Investigating the physical controls of monsoon seasonality in a hierarchy of climate models

Abstract

Monsoons supply the majority of annual rainfall to land regions across the tropics and play a prominent role in global climate. Despite this, we lack a complete mechanistic understanding of monsoon characteristics and how they might change with warming. This dissertation draws on theory and a range of climate models to elucidate the physics governing monsoon circulations. First, we investigate the seasonally dependent responses of the South American monsoon to warming. In the latest Geophysical Fluid Dynamics Laboratory Atmospheric Model (AM4), uniform 2-K sea surface temperature warming reduces precipitation by 11%, and precipitation minus evaporation by 40%, in the monsoon onset season, producing a more severe dry season. A column-integrated moist static energy budget analysis helps elucidate the drying mechanism. By comparing the response across seasons we identify two necessary conditions for a region to exhibit a substantial drying response to warming. To better understand the seasonal and global variations in monsoon circulations, Chapter 3 utilizes an idealized modeling framework. We systematically vary the moisture and albedo parameters over an isolated South American continent with reduced heat capacity, but no topography. Irrespective of the local moisture availability, the seasonal cycles of precipitation and circulation over the monsoon sector are distinctly monsoonal with the default surface albedo, but with differing characteristics and dynamical mechanisms. The analysis supports that when the land-ocean thermal contrast is strong enough, inertial instability is sufficient for producing a shallow but vigorous circulation and converging a large amount of moisture from the ocean. Once the land is sufficiently moist, convective instability takes hold and the shallow circulation deepens. Chapter 4 investigates the monsoonal precipitation over a dry continent on synoptic timescales, and provides additional evidence for the importance of inertial instability. In Chapter 5, we utilize an Earth System Model (ESM4.1) to investigate changes in precipitation and aridity over the South American monsoon sector in future emissions scenarios. The projected runoff reductions have implications for energy system planning in Brazil, where 60-80% of monthly power generation comes from hydropower. The possibility of seasonal power generation deficits underscores the importance of climate risk management in the energy sector.