressure gradient term inside the monsoon region itself, the transformation from transient eddy kinetic energy being very small. The proper evaluation of the pressure gradient appears to be the main stumbling block in the present study, preventing us from estimating the generation and thereby, as a residual, the frictional dissipation in the two regions.
In summer, the extramonsoon region remains a source of angular momentum, but the monsoon region with its surface westerlies acts as a sink, leading to a sharp reduction (and even a midsummer reversal) of the export into middle latitudes. Also the export of mean kinetic energy almost vanishes in summer, except for a small southward transfer across the equator. The calculations for two 5-year periods give very similar estimates and thereby show the reliability of the results.
Oort, Abraham H., 1975: On the variability of the general circulation of the atmosphere as deduced from aerological data In The Physical Basis of Climate and Climate Modelling, Report of the International Study Conference, GARP Publications Series No. 16, World Meteorological Organization, 95-105.
Gray, Jr, T I., and Abraham H Oort, 1974: Interannual variations in convective activity over the GATE area. Bulletin of the American Meteorological Society, 55(3), 220-226. Abstract PDF
sed on five years of satellite brightness data the year-to-year variations in convective activity are investigated over the GATE area.
The monthly-mean brightness data suggest that in the B-scale region, centered at 8.5N, 23.5W, the probability of intense convective activity during the summer of 1974 (the GATE period) is quite large. During the majority of the summer months investigated the intertropical convergence zone was found close to the latitude of the B-scale network.
The daily brightness records show that one may expect a high day-to-day variability with a predominant period of about 4 to 5 days. This may limit the range of extrapolative predictibility of cloud clusters in the B-scale area to only a few days.
Oort, Abraham H., 1974: Year-to-year variations in the energy balance of the Arctic atmosphere. Journal of Geophysical Research, 79(9), 1253-1260. Abstract
In order to understand better the processes responsible for maintaining the arctic climate, a study has been made of the energy budget at high latitudes. For simplicity this study is restricted to budget calculations for a polar cap that contains almost the entire atmospheric mass north of latitude 60 degrees N. The computations are based on 5 years of daily upper air observations of wind, temperature, geopotential height, and specific humidity for the period May 1958-April 1963, contained in the so-called MIT general circulation library. From the hemispheric network of radiosonde stations available in the library, more than 200 stations are located north of 50 degrees N. All these stations were used in the analyses to define the meteorological parameters in the Arctic. Averaged over the year, about 75% of the energy needed to balance radiative cooling in the polar cap is found to be transported by atmospheric processes across the southern boundary. The remaining 25% is probably supplied through a direct exchange of sensible and latent heat at the earth's surface in the polar cap. Throughout the year the influx of sensible heat plus potential energy dominates over that of latent heat. A further breakdown according to the mechanism of transfer shows that at 60 degrees N transient and stationary disturbances are far more effective than the mean meridional circulation in transporting energy poleward. By using Budyko's values for the surface energy flux, indirect estimates could be made of the radiative heat loss in the polar cap that are in good agreement with earlier direct estimates. The computed cooling curve has an interesting asymmetry; it shows a rapid decrease of cooling from about -1.4 degrees C/day in winter to about -0.6 degrees C/day in May, followed by a slow increase throughout the summer and fall. The year-to-year variability in inflow of energy from middle latitudes seems to be large, especially during the winter half year. Since observed temperature deviations in the polar cap appear to be, on the average, a factor of 5 smaller than the variations in inflow, these deviations must be largely compensated by variations in radiative cooling or in heat supply from the earth's surface. The present evidence tends to underline the complexity of the mechanism responsible for climatic anomalies in the Arctic.
Oort, Abraham H., and H D Bowman II, 1974: A study of the mountain torque and its interannual variations in the northern hemisphere. Journal of the Atmospheric Sciences, 31, 1974-1982. Abstract PDF
Values of the large-scale mountain torque between 10S and 80N are computed for each month of the five-year period, May 1958 through April 1963. A careful comparison with earlier midseason values presented by Newton using different data sources and different analysis techniques is made. The calculations appear to be equally sensitive to differences in smoothing of the mountain profiles as to differences in basic geopotential height data used. Interannual variations in the monthly-mean mountain torques are found to be large, especially during the winter half-year.
In general, the large-scale mountain torque gives only a minor contribution to the total surface torque as estimated by other investigators. However, when integrated over the Northern Hemisphere as a whole, the mountain and frictional torques appear to be equally important. Both torques seem to act as a sink of angular momentum in summer and a source in winter.
Oort, Abraham H., and J P Peixoto, 1974: The annual cycle of the energetics of the atmosphere on a planetary scale. Journal of Geophysical Research, 79(18), 2705-2719. Abstract
A fairly complete description of the annual cycle of the energetics of the atmosphere is given for the entire northern hemisphere. The basis for the present study is the collection of 5 years of daily upper air data from more than 600 radiosonde stations contained in the so-called MIT general circulation library. The mathematical expressions are formulated in the mixed space-time domain using Lorenz's approximate expression for available potential energy. However, for comparison, some terms have been computed in the time domain. Aside from the usual generation, conversion, and dissipation integrals the contribution by boundary terms at the equator must be included in the monthly budgets. Some of the main results are as follows. (1) Tentative box diagrams are given for the complete energy cycle for the 60-month, January, and July samples. (2) The exchange of mean available potential energy at the equatorial boundary plays an important role in the hemispheric energetics, especially in winter. (3) A maximum in generation of mean available potential energy (nearly 3 W m-3) takes place in the fall. Earlier investigators have found a maximum in midwinter; the discrepancy is explained. (4) It appears that the generation of eddy available potential energy constitutes the main driving force of the summer circulation. This generation is probably largely due to the release of latent heat in the intertropical regions. (5) Rather reliable estimates of the hemispheric dissipation rate of kinetic energy are made of about 2 W m-2 in the long-term mean, 3.5 W m-2 in winter, and about 1 W m-2 in summer. (6) The energy amounts and conversion terms appear to have about the same numerical value in the time domain, in the mixed space-time domain, and most likely also in the space domain.
