“Cumulus Friction”: Estimated Influence on the Tropical Mean Meridional Circulation

Abstract

A simple diagnostic study is performed in order to examine the influence of parameterized westerly momentum exchange by cumulus convection on the observed Hadley circulation. Observed precipitation and 200 mb winds are used to diagnose the mean 100?200 mb tropical cumulus friction using the parameterization proposed by Schneider and Lindzen (1976). By using seasonal data on a latitude-longitude grid, we estimate not only the annual (standing) zonally symmetric cumulus friction but the seasonal (transient) and zonally asymmetric contributions as well. In general, the standing zonally symmetric contribution dominates the total cumulus friction, but other components are significant at certain latitudes. The meridional mass flux induced by cumulus friction is estimated through the use of an approximate u-momentum equation. The diagnosed meridional mass transports are about an order of magnitude smaller than those which are observed by direct measurements. Therefore it appears unlikely that cumulus friction plays a dominant role in maintaining the tropical mean meridional circulation.