Processes and Timescales in Onset and Withdrawal of 'Aquaplanet Monsoons'

Abstract

Aquaplanets with low heat capacity slab ocean boundary conditions can exhibit rapid changes in the regime of the overturning circulation over the seasonal cycle, which have been connected to the onset of Earth’s monsoons. In spring, as the ITCZ migrates off the Equator, it jumps poleward and a sudden transition occurs from an eddy-driven, equinoctial regime with two weak Hadley cells, to a near angular momentum conserving, solstitial regime with a strong, cross-equatorial winter hemisphere cell. Here, the controls on the transition latitude and rate are explored in idealised moist aquaplanet simulations. It is found that the transition remains rapid relative to the solar forcing when year length and slab ocean heat capacity are varied, and, at Earth’s rotation rate, always occurs when the ITCZ reaches approximately 7°. This transition latitude is, however, found to scale inversely with rotation rate. Interestingly, the transition rate varies non-monotonically with rotation, with a maximum at Earth’s rotation rate, suggesting that Earth may be particularly disposed to a fast monsoon onset. The fast transition relates to feedbacks in both the atmosphere and the slab ocean. In particular, an evaporative feedback between the lower-level branch of the overturning circulation and the surface temperature is identified. This accelerates monsoon onset and slows withdrawal. Lastly, comparing eddy-permitting and axisymmetric experiments shows that, in contrast with results from dry models, in this fully moist model the presence of eddies slows the migration of the ITCZ between hemispheres.