AN AQUAPLANET MONSOON

A hypothesis is proposed that the seasonal evolution of sea surface te mperature (SST) is the major forcing to control both the onset and the life cycle of the monsoon. A sensitive coupling of surface heat flux and cumulus convection is the central process and, in the current mode l, is realized by wind-induced surface-heat exchange. The modal adopte d is a shallow water analog in dynamics with two vertical levels for t hermodynamics. The land forcing effect is neglected as a crucial simpl ification of the model experiments, along with the absence of the dyna mical feedback to the SST in the model. Experiments with steady SST fo rcing reveal the presence of three regimes of response. Weak SST forci ng realizes two unsteady regimes, depending on the latitude of the for cing: (i) the supercluster regime, characterized by equatorially trapp ed eastward propagating convective coherencies akin to the Madden-Juli an waves, and (ii) the monsoon regime, characterized by an intermitten t planetary-scale standing convective oscillation at the subtropics. F or a large SST forcing, a steady response is found similar to the earl ier solutions of Matsuno, Webster, and Gill. Experiments with a season ally varying SST anomaly simulate both the sudden onset and the active -break cycle of the monsoon. In particular the onset is interpreted as the atmosphere undergoing a sudden switch from one dynamical regime t o the other. The two unsteady regimes are seen to be in competition, a s in the model they cannot coexist. Implications for the atmospheric m onsoon are (i) that the SST forcing away from the equator should prece de monsoon onset and (ii) that equatorial intraseasonal convective act ivity should be less active at the time of year when monsoon activity occurs away from the equator.