Investigating the Impact of Western Disturbance Absence on Spring Temperatures in India

Heatwaves at the cusp of winter and pre-monsoon seasons pose a significant threat to agriculture in India, particularly affecting winter crops like wheat, as seen in the 2022 March heatwave. Understanding the drivers of these extreme weather events is crucial for developing effective mitigation strategies. This study focuses on the role of Western Disturbances (WDs) in influencing cool season heatwave patterns. Western Disturbances are cyclonic storms that travel through the westerly jet stream to bring rain and snow to northwestern India. Recent studies have indicated increased variability in their frequency, intensity and tracks. While WDs have been extensively studied in the context of precipitation and floods, their role in winter heatwaves, especially their prolonged absence, has not been thoroughly examined. Thus investigating changes in WD activity, differences between high and low WD periods and their impact on weather is essential for predicting these heatwaves.

In this study, we classify events into positive, negative and neutral categories based on WD activity and vorticity anomalies for March from 1951-2022. Our composite analysis reveals that during positive events, with strong WDs, cold and wet anomalies dominate at the surface and mid-troposphere. The passage of WDs is preceded by moisture incursion from the Arabian Sea. In contrast, negative events feature dry advection from neighboring arid regions, a maximum heat anomaly with subsidence in the mid-troposphere, and reduced cloud cover. These periods, linked to an anticyclone, result in a lack of moisture from the Arabian Sea, exacerbating heatwave conditions. To further understand the factors contributing to temperature variations, we conduct a temperature budget analysis at the lower (950 hPa) and mid (500 hPa) troposphere. This analysis examines the role of advection, diabatic and adiabatic processes, and identifies the dominant processes in modulating temperatures during positive, negative and neutral events.

This study emphasizes the need for continuous monitoring of WD activity and its implications for regional climate dynamics. Policymakers and stakeholders must consider these insights for managing the risks associated with pre-monsoon heatwaves and ensuring the resilience of winter crops like wheat under climate change.