Diagnostic study of the structure of inter tropical convergence zone in an aquaplanet general circulation model

Abstract

The Intertropical Convergence Zone (ITCZ) is a narrow region of cloudiness and rainfall that has a predominant east–west orientation. In the present study, the response of the ITCZ to latitudinal variation of the sea surface temperature (SST) gradient, magnitude of SST maxima, and location of SST maxima was investigated in an ocean-covered planet (aquaplanet) using a General Circulation Model with two different cumulus parameterization schemes, namely the Kuo scheme, which is a one-dimensional cloud model with convection extending throughout the troposphere in all instances, and the SAS scheme (Simplified Arakawa–Schubert scheme), which includes downdraft mechanisms and entrainment processes with evaporation of precipitation.

The main objective of the thesis was to unravel the factors that control the strength, location, and width of the ITCZ using a diagnostic model based on the conservation of moist static energy and moisture.

The strength and width of the ITCZ were found to be sensitive to variations in the SST gradient and magnitude of the SST maxima when the SST maximum was at the equator, whereas they were largely insensitive to these variations when the SST maximum was at 20°N. The strength of the ITCZ increases with an increase in SST gradient and magnitude of SST maxima. For a 16 K increase in SST maxima, a sevenfold increase in ITCZ precipitation was observed. The major contributions came from the net energy available in the atmospheric column (which showed a threefold increase) and the vertical stability of the atmosphere (which showed a twofold increase).

The major factor controlling the strength of the ITCZ was found to be the net energy available in the atmospheric column. A comparison of the ITCZ strength for the two cumulus schemes (SAS and Kuo) with increasing SST gradient showed that for a tenfold increase in SST gradient, there was a fourfold increase in ITCZ precipitation with the SAS scheme, whereas a twentyfold increase in ITCZ precipitation was observed with the Kuo scheme for the same increase in SST gradient. This difference was due to changes in the net energy available in the atmospheric column, which increased sevenfold in the SAS scheme and sixteenfold in the Kuo scheme. The vertical stability of the atmosphere also showed major differences between the two schemes: vertical stability decreased with increasing SST gradient in the SAS scheme, whereas it increased with increasing SST gradient in the Kuo scheme.

The width of the ITCZ decreases as the SST maximum shifts from the equator to 20°N. For similar SST gradients at the equator and 20°N, the strength of the ITCZ was found to be greater at 20°N. This increase in strength is due to differences in the structure and magnitude of the net energy available in the atmospheric column at the equator and 20°N, which depend upon wind speed. The wind speed at the equator is governed by frictional convergence and SST gradient, whereas at 20°N it is controlled by geostrophic balance (i.e., a balance between the Coriolis force and pressure gradient force).

The position of the ITCZ in simulations with the SST maximum away from the equator lies between the location where net energy availability has a maximum and where vertical stability has a maximum.

In the aquaplanet model, surface fluxes were found to play a more important role in maintaining the net energy available in the atmospheric column than solar radiation. Thus, the magnitude of SST maxima, its location, and the latitudinal gradient of SST play crucial roles in determining the strength, location, and width of the ITCZ.