Transient Eddy Kinetic Energy Research Articles

Decadal co‐variation of the aerosols over East Asia and the East Asian jet streams in the boreal winter

AbstractThe changes in the East Asian polar‐front jet (EAPJ) and the East Asian subtropical jet (EASJ) profoundly impact the weather and climate in East Asia. However, the link between aerosols and the jet streams is still unclear. Here, we investigated the decadal co‐variation of aerosols and the East Asian jet streams in the boreal winter during the period of 1980–2019. In synch with a positive change in aerosol optical depth over East Asia, 300‐hPa winds show an equatorward shift of the land branch of the EASJ, and weakened EAPJ and oceanic branch of the EASJ. This can be linked to the enhanced meridional temperature gradient along 30°–50° N but weakened in northern regions and the decreasing synoptic‐scale transient eddy kinetic energy over subtropical Pacific. Relative importance estimation of aerosols and ocean signals emphasized the contributions of aerosols in jet variations. In turn, meteorological conditions related to jet streams also contribute to variations in aerosols, the decadal co‐variations are a result of their interactions, particularly for the oceanic branch of EASJ. The findings would be helpful in providing potential indicators of climate change.

The Future Intensification of the North Atlantic Winter Storm Track: The Key Role of Dynamic Ocean Coupling

AbstractClimate models project an intensification of the wintertime North Atlantic Ocean storm track, over its downstream region, by the end of this century. Previous studies have suggested that ocean–atmosphere coupling plays a key role in this intensification, but the precise role of the different components of the coupling has not been explored and quantified. In this paper, using a hierarchy of ocean coupling experiments, we isolate and quantify the respective roles of thermodynamic (changes in surface heat fluxes) and dynamic (changes in ocean heat flux convergence) ocean coupling in the projected intensification of North Atlantic transient eddy kinetic energy (TEKE). We show that dynamic coupling accounts for nearly all of the future TEKE strengthening as it overcomes the much smaller effect of surface heat flux changes to weaken the TEKE. We further show that by reducing the Arctic amplification in the North Atlantic, ocean heat flux convergence increases the meridional temperature gradient aloft, causing a larger eddy growth rate and resulting in the strengthening of North Atlantic TEKE. Our results stress the importance of better monitoring and investigating the changes in ocean heat transport, for improving climate change adaptation strategies.Significance StatementBy the end of this century, the North Atlantic Ocean storm track is projected to intensify on its eastward flank. Such intensification will have large societal impacts, mostly over western Europe. Thus, it is critical to better understand the mechanism underlying the intensification of the storm track. Here we investigate the role of ocean coupling in the future intensification of the North Atlantic storm track and find that ocean heat transport processes are responsible for the strengthening of the storm track. Our results suggest that better monitoring the changes in ocean heat transport will hopefully improve climate change adaption strategies.

Limited-area atmospheric energetics: illustration on a simulation of the CRCM5 over eastern North America for December 2004

The seminal work of Lorenz on global atmospheric energetics has allowed understanding much on the physical processes responsible for the general circulation, its maintenance and the development of synoptic-scale weather systems. In mid-latitudes, potential energy generated by the differential heating of the planet by the Sun is converted into kinetic energy by the weather systems and eventually dissipated by friction. While a corresponding study of atmospheric energetics over a limited region would have the advantage of focusing on the details of individual storms and the processes taking place over a domain of interest, such study has encountered several pitfalls and has been challenging. Here we build upon our earlier work on the energy cycle of inter-member variability in ensembles of limited-area model simulations to develop a regional-scale atmospheric energy cycle formulated in terms of available enthalpy and kinetic energy. The approach is then applied to study the energetics of a short simulation made with the fifth-generation of the Canadian Regional Climate Model (CRCM5) over an eastern North American domain for December 2004. The results obtained for a specific storm and monthly mean climatology confirm the current understanding that available enthalpy of the atmospheric time-mean state is mainly generated by the covariance of diabatic heating and temperature, and that the most important conversions of energy are found to correspond to baroclinic processes that take place along the storm track, where perturbations of temperature and wind are important. Finally, transient-eddy kinetic energy is mainly dissipated by friction in the boundary layer.

