PAOC Colloquium: Simona Bordoni (Caltech)
A new perspective on an old problem: understanding the observed variability of the South Asian monsoon
Recent theoretical advances indicate that the South Asian monsoon (SAM) should be viewed as an energetically-direct cross-equatorial Hadley circulation, with the monsoonal precipitation primarily occurring in its ascending branch, rather than as a large land-sea breeze circulation. In this talk, we explore the implications of these emerging theories for the observed variability of the SAM on seasonal and longer time scales. We start by using the atmospheric moisture budget to introduce a novel objective index for the onset and retreat of the SAM, which robustly captures the expected seasonal transitions in precipitation and winds and eliminates the need for arbitrarily selected thresholds. Using this index, we show how the SAM onset and retreat are associated with a coherent set of seasonal transitions in circulation, jet streams, precipitation, energetics, and momentum balance throughout the SAM sector. We also use the atmospheric moisture budget to define a new index for the SAM strength on interannual time scales. We show that interannual variability in SAM net precipitation is primarily caused by variations in winds rather than variations in humidity, and that strong monsoons are associated with a northward expansion of the overturning circulation and a decreased near-surface land-sea thermal contrast, in disagreement with the traditional view of the SAM as a sea-breeze circulation. We also find teleconnections between SAM strength and temperatures, winds, and momentum and energy transports in the southern hemisphere extra tropics, which suggest new directions for future research to improve our understanding of the mechanisms involved in SAM variability.