Assessing the role of boreal summer intraseasonal oscillation on the severe heatwave of May 2015 over India

Abstract In May 2015, the southeastern coastal states of India encountered one of the deadliest heatwaves in Indian history. Researchers have extensively studied the event to understand the underlying mechanisms and concluded that horizontal warm‐air advection from northwestern parts of India and adiabatic heating were the main attributing factors for the event. However, the large‐scale atmospheric processes that led to these conditions have not been thoroughly explored. In the present study, we show that this event was largely associated with the boreal summer intraseasonal oscillation (BSISO) that is prominent during this season. Our analysis shows that the BSISO dry phase lead to a persistent high‐pressure system, with anomalous subsidence favoring adiabatic heating and anticyclonic circulation anomalies increasing the northwesterly warm‐air advection. It is shown that a 55% to 75% contribution to the maximum surface air temperature (SAT) anomalies during the heatwave period can be attributed to BSISO‐related temperature anomalies. Furthermore, the results show that, in the absence of BSISO, the heat event would have dissipated with 1–2 hot days with much less intensity and the presence of the BSISO dry phase extended the heatwave duration by six days. The impact of BSISO on this heatwave was further substantiated by sensitivity experiments using the Weather Research and Forecasting (WRF) model. This analysis emphasizes that improving the forecasting skills of BSISO may facilitate the subseasonal forecast of local heatwave events.