The Instabilities and Multiscale Energetics Underlying the Mean–Interannual–Eddy Interactions in the Kuroshio Extension Region

AbstractUsing a recently developed energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), this study investigates the intricate nonlinear mutual interactions among the decadally modulating mean flow, the interannual fluctuations, and the transient eddies in the Kuroshio Extension region. It is found that the mean kinetic energy maximizes immediately east of the Izu–Ogasawara Ridge, while the transient eddy kinetic energy does not peak until 400 km away downstream. The interannual variabilities, which are dominated by a jet-trapped Rossby wave mode, provide an energy reservoir comparable to the other counterparts. In the upstream, strong localized barotropic and baroclinic transfers from the mean flow to the eddies are observed, whereas those from the interannual variabilities are not significant. Besides fueling the eddies, the unstable mean jet also releases energy to the interannual-scale processes. Between 144° and 154°E, both transfers from the mean flow and the interannual variabilities are important for the eddy development. Farther downstream, eddies are found to drive the mean flow on both the kinetic energy (KE) and available potential energy (APE) maps. They also provide KE to the interannual variabilities but obtain APE from the latter. The gained eddy APE is then converted to eddy KE through buoyancy conversion. Upscale energy transfers are observed in the northern and southern recirculation gyre (RG) regions. In these regions, the interannual–eddy interaction exhibits different scenarios: the eddies lose KE to the interannual processes in the northern RG region, while gaining KE in the southern RG region.

Effect of horizontal resolution on ECHAM6-AMIP performance

This study analyzes the effect of increasing horizontal resolution in the atmospheric model ECHAM6 on the simulated mean climate state and climate variability. For that purpose three AMIP-style simulations with the resolutions T63L95, T127L95, and T255L95 are compared to reanalysis data and observations. Biases in atmospheric fields as well as tropospheric and stratospheric biases individually are analyzed. Besides mean errors of the climate state and the variance, some atmospheric phenomena with different time scales are studied at the three horizontal resolutions: the transient eddy kinetic energy, storm tracks, atmospheric teleconnections, the Madden–Julian-Oscillation (MJO), and the Quasi-Biennial Oscillation (QBO). The main result is that, overall, the bias of the simulated climate is reduced with increasing resolution when considering the mean state and the variance. A greater improvement takes place in the extra-tropical than in the tropical troposphere. The errors in the stratosphere are generally larger but the relative benefit of increasing resolution is greater than in the troposphere and we find that stratospheric phenomena, like the QBO, are sensitive to horizontal resolution. Globally, the bias of the mean state improves by 19 %, while the bias of the variability improves by 15 % (from T63 to T255). Major challenges remain the simulation of the precipitation and climate features like the MJO, which might require a coupled atmosphere–ocean model.

Kinetic and available potential energy transport during the stratospheric sudden warming in January 2009

The local features of transient kinetic energy and available potential energy were investigated using ECMWF (European Centre for Medium-Range Weather Forecasts) Interim Reanalysis data for the stratospheric sudden warming (SSW) event of January 2009. The Western Europe high plays important roles in the propagation of the energy from North America to Eurasian. When the Western Europe high appeared and shifted eastward, energy conversions increased and energy propagated from North America to Eurasian as a form of interaction energy flow. The baroclinic conversion between transient-eddy kinetic energy (Ke) and transient-eddy available potential energy (Ae) and the horizontal advection of geopotential height were approximately one order of magnitude less than Ke and Ae generation terms. So, these terms were less important to this SSW event.

