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Supported by numerical experiment results, the abrupt change of the location of the intertropical convergence zone (ITCZ), from the equatorial trough flow regime to the monsoon trough flow regime, or the monsoon onset, is interpreted as a subcritical instability. There are two balancing ''forces'' acting on the ITCZ. One toward the equator, or an equatorial latitude depending on the convection scheme, due to the earth's rotation, has a nonlinear latitudinal dependence; and the other toward a latitude close to the sea surface temperature peak has a relatively linear latitudinal dependence. The highly nonlinear latitudinal dependence of the first ''force'' is crucial for the existence of the multiple equilibria. This work pivots on the finding that the ITCZ and Hadley circulation can still exist without the pole-to-equator gradient of radiative-convective equilibrium temperature. The numerical experiments are done with an atmospheric general circulation model over an aquaplanet with zonally uniform sea surface temperature. The existence of the two flow regimes, the two ''forces,'' and the abrupt transition are all demonstrated in the experiments. Experimental results show high dependence on the choice of cumulus parameterization scheme, especially during the equatorial trough circulation regime. Although the proposed interpretation is more suitable for explaining the monsoon trough onset in the western Pacific, it is hypothesized that the same basic mechanism is also at the core of monsoon onset in other parts of the Tropics.

Multiple Quasi Equilibria of the ITCZ and the Origin of Monsoon Onset