Abstract
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. The existence of these multiple quasi-equilibria is due to the balance of two "forces" on the ITCZ. One toward the equator, due to the earth's rotation, has a nonlinear latitudinal dependence; and the other toward the latitude of the sea surface (or ground) temperature peak has a relatively linear latitudinal dependence. 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 aqua-planet with zonally uniform sea surface temperature. The existence of the two flow regimes, the two "forces", and the abrupt transition are all simulated 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 at the core of monsoon onset in other parts of the world.
Close window or click on previous window to return to the Conference Program.
12th Conference on Atmospheric and Oceanic Fluid Dynamics