Influence of Continental Geometry on the Onset and Spatial Distribution of Monsoonal Precipitation

Abstract

Recent studies have shown that the rapid onset of the monsoon can be interpreted as a switch in the tropical circulation, which can occur even in the absence of land-sea contrast, between a dynamical regime controlled by eddy momentum fluxes to a monsoon regime more directly controlled by energetic constraints. Here we investigate the influence of continental geometry on such transitions. We conduct experiments with an aquaplanet model with a slab ocean, in which different zonally symmetric continents are prescribed in the Northern Hemisphere poleward from southern boundaries at various latitudes, with “land” having a mixed layer depth two orders of magnitude smaller than ocean. For continents extending to tropical latitudes, the simulated monsoon features a rapid migration of the convergence zone over the continent, similar to what is seen in the Asian monsoon. For continents with more poleward southern boundaries, the main precipitation zone remains over the ocean, moving gradually into the summer hemisphere. We show that the absence of land at tropical latitudes prevents the establishment of a reversed meridional gradient in lower-level moist static energy and, with it, an overturning circulation with poleward displaced convergence zone that can transition rapidly into an angular momentum conserving monsoon regime. Implications for observed monsoons are discussed.