The Globally Coherent Pattern of Autumn Monsoon Precipitation

Abstract:

Over most tropical land regions, the annual peak in precipitation occurs during summer, associated with the local monsoon circulation. However, in a few coastal regions in the tropics the bulk of annual precipitation occurs in autumn, after the low-level summer monsoon westerlies have abated. Examples include the Nordeste region of Brazil, southeastern India and Sri Lanka, and coastal Tanzania. Unlike equatorial regions, they receive little rainfall during local spring. Such regions are present along the eastern coasts of all continents except Australia, suggesting that they comprise a coherent yet previously unrecognized global phenomenon.

In this study, we identify eight locations in the tropics that experience an “autumn monsoon” and show that this unusual seasonal cycle is generated by similar mechanisms in all of them. When these regions receive their peak rainfall, they lie poleward of the ITCZ in easterly low-level winds. The spatial structure of precipitation in these regions can be explained by their placement to the east of mountain ranges that organize deep convection on their windward sides. However, orographic forcing alone cannot explain their unique seasonal cycle: despite similarities in the wind direction, surface humidity, and sea surface temperatures (SSTs) between autumn and spring, these regions receive significantly more rainfall in autumn than in spring. We show that this is due to differences in the large-scale atmospheric stability between the equinoctial seasons, which can be captured by a relative SST metric and is influenced by SSTs in the remote eastern upwelling zones of the Pacific and Atlantic Oceans.