Towards the development of a robust model hierarchy: investigation of dynamical limitations at low resolution and possible solutions

AbstractIn order to investigate questions concerning changes in the atmosphere and other components of the Earth system, it is desirable to have a seamless modelling system that operates across a wide range of time‐scales and horizontal resolutions. The development of a low‐resolution configuration of the model would greatly assist in addressing these scientific questions whose long time‐scales or complexity are computationally demanding—a cost that cannot be sustained by higher resolutions. However, in order to be confident that results remain robust to changes in resolution, it is essential that such models include the same underlying fundamental processes as those at higher resolution.Some resolution‐dependent biases are found to grow (notably when horizontal resolution decreases below ∼150 km) as a result of the lack of transient eddy kinetic energy (TEKE). The advection scheme used by many of the current general circulation models follows the semi‐Lagrangian (SL) dynamics approach. Because of the need for interpolation to the departure point, the scheme is highly diffusive when low‐order interpolation schemes are used. We analyse how the impact of the high diffusivity of SL schemes leads to a lack of TEKE, which inhibits the development of mid‐latitude variability phenomena such as synoptic cyclones or blocking events. Conversely, some examples where the performance of the low resolution is acceptable are provided.We investigate two different traceable solutions to partially offset the misrepresentation of dynamical features in the low‐resolution version: a change of the interpolation scheme from quasi‐cubic to quintic interpolation; and vorticity confinement: a parametrization designed to preserve vorticity features in dissipative numerical schemes. Both solutions are found to increase TEKE and improve many of the mid‐latitude metrics considered. Copyright © 2012 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.

Comments on “The Role of the Central Asian Mountains on the Midwinter Suppression of North Pacific Storminess”

We thank Chang and Lin for their thoughtful and constructive comments on our study (Park et al. 2010). In Park et al. (2010), we did not explicitly state that the topography-forced stationary waves are the direct cause for the reduced downstream transient eddy kinetic energy (EKE). The response of stationary waves to topography may saturate even with a relatively small mountain (Cook and Held 1992); furthermore, their magnitudes are much smaller than thermally forced stationary waves (Chang 2009; Held et al. 2002). Instead, we suggest that quasistationary waves generated by the central Asian mountains may strongly affect North Pacific storminess by changing the year-to-year variability of westerly winds over the eastern Eurasian continent. Observational analyses indicate that the midwinter suppression of North Pacific storminess does not occur every year. Some years experience stronger and more meridionally confined zonal winds over the western North Pacific, leading to stronger midwinter suppression (Harnik and Chang 2004; Nakamura and Sampe 2002).

The Role of the Central Asian Mountains on the Midwinter Suppression of North Pacific Storminess

Abstract The role of the central Asian mountains on North Pacific storminess is examined using an atmospheric general circulation model by varying the height and the areas of the mountains. A series of model integrations show that the presence of the central Asian mountains suppresses the North Pacific storminess by 20%–30% during boreal winter. Their impact on storminess is found to be small during other seasons. The mountains amplify stationary waves and effectively weaken the high-frequency transient eddy kinetic energy in boreal winter. Two main causes of the reduced storminess are diagnosed. First, the decrease in storminess appears to be associated with a weakening of downstream eddy development. The mountains disorganize the zonal coherency of wave packets and refract them more equatorward. As the zonal traveling distance of wave packets gets substantially shorter, downstream eddy development gets weaker. Second, the central Asian mountains suppress the global baroclinic energy conversion. The decreased baroclinic energy conversion, particularly over the eastern Eurasian continent, decreases the number of eddy disturbances entering into the western North Pacific. The “barotropic governor” does not appear to explain the reduced storminess in our model simulations.

Representation of Northern Hemisphere winter storm tracks in climate models

Northern Hemisphere winter storm tracks are a key element of the winter weather and climate at mid-latitudes. Before projections of climate change are made for these regions, it is necessary to be sure that climate models are able to reproduce the main features of observed storm tracks. The simulated storm tracks are assessed for a variety of Hadley Centre models and are shown to be well modelled on the whole. The atmosphere-only model with the semi-Lagrangian dynamical core produces generally more realistic storm tracks than the model with the Eulerian dynamical core, provided the horizontal resolution is high enough. The two models respond in different ways to changes in horizontal resolution: the model with the semi-Lagrangian dynamical core has much reduced frequency and strength of cyclonic features at lower resolution due to reduced transient eddy kinetic energy. The model with Eulerian dynamical core displays much smaller changes in frequency and strength of features with changes in horizontal resolution, but the location of the storm tracks as well as secondary development are sensitive to resolution. Coupling the atmosphere-only model (with semi-Lagrangian dynamical core) to an ocean model seems to affect the storm tracks largely via errors in the tropical representation. For instance a cold SST bias in the Pacific and a lack of ENSO variability lead to large changes in the Pacific storm track. Extratropical SST biases appear to have a more localised effect on the storm tracks.

