Intensifying Humid-Heat Extremes in Asia

Abstract

This dissertation investigates the intensification of humid-heat extremes across monsoon Asia, emphasizing the roles of monsoon dynamics and external forcings such as greenhouse gases (GHGs) and aerosols. The first part demonstrates that humid-heat extremes (ExTw) are rising rapidly in the monsoon-influenced parts of Asia due to increasing specific humidity (Q). Diagnostics based on RH and Q indicate that moisture availability, modulated by SST warming, urbanization, irrigation, and circulation changes including the westward expansion of the WPSH, is not just a modifier but a central driver f heat stress. The second part focuses on the Indo-Gangetic Plain (IGP) and finds that long-term RH increases are driven mainly by rising Q, aerosol-induced surface cooling, and stabilized T, while GHG-driven warming tends to suppress RH by enhancing saturation vapor pressure. CESM2-LE, CESM1-LE, and CMIP6 simulations confirm these opposing thermodynamic responses. Future projections show RH increase under high aerosol loadings (e.g., early SSP3-7.0), followed by reversals as aerosol decline and GHG warming accelerates. This scenario contrast is clearly seen across SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5, emphasizing that future RH trajectories over the IGP are shaped by a delicate balance between competing forcings, and that their evolution remains highly uncertain, contingent on the pace of aerosol decline and the strength of GHG-driven warming. The third part examines structural intensification of the East Asian summer monsoon (EASM) lifecycle, characterized by stronger active phases and prolonged breaks. Observation shows this intensification is ties to WPSH expansion, which multiple studies suggest, via a thermodynamic response to greenhouse warming. This altered lifecycle increases the likelihood of sequential extremes, such as flood-to-heatwave transitions, and is better captured by models that realistically simulate the EASM rainband and its migration. Together these findings highlight that rising humid- heat risks result from the coupled effect of atmospheric moistening, seasonal reorganization of monsoon system, and externally forced thermodynamic shifts. While RH trends are governed by a balance between moisture accumulation and temperature-driven saturation, changes in the EASM lifecycle, particularly the intensification of monsoon break, further compound the risks of sequential extremes. Effective climate risk assessments must account for both humidity and temperature, along with structural changes in the monsoon, especially under shifting emission pathways. Mitigation strategies should jointly address GHG and aerosol emissions, and heat risk frameworks must incorporate humidity to more accurately represent compound heat stress.