Monsoons as eddy-mediated regime transitions of the tropical overturning circulation: II. The Asian-Australian monsoon system

Simulations with an aquaplanet GCM presented in Part I show that feedbacks between large-scale eddies and the tropical overturning circulation mediate rapid transitions between circulation regimes that are distinguishable according to the degree to which eddy momentum fluxes control the strength of the circulation. Here we use 20 years (1981-2000) of reanalysis data to demonstrate that similar transitions between the two circulation regimes mark the onset and end of monsoons over Asia and Australia.

At monsoon onset, the meridional overturning circulation zonally averaged over the Asian and Australian sectors rearranges itself from an equinox pattern, with two cells almost symmetric about the equator, to a monsoon pattern dominated by a single cross-equatorial cell, with ascent in the subtropics in the summer hemisphere and descent in the winter hemisphere. In the equinox regime, eddy momentum fluxes of midlatitude origin extend into the upper branches of the equinox cells, forcing them to deviate from angular-momentum conservation and strongly influencing their strength. In the monsoon regime, eddy momentum fluxes are confined to the descending branch of the cross-equatorial cell, allowing it to more closely conserve angular momentum and respond more directly to the thermal forcing. An analysis of the upper tropospheric zonal momentum budget will be presented, to quantify the nature of monsoon transitions as shifts in the dominant zonal momentum balance at the center of the overturning circulations.