An Aquaplanet Monsoon

Abstract

A hypothesis is proposed that the seasonal evolution of sea surface temperature (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 model, is realized by wind-induced surface-heat exchange. The model adopted is a shallow water analog in dynamics with two vertical levels for thermodynamics. The land forcing effect is neglected as a crucial simplification of the model experiments, along with the absence of the dynamical feedback to the SST in the model. Experiments with steady SST forcing reveal the presence of three regimes of response. Weak SST forcing realizes two unsteady regimes, depending on the latitude of the forcing: (i) the supercluster regime, characterized by equatorially trapped eastward propagating convective coherencies akin to the Madden?Julian waves, and (ii) the monsoon regime, characterized by an intermittent planetary-scale standing convective oscillation at the subtropics. For a large SST forcing, a steady response is found similar to the earlier solutions of Matsuno, Webster, and Gill. Experiments with a seasonally 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 to the other. The two unsteady regimes are seen to be in competition, as in the model they cannot coexist. Implications for the atmospheric monsoon are (i) that the SST forcing away from the equator should precede monsoon onset and (ii) that equatorial intraseasonal convective activity should be less active at the time of year when monsoon activity occurs away from the equator.