Peixoto, J P., and Abraham H Oort, 1974: The annual distribution of atmospheric energy on a planetary scale. Journal of Geophysical Research, 79(15), 2149-2159. Abstract
A formulation of the general problem of the energetics of the atmosphere including boundary terms is given. The distribution and the annual variation of the various components of available potential and kinetic energy based on 5 years of daily observations are evaluated. The results are shown in tabular form and in many meridional cross sections for both the 60-month mean conditions and the 12 composite calendar months. Latitudinal and vertical profiles of the different quantities are also included. The results obtained agree on the whole with previous findings, although the present ones are more consistent owing to the homogeneity of the data set and the objective analysis scheme used. A more than twofold increase is found in the ratio of eddy versus mean energy from winter to summer. The increase suggests that besides the baroclinic conversion process there exists in summer an important additional source of eddy potential energy.
Vonder Haar, T H., and Abraham H Oort, 1973: New estimate of annual poleward energy transport by Northern Hemisphere oceans. Journal of Physical Oceanography, 3(2), 169-172. Abstract PDF
Recent measurements of the earth's radiation budget from satellites, together with extensive atmospheric energy transport summaries based on rawinsonde data, allow a new estimate of the required poleward energy transport by Northern Hemisphere oceans for the mean annual case. In the region of maximum net northward energy transport (30-35 N), the oceans transport 47% of the required energy (1.7 x 1022 cal year-1). At 20 N, the peak ocean transport accounts for 74% at that latitude; for the region 0-70 N the ocean contribution averages 40%.
Oort, Abraham H., 1971: The observed annual cycle in the meridional transport of atmospheric energy. Journal of the Atmospheric Sciences, 28(3), 325-339. Abstract PDF
The annual cycles in the atmospheric storage and in the meridional transport of energy are discussed. The calculations are based on a five-year sample from more than 500 radiosonde stations mainly located in the Northern Hemisphere. All statistics represent values integrated vertically between the earth's surface and 75 mb and horizontally along a latitude circle.
Several new and interesting features of the eddy and mean transports of potential energy, sensible heat, latent heat and kinetic energy become apparent by the breakdown according to calendar month.
In December through February more than half of the sensible heat is tranported poleward by the standing eddies. The transient eddy heat flux does not peak in winter but in April and November.
The strong annual cycle in the tropical Hadley circulation does not contribute to the poleward transfer of energy for the year as a whole.
Oort, Abraham H., and E Rasmusson, 1970: The annual variation of the monthly mean meridional circulation. Monthly Weather Review, 98(6), 423-442. Abstract PDF
Data for a 5-yr. period from a dense network of upper air stations have been used to determine the annual cycle in the mean meridional circulation north of 15 degrees S. Only during the transition months April, May and October, November is there some degree of symmetry with respect to the Equator. During the other months of the year, the Hadley cell of the winter hemisphere with a maximum strength of about 23 x Data for a 5-yr. period from a dense network of upper air stations have been used to determine the annual cycle in the mean meridional circulation north of 15 degrees S. Only during the transition months April, May and October, November is there some degree of symmetry with respect to the Equator. During the other months of the year, the Hadley cell of the winter hemisphere with a maximum strength of about 23 x 10 13 gm sec-1 appears always to dominate the circulation. The Hadley cell of the summer hemisphere practically disappears, except possibly near the surface. Maximum meridional velocities connected with the winter Hadley cell are about 2.5 m sec-1 near 1000 mb and over 3 m sec-1 near 200 mb. Mean vertical velocities attain values of about 5 and 8 mm sec-1 in the downward and upward branches of the winter Hadley cells. A rather weak Ferrel circulation (about 4 x 10 13 gm sec-1) and a very weak polar circulation (about 1 x 10 13 gm sec-1) are computed in middle and high latitudes throughout the year.
With the aid of several diagrams giving the variability of the south-north wind components both in time and space, it is shown that the tropical circulation as presented almost certainly gives a representative picture of the true situation. Much more uncertainty is involved in the circulation at middle and high latitudes.
The transport of angular momentum, potential energy, and sensible and latent heat connected with the calculated mean meridional circulations are presented for January and July. The transports agree quite well with those computed in earlier investigations.
Oort, Abraham H., and A Taylor, 1969: On the kinetic energy spectrum near the ground. Monthly Weather Review, 97(9), 623-636. Abstract PDF
For six stations in the northeastern United States, the spectrum of horizontal wind speed was analyzed using 10 yr of 1-min averaged, hourly surface reports. The fast Fourier transform technique was employed to estimate the spectrum between 1 cycle/2 hr and 1 cycle/2 yr.
The kinetic energy spectra show two major spikes at periods of 24 hr and 1 yr. However, most of the energy is contained in the traveling cyclones and anticyclones with periods between 2 and 7 days. The apparent discrepancy between Van der Hoven's results and our results concerning the existence of an important diurnal cycle in the kinetic energy can be explained by Blackadar's theory of the diurnal wind variation with height. Van der Hoven's spectrum represents conditions near the top of the surface layer, while our data were taken well within the surface layer. A line-by-line investigation of the diurnal peak reveals a very sharp line at 2400 hr with two side lobes 3.9 min away from the main line. These side lobes are probably caused by an annual modulation of the diurnal cycle.
The spectra tentatively corrected for aliasing give some indication of the existence of a spectral gap between small-scale turbulence and mesoscale phenomena .
