INTERANNUAL VARIATIONS OF SUMMER MONSOONS - SENSITIVITY TO CLOUD RADIATIVE FORCING

The sensitivity of the interannual variations of the summer monsoons t o imposed cloudiness has been studied with a general circulation model using the initial conditions Prepared from the European Centre for Me dium-Range Forecasts analyses of 1 May 1987 and 1988. The cloud optica l properties in this,global model are calculated from prognostically c omputed cloud liquid water. The model successfully simulates the contr asting behavior of these two successive monsoons. However, when the op tical properties of the observed clouds are specified in the model run s, the simulations show some degradation over India and its vicinity. The main cause of this degradation is the reduced land-sea temperature contrast resulting from the radiative effects of the observed clouds imposed in such simulations. It is argued that the high concentration of condensed water content of clouds over the Indian land areas will s erve to limit heating of the land, thereby reducing the thermal contra st that gives rise to a weak Somali jet. A countermonsoon circulation is, therefore, simulated in the vector difference field of 850-hPa win ds from the model runs with externally specified clouds. This counterm onsoon circulation is associated with an equatorial heat source that i s the response of the model to the radiative effects of the imposed cl ouds. Indeed, there are at least two clear points that can be made: 1) the cloud-SST patterns, together, affect the interannual variability; and 2) with both clouds and SST imposed, the model simulation is less sensitive to initial conditions. Additionally, the study emphasizes t he importance of dynamically consistent clouds developing in response to the dynamical, thermal, and moist stare of the atmosphere during mo del integrations.