Seasonal and Interannual Variability in South Asian Monsoon Dynamics

Abstract

Recent theoretical advances indicate that the South Asian summer monsoon (SASM) should be viewed as an energetically-direct cross-equatorial Hadley circulation, with the monsoonal precipitation primarily occurring in its ascending branch, rather than its traditional interpretation as a large land-sea breeze circulation. Despite these developments, very few studies have explored the implications of these emerging theories for the observed variability of the SASM. This is the goal of this PhD thesis, which uses atmospheric reanalysis data from recent decades to investigate seasonal transitions and interannual variability of the SASM. Our approach differs from previous studies in its focus on the large-scale atmospheric dynamics of the SASM: more specifically, we analyze the tropical circulation throughout the SASM sector and its relationship with extratropical weather systems in both hemispheres that can affect transports of momentum and energy. The atmospheric moisture budget over the SASM region provides a crucial starting point for our work. We use this budget to introduce a novel objective index for the onset and retreat of the SASM, 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 SASM onset and retreat are associated with a coherent set of seasonal transitions in circulation, jet streams, precipitation, energetics, and momentum balance throughout the SASM sector. These transitions closely resemble those of the zonal mean Hadley circulation, indicating that the SASM projects strongly onto the zonal mean and that the observed SASM is consistent with new theoretical interpretations. We also use the atmospheric moisture budget to define a new index for the SASM strength on interannual time scales. We show that interannual variability in SASM net precipitation is primarily caused by variations in winds rather than variations in humidity, highlighting the importance of understanding drivers of the large-scale circulation and its changes. We then use linear regression analysis to identify robust changes in the circulation associated with SASM year-to-year variability. We find 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 SASM as a sea-breeze circulation. We also find teleconnections between SASM strength and temperatures, winds, and momentum and energy transports in the southern hemisphere extratropics. These interhemispheric teleconnections, which have not been previously identified, suggest new directions for future research to improve our understanding of the mechanisms involved in SASM variability. Important advances in modeling and predicting the SASM at all timescales require a deeper understanding of the fundamental processes driving this system. Through theoretically-guided analyses of the SASM observed variability, this dissertation work takes an important step in this direction and provides novel insight into long-standing open questions on the SASM, with crucial implications for its predictability.