Tropical East–West Circulations During the Northern Winter

Abstract

In this Paper we present the geometry and intensity of the mean east?west circulation during the northern winter. We show that near the equatorial latitudes two pronounced regions of divergent mass outflow in the upper troposphere are found near the convective regions over the northwestern part of South America and Indonesia. The intensity of the east?west circulation is shown to be of the order of 1 m sec?1 which is comparable to the intensity of the Hadley circulation. The divergent streamlines are shown to be important for the maintenance of the three waves of the subtropical westerly jet in the Northern Hemisphere, and are shown to exhibit asymptotes of convergence in the regions of mid-oceanic upper tropospheric troughs over the tropical southern oceans. Kinetic energy exchanges for a tropical belt 15S to 15N at 200 mb are expressed as a function of zonal wavenumber. Results for northern summer and winter seasons are compared. We find that wave interactions with the mean zonal flow differ in the two seasons. During the northern summer the long waves (wavenumbers 1 and 2) transfer kinetic energy to the zonal flow which in turn transfers kinetic energy to the short waves (wavenumbers 6, 7 and 8). During the northern winter the opposite occurs: long waves receive kinetic energy from the zonal flow while short waves transfer kinetic energy to the zonal flow. Finally, we evaluate the generation of eddy kinetic energy by the mean east?west circulations during the two seasons. We show that the east?west circulations are thermally direct, i.e., there is a generation of eddy kinetic energy, on horizontal scales >10,000 km. We furthermore find that this generation during the northern summer is about an order of magnitude larger than for the northern winter.