The Physical Properties of the Atmosphere in the New Hadley Centre Global Environmental Model (HadGEM1). Part II: Aspects of Variability and Regional Climate

Abstract The performance of the atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is assessed in terms of its ability to represent a selection of key aspects of variability in the Tropics and extratropics. These include midlatitude storm tracks and blocking activity, synoptic variability over Europe, and the North Atlantic Oscillation together with tropical convection, the Madden–Julian oscillation, and the Asian summer monsoon. Comparisons with the previous model, the Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3), demonstrate that there has been a considerable increase in the transient eddy kinetic energy (EKE), bringing HadGEM1 into closer agreement with current reanalyses. This increase in EKE results from the increased horizontal resolution and, in combination with the improved physical parameterizations, leads to improvements in the representation of Northern Hemisphere storm tracks and blocking. The simulation of synoptic weather regimes over Europe is also greatly improved compared to HadCM3, again due to both increased resolution and other model developments. The variability of convection in the equatorial region is generally stronger and closer to observations than in HadCM3. There is, however, still limited convective variance coincident with several of the observed equatorial wave modes. Simulation of the Madden–Julian oscillation is improved in HadGEM1: both the activity and interannual variability are increased and the eastward propagation, although slower than observed, is much better simulated. While some aspects of the climatology of the Asian summer monsoon are improved in HadGEM1, the upper-level winds are too weak and the simulation of precipitation deteriorates. The dominant modes of monsoon interannual variability are similar in the two models, although in HadCM3 this is linked to SST forcing, while in HadGEM1 internal variability dominates. Overall, analysis of the phenomena considered here indicates that HadGEM1 performs well and, in many important respects, improves upon HadCM3. Together with the improved representation of the mean climate, this improvement in the simulation of atmospheric variability suggests that HadGEM1 provides a sound basis for future studies of climate and climate change.

The Role of Wave Packets in Wave–Mean Flow Interactions during Southern Hemisphere Summer

Abstract In this study, reanalysis data produced by the European Centre for Medium-Range Weather Forecasts for 14 Southern Hemisphere (SH) summer seasons have been analyzed. All cases of hemispheric transient eddy kinetic energy (TEKE) maxima have been identified, and the evolution of the local energetics and planetary-scale flow anomalies accompanying these TEKE growth/decay episodes are composited. The longitude–time evolution of the composite energetics shows the clear signature of a wave packet propagating eastward at a group velocity of about 27° longitude per day and undergoing a life cycle of growth and decay, with the energetics within a volume close to the wave packet center dominating the hemispheric mean energetics. When individual cases are examined, 52% are found to resemble the composite and have the energetics life cycle dominated by the evolution of a single wave packet, and an additional 21% are found to be dominated by the evolution of two wave packets having similar amplitudes. Only the remaining 27% can be regarded as having experienced TEKE growth and decay throughout much of the hemisphere. The zonal mean flow and eddy feedback anomalies (i.e., reduction in the meridional temperature gradient due to the effects of the eddy heat fluxes, as well as increase in the barotropic shear due to a narrowing of the midlatitude jet through the effects of the eddy momentum fluxes) associated with the cases dominated by the evolution of a single wave packet are also found to be dominated by anomalies close to the wave packet center. The fact that hemispheric wave growth/decay is often dominated by the evolution of a single wave packet has interesting dynamical consequences when the climatological basic flow is not zonally symmetric. When a wave packet propagates over regions of enhanced baroclinicity, it can extract more energy from the mean flow via baroclinic conversion, leading to its preferential growth. On the other hand, when a wave packet propagates over regions of weak baroclinicity, baroclinic conversion is suppressed; hence any packet growth must be due to other processes. By examining the location of wave packet peaks when hemispheric TEKE is at a maximum, it is observed that hemispheric mean TEKE peaks much more frequently when the dominant wave packet is located downstream of the region with strongest baroclinicity. In addition, the growth in TEKE for these cases is usually dominated by an increase in baroclinic conversion. In contrast, for the small number of cases in which the hemispheric mean TEKE maximum occurs when the dominant wave packet is located downstream of the region with weakest baroclinicity, the growth of the hemispheric TEKE is instead dominated by a reduction in barotropic dissipation.

