The role of hemispheric flow patterns and land-atmosphere interactions in South Asian Heatwaves

In South Asia (SA) heatwaves during spring season have become a reoccurring phenomenon in the recent decade with severe socioeconomic losses reported and unprecedented ecosystem losses. More than 24% of the global population and 3.85% of global GDP (as of 2021) is exposed to severe heatwaves during pre-monsoon season. In this study, we explored the physical process and mechanism triggering these events and those amplifying these heatwaves. We used the ERA5 reanalysis data from the ECMWF and a suit of statistical and process-based techniques to understand the physical mechanism of the heatwaves in SA. We discovered that the heatwaves occur during April and May and end in early June. The heatwaves are comprised of two stages including the early stage where thermal stress exists followed by absolute heat driven by high temperature and energy accumulation. The zonally transmitted meridionally oscillating quasi-resonant Rossby waves transport energy and momentum into SA triggering the heatwave and control the early stage of the heatwaves. Afterwards when the soil moisture is depleted due to higher evaporative demands the strong land-atmosphere coupling exerts positive feedback and amplifies the second stage of the heatwaves.