Idealized Modelling of Monsoon Dynamics

Recent idealized aquaplanet studies suggest that the fast onset of the monsoon may correspond to a sudden transition in the dynamical regime of the Hadley circulation. During monsoon onset, the tropical circulation changes from an equinoctial regime with two near symmetric eddy driven cells, to a monsoon regime with a strong, thermally direct cross-equatorial cell, and a weak summer hemisphere cell. Here, it is proposed that the fast transition to the thermally direct monsoon regime relates to a change in the balance of terms in the upper level vorticity budget. In particular, as the ITCZ shifts off the Equator into a region of non-negligible planetary vorticity, the tendency due to vortex stretching (a product of absolute vorticity and horizontal divergence) increases in magnitude and acts to lower upper level absolute vorticity.

A versatile idealized model, GFDL-MiMA, is used to investigate this behaviour. Experiments are performed in both a realistic configuration including Earth’s land and topography, and in a range of aquaplanet configurations. Evidence for a change in dynamical behaviour is found in both the aquaplanets and the more realistic experiment, with a change in the relationship between ITCZ latitude and overturning circulation strength observed as the ITCZ shifts poleward. In order to build a theoretical framework for understanding the transition, aquaplanet simulations in which orbital period and rotation rate are varied are analysed. Initial results support the mechanism proposed above. Implications for real world monsoon onset in present and future climates are discussed.