A New Perspective on Blocking

Abstract It is argued that the essential aspect of atmospheric blocking may be seen in the wave breaking of potential temperature (θ) on a potential vorticity (PV) surface, which may be identified with the tropopause, and the consequent reversal of the usual meridional temperature gradient of θ. A new dynamical blocking index is constructed using a meridional θ difference on a PV surface. Unlike in previous studies, the central blocking latitude about which this difference is constructed is allowed to vary with longitude. At each longitude it is determined by the latitude at which the climatological high-pass transient eddy kinetic energy is a maximum. Based on the blocking index, at each longitude local instantaneous blocking, large-scale blocking, and blocking episodes are defined. For longitudinal sectors, sector blocking and sector blocking episodes are also defined. The 5-yr annual climatologies of the three longitudinally defined blocking event frequencies and the seasonal climatologies of blocking ...

A Simple GCM Based on Dry Dynamics and Constant Forcing

A dry spectral primitive equation model is used to simulate the global atmospheric circulation during northern winter. The resolution is T21 in the horizontal, with five equally spaced sigma layers. The only additional terms in the equations are those describing linear damping, linear scale-selective diffusion, and time-independent forcing. The damping and diffusion act on temperature and momentum. The forcing acts on all prognostic variables. It is calculated objectively from the tendencies produced when the model is initialized with a long time series of observational analyses, and separated into components to ease comparison with time-dependent perturbation models. The simulated climate reproduces observed features of the circulation, both time-mean fields and transient-eddy covariances, with remarkable success. The accurate simulation of tropical divergent flow is a particularly useful result. The main deficiencies are an underestimation of transient-eddy kinetic energy and a lack of transient activity in the Southern Hemisphere. In an attempt to reduce the forcing of divergent flow, a modified vertical scheme and modified forcing functions based on a calculation of balanced flow are introduced. The former still has significant divergence forcing and makes little difference to the final result. The latter tends to give solutions that are unrealistic in the Tropics. The model’s sensitivity to variations in forcing functions and damping parameters is further explored. The Southern Hemisphere transient behavior can be improved by boosting the local forcing of baroclinicity by up to a factor of 2, and a simulation of the Southern Hemisphere winter is relatively successful. The applications and limitations of such a simple fast-running climate model with a relatively realistic simulated climate are discussed.

The generation of available potential energy: a comparison of results from a general circulation model with observations

The generation of available potential energy (APE) in the space-time domain in January and July in a present-day climate simulation with a general circulation model (GCM) is compared with observations. An attempt is made to explain the differences. The generation is computed from the fields of diabatic heating and temperature. The heating is computed with the residual method, using UKMO (United Kingdom Meteorological Office) Unified Model GCM circulation data and ECMWF (European Centre for Medium-Range Weather Forecasts) initialized analyses for the period 1989–1992. The global value of the generation of APE is about 35% larger in the GCM climate than the value computed from the analyses. This is mainly because the generation of transient eddy APE in the GCM is too large, due to the more than 40% too large generation of transient eddy kinetic energy by baro-clinic processes. In most of the extratropics the local contribution to the generation of transient eddy APE in the GCM climate is more than twice as large as the contribution computed from the analyses. A possible qualitative explanation for this difference is presented.

