Influence of intraseasonal oscillations on Indian heat waves: assessing the prominence of BSISO and MJO

Abstract The Boreal Summer Intraseasonal Oscillation (BSISO) and Madden–Julian Oscillation (MJO) are two significant intraseasonal modulators of tropical weather. The effects of these two oscillations are mostly limited to particular seasons and regions. While the role of these two events on heatwave occurrence is explored in multiple studies individually. The relative importance of these oscillations on the extreme heat events over the Indian region is not explored. It is relevant over Indian subcontinent as the peak heatwave season of the region falls during the transition period between MJO and BSISO. The present study attempts to bridge this gap in the understanding of their relative roles in modulating the heat wave events over India. The results show that BSISO has a more pronounced effect on the heatwave events over the Indian region compared to the MJO. Specifically, the BSISO dry phase is associated with a higher frequency and higher intensity heatwaves, and with medium-duration events dominating. In contrast, the MJO dry phase promotes longer-duration heatwaves. Further, the study explores the dynamics and thermodynamic processes underlying these effects. The modulation of heatwaves by BSISO is attributed to the increased adiabatic heating due to anomalous subsidence associated with the clear sky condition and a reduction in latent heat flux, which collectively leads to warmer surface conditions. While the horizontal temperature advection is the main cause of an increase in heatwave events associated with MJO. Further BSISO dry phase induces stronger diabatic heating and a greater modulation of surface energy fluxes compared to MJO, leading to more intense and frequent heatwave events.