Alternative mechanism of the Indian summer monsoon formation

Abstract

The Indian summer monsoon is characterized by exceptionally intense cross-equatorial southwest winds, with its underlying mechanism vigorously debated. From the perspective of the general circulation, changes in regional atmospheric circulation are also ambiguous. Here, we present our latest research results, which show that the Hadley cells over the Indian Ocean change from two cells in boreal winter to a single cell in boreal summer, due mainly to a strong meridional shift of the ascending branch of the Hadley cells. Simultaneously, an enhancement of the Mascarene High also occurs over the south Indian Ocean. The coincidence of a deepening of the Indian Low and an uncharacteristically enhancing of the Mascarene High is likely to contribute to a northward expansion of the southern cell, with the northern cell totally vanished in the north Indian Ocean in summer, resulting in the Indian summer monsoon. This is the first time that the Hadley cells have been identified as having considerable inter-hemispheric variability, calling into question the long-held belief that the Hadley cells only occur in one hemisphere.