ANALYTIC MODEL FOR ZONAL WINDS IN THE TROPICS

Abstract

An equatorial ?-plane model for the tropospheric zonal circulation is used to examine the consequences of the seasonal and hemispheric variation both of the tropical rain belt as a zonal mean heat source and of the horizontal eddy momentum fluxes as a zonal mean momentum source. The model calculations show variations of Hadley circulations with hemisphere and season. The winter hemisphere Hadley cell is more intense in July than in January because of the greater mean displacement of the tropical rain belt from the Equator and hence the greater asymmetry between hemispheres. The essential differences between the zonal winds in the two hemispheres during summer are reproduced by differences in the eddy momentum transports and in the mean meridional circulation. The model indicates how the annual oscillation of temperature in the equatorial lower stratosphere, with lowest temperatures in January, derives from the difference between the upward branches of the July and the January winter hemisphere Hadley cell. The semiannual oscillation in winds and temperatures in the Tropics is largely accounted for by the model in terms of the longitudinally averaged tropical rain belt migrating between summer hemispheres.