Modes of summer temperature intraseasonal oscillations and heatwaves over the Indian region

Abstract

Over the Indian region, the pre-monsoon (i.e. April–May) is a dry summer season. The heatwaves, as well as local temperature variations during this season, are not associated with significant large-scale convective heating like the monsoonal modes, and several studies identified several drivers of heatwaves. Heatwaves are extreme events. Are these extremes arising from low-frequency intraseasonal modes, in the same way, extreme rainfall occurs on a synoptic or intraseasonal mode during monsoon? Studies do not explicitly point out the existence of temperature intraseasonal modes during April–May over the Indian region, and it is not clear if some of the drivers of heatwaves can also explain the April–May temperature variations as derivative of some modes. This study identifies the dominant pair of the intrinsic mode of temperature intraseasonal oscillations, which can also explain the heatwave spikes. The empirical orthogonal function based modes are isolated in the detrended surface temperature data to remove the global warming mode. It was found that the subtropical jet acting as a Rossby wave guide drives the first mode with pan India spatial modal signature, while the second mode is driven by the extratropical Rossby wave modes originating from the latitudes of the eddy-driven jet. Another important result is that the first (second) mode principal component shows a significant decreasing (increasing) trend from 1981–2020 period. The observed spatial heterogeneity in warming and the trend in the spatial distribution of extreme temperature events in India could also be explained by the trend in the two modes of oscillation.