Energetic constraints unify the Breeze and ITCZ interpretations of monsoons and explain regional monsoon variability

Monsoons are historically understood as continental-scale land-ocean Breeze. Modern studies, however, link monsoons to seasonal migrations of the inter-tropical convergence zone (ITCZ) -- a band of intense precipitation that lies along the rising limb of the tropical overturning circulation. Here, we explore in reanalysis data the relative role of zonal vs. meridional migrations of tropical convergence zones in the Asian-Australian monsoon, employing energetic constraints. Both seasonal ITCZ shifts and seasonal land-ocean energetic contrasts are shown to have a critical influence on monsoons. Energetic constraints, therefore, merge the Breeze and ITCZ interpretations of monsoons and provide a simple analytic framework for understanding monsoon variations. Specifically, we provide energetic constraints on South Asian Summer Monsoon (SASM) onset, retreat, and strength, which yield a mechanism explaining the known tendency for enhanced SASM during La Niña episodes. Similarly, the tendency for enhanced Australian monsoon during La Niña episodes is shown to be related to energetically constrained zonal shifts of the Indo-Pacific regional overturning circulation. Moreover, we show that meridional and zonal energetic contrasts in the Indo-Pacific sector are both statistically independent and precede SASM variations by up to two months. Regional energetic contrasts may therefore be used for predictive applications of seasonal SASM variability.