Sensitivity of Simulated Climate to Model Resolution

Dynamical measures of the climate (e.g., winds, eddy fluxes) simulated by a general circulation model are compared at different horizontal and vertical resolutions for the December, January, and February period. The simulations of the troposphere are found to improve significantly as the horizontal resolution increases in the range of spectral truncations from T21 to T63. Little sensitivity is found to changes in vertical resolution between about 2.8 km and 0.7 km vertical grid spacing. The improvements in the Southern Hemisphere troposphere are greater than in the Northern Hemisphere as the horizontal resolution increases. The eddy momentum fluxes and kinetic energies in both hemispheres increase monotonically with horizontal resolution. At T63, the Southern Hemisphere winds, eddy fluxes, and eddy kinetic energies agree favorably with observations, while serious discrepancies are present at lower resolutions. In the Northern Hemisphere, the eddy momentum flux at T63 is slightly larger than observed, while the transient eddy kinetic energy is still barely half of the observed. The gravity wave drag parameterization plays a significant role in the simulations at all resolutions. Eddy momentum fluxes increase when gravity wave drag is removed with a corresponding increase in low-level winds and surface stress.

Intraseasonal fluctuations in low-level meridional winds over the Indian Ocean and monsoonal convection over South Asia

Some aspects of 30-60 day meridional wind fluctuations over the mid-latitude Indian Ocean were investigated using lag correlation maps for the 5 summers of 1979-1983. At times, the equatorial low-frequency (30-60 day) modes exhibit a systematic eastward propagation. However, they are occasionally interrupted by irregular movements or even westward propagation. This study concentrates only on those periods with a systematic eastward propagation and a well-defined tropical-extratropical atmospheric teleconnection pattern. Low-level meridional wind fluctuations in the low-frequency domain are prominent over the western Indian Ocean off the east coast of Africa. The anomalous southerly flows, which are initiated around northern Madagascar move northward and, after crossing the equator, contribute to the intensification of the monsoonal westerlies over South Asia, thus, enhancing convection over the monsoon region. In comparison, 30-60 day southerlies (anomaly) over the eastern Indian Ocean off the west coast of Australia tend to enhance equatorial convection over the western Pacific between about 150°E and the date line. Perturbations with periods shorter than 30 days are considered transient disturbances. Spectral analysis of transient eddy kinetic energy in the extratropical regions reveals a significant peak in the low-frequency range. This is associated with an amplitude modulation of transient disturbances. Thus, it is plausible that transient disturbances, as a group, provide energy for the enhancement of 30-60 day planetary-scale perturbations over the subtropics. DOI: 10.1111/j.1600-0870.1988.tb00411.x

A Comparative Study of Observed Northern Hemisphere Circulation Statistics Based on GFDL and NMC Analyses. Part II: Transient Eddy Statistics and the Energy Cycle

Abstract 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 pres...

The Effects of Condensational Heating on Midlatitude Transient Waves in Their Mature Stage: Control Experiments with a GFDL General Circulation model

Abstract The effects of condensational heating on midlatitude transient waves in their mature stage are re-examined by comparing moist and dry GFDL spectral general circulation models, both of which have all ocean surfaces with prescribed zonally uniform temperature. The zonal mean states of both models are fixed in time. so as to he identical throughout the time integration. It is found that the transient eddy kinetic energy is significantly enhanced for all wavenumbers by the effect of latent beat release. This increase is primarily due to an increase in baroclinic conversion from the zonal available potential energy and only partly due to the generation of eddy available potential energy by condensational heating.

On the role of the Asian monsoon in the angular momentum and kinetic energy balances of the tropics

The balance conditions of 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-sized 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 relative westerly angular momentum for the middle latitude circulation. However, it is found that the angular momentum gained in the extramonsoon region of the tropics (26 Hadley) is mostly destroyed by a net southward flow of mass in that region (on the order of 30 cm sec-l), 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 (17 Hadley) is almost all exported across its northern boundary (35 Hadley). 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 (about 4 watt m-2), the transformation from transient eddy kinetic energy being very small. The proper evaluation of the zonal 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 (20 Hadley), but the monsoon region with its surface westerlies acts as a sink (-18 Hadley), 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. Probably most of the summer generation and dissipation of mean kinetic energy in the tropics (about I watt m-2) occur in the monsoon sector itself without appreciable exchange with other regions. The calculations for two 5-year periods give very similar estimates and thereby show the reliability of the results.