Marine heatwaves (MHWs) are extreme oceanic warm water events (above 90th percentile threshold) that significantly impact the marine environment. Several studies have recently explored the genesis and impacts of MHWs though they are least understood in the tropical Indian Ocean. Here we investigate the genesis and trend of MHWs in the Indian Ocean during 1982–2018 and their role in modulating the Indian monsoon. We find that the rapid warming in the Indian Ocean plays a critical role in increasing the number of MHWs. Meanwhile, the El Nino has a prominent influence on the occurrence of MHWs during the summer monsoon. The Indian Ocean warming and the El Nino variability have synergistically resulted in some of the strongest and long-lasting MHWs in the Indian Ocean. The western Indian Ocean (WIO) region experienced the largest increase in MHWs at a rate of 1.2–1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.4–0.5 events per decade. Locally, the MHWs are induced by increased solar radiation, relaxation of winds, and reduced evaporative cooling. In the western Indian Ocean, the decreased winds further restrict the heat transport by ocean currents from the near-equatorial regions towards the north. Our analysis indicates that the MHWs in the western Indian Ocean and the north Bay of Bengal lead to a reduction in monsoon rainfall over the central Indian subcontinent. On the other hand, there is an enhancement of monsoon rainfall over southwest India due to the MHWs in the Bay of Bengal.

Cyclones in the north Indian Ocean evolve differently during the pre-monsoon (April-June) and post-monsoon (October-December) seasons. While several studies have investigated the near-surface ocean-atmospheric interactions, there is a lack of understanding of the upper-atmospheric response during cyclones. In the current study, we find that cyclones in this basin induce warming of 3–4°C at the upper tropospheric levels (300–400 hPa) during the pre-monsoon season, for the period 1982–2019. However, during the post-monsoon season, the upper-level warming is only ~1°C. The contrasting atmospheric response to cyclones in the two seasons is attributed to the contrasting ocean-cyclone-atmosphere coupled interaction. In the pre-monsoon season, higher SSTs coupled with higher wind forcing and moisture disequilibrium enhance the latent heat flux from the ocean to the atmosphere during the cyclones. This enhances convection resulting in enhanced latent heat release and anomalous upper-level warming in the atmosphere. During the post-monsoon season, the SSTs are cooler, and wind forcing and moisture disequilibrium is less than in the pre-monsoon season. As a result, the latent heat flux exchange is weak, leading to weaker convection, reduced latent heat release and weak upper-level warming. The lower atmospheric response to cyclones is also different in the two seasons, with enhanced evaporative cooling due to a drier lower atmosphere in the pre-monsoon season as compared to the post-monsoon season. Since the Indian Ocean is warming rapidly, it is essential to closely monitor the atmospheric temperature changes accompanying the cyclones in this basin since they can potentially influence largescale atmospheric dynamics and circulation.

The cyclones during November in the Bay of Bengal follow two distinct tracks. Some cyclones move west-northwestward and make landfall at Odisha, Andhra Pradesh, Tamil Nadu or Sri Lanka coast while others move north-northeastwards and make landfall at West Bengal, Bangladesh or Myanmar coast. We found that there is a difference in the steering winds governing these two different cyclone tracks. The north-northeastward moving cyclones are associated with an anomalous upper-level cyclonic circulation over India which is a part of a sub-tropical wave train. This wave train is triggered by an anomalous upper-level convergence over the Mediterranean region near the subtropical westerly jet entrance. The jet acts as a Rossby waveguide and excites an eastward propagating wave train which propagates from the East Atlantic/Mediterranean region and reaches the Indian subcontinent in four days. This wave train induces an anomalous cyclonic circulation over Indian landmass and provides south-to-north and west-to-east steering over the Bay of Bengal causing the cyclones to move in a north-northeastward direction. On the other hand, for west-northward moving cyclones, there is no Rossby wave intrusion over the Indian subcontinent, hence the cyclones move in west-northwestward direction assisted by the climatological winds which are from east to west over the south and central Bay of Bengal. This shows that the track of the cyclone in the north Indian Ocean can be modulated by the atmospheric changes in the extratropics and can act as a precursor for the prediction of the track of the cyclone in this region.