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The Indian summer monsoon rainfall (ISMR) exhibits significant variability, affecting the food and water security of the densely populated Indian subcontinent. The two dominant spatial modes of ISMR variability are associated with the El Niño Southern Oscillation (ENSO) and the strength of the semi-permanent monsoon trough along with related variability in monsoon depressions, respectively. Although the robust teleconnection between ENSO and ISMR has been well established for several decades, the major drivers leading to the time-varying relationship between ENSO and ISMR patterns across different regions of the country are not well understood. Our analysis shows a consistent increase from a moderate to substantially strong teleconnection strength between ENSO and ISMR from 1901 to 1940. This strengthened relationship remained stable and strong between 1941 and 1980. However, in the recent period from 1981 to 2018 the teleconnection decreased consistently again to a moderate strength. We find that the ENSO–ISMR relationship exhibits distinct regional variability with time-varying relationship over the north, central, and south India. Specifically, the teleconnection displays an increasing relationship for north India, a decreasing relationship for central India and a consistent relationship for south India. Warm SST anomalies over the eastern Pacific Ocean correspond to an overall decrease in the ISMR, while warm SST anomalies over the Indian Ocean corresponds to a decrease in rainfall over the north and increase over the south of India. The central Indian region experienced the most substantial variation in the ENSO–ISMR relationship. This variation corresponds to the variability of the monsoon trough and depressions, strongly influenced by the Pacific Decadal Oscillation and North Atlantic Oscillation, which regulate the relative dominance of the two spatial modes of ISMR. By applying the PCA-Biplot technique, our study highlights the significant impacts of various climate drivers on the two dominant spatial modes of ISMR which account for the evolving nature of the ENSO–ISMR relationship. [ABSTRACT FROM AUTHOR]

The cyclones during November in the Bay of Bengal follow two distinct tracks. Analysis for the period 1982–2019 shows that some cyclones move west‐northwestward and make landfall at the Odisha, Andhra Pradesh or Tamil Nadu coast of India, or the Sri Lanka coast, while others move north‐northeastwards and make landfall at the West Bengal, Bangladesh or Myanmar coast. Our analysis shows there is a significant 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 part of a subtropical Rossby wave train triggered by