Regime Change Behavior During Asian Monsoon Onset

AbstractAs the ITCZ moves off the equator on an aquaplanet, the Hadley circulation transitions from an equinoctial regime with two near symmetric, significantly eddy-driven cells, to a monsoon-like regime with a strong, thermally direct cross-equatorial cell, intense low-latitude precipitation, and a weak summer hemisphere cell. Dynamical feedbacks appear to accelerate the transition. This study investigates the relevance of this behavior to monsoon onset by using primitive-equation model simulations ranging from aquaplanets to more realistic configurations with Earth’s continents and topography.A change in the relationship between ITCZ latitude and overturning strength is identified once the ITCZ moves poleward of about ∼ 7°. Monsoon onset is associated with off-equatorial ascent, in regions of non-negligible planetary vorticity, and this is found to generate a vortex stretching tendency that reduces upper level absolute vorticity. In an aquaplanet, this causes a transition to the cross-equatorial, therm...