Causal analysis of Heatwaves in India: Impact of Remote Soil Moisture

Heatwaves are a significant threat to human health, agriculture, and infrastructure; particularly in India, where they are prevalent during the pre-monsoon months. May is a critical period for heatwave occurrences, severely impacting the Indian subcontinent. This work delves into the underlying mechanisms driving heatwaves in India, specifically focusing on those that occur in May. Utilizing an intermediate complexity earth system model, PLASIM¹, and its adjoint² for sensitivity analysis³, we systematically investigate the causal role of remote soil moisture in heatwave formation. We find that variations in remote soil moisture significantly influence the strength and duration of pre-monsoon heat waves in India. Our analysis shows that at a lead time of 10-15 days, higher soil moisture particularly over the Middle East, can prolong heatwave conditions over India. On the other hand, high soil moisture over India suppresses the development of heatwaves with no lag. The delayed mechanism of remote soil moisture works through the altered atmospheric circulation patterns induced by heat flux forcing modulated by soil moisture anomalies, leading to enhanced subsidence and reduced moisture transport to India. Our study provides valuable insights into the mechanisms driving heatwaves in India, particularly those in May. These insights are crucial for developing effective early warning systems, enhancing disaster preparedness, and implementing mitigation strategies to reduce the adverse impacts of these extreme events.

¹The Planet Simulator (PlaSim): a climate model of intermediate complexity for Earth, Mars and other planets.

²Marotzke, Jochem, et al. "Construction of the adjoint MIT ocean general circulation model and application to Atlantic heat transport sensitivity." Journal of Geophysical Research: Oceans 104.C12 (1999): 29529-29547.

³Köhl, Armin, and Andrey Vlasenko. "Seasonal prediction of northern European winter air temperatures from SST anomalies based on sensitivity estimates." Geophysical Research Letters 46.11 (2019): 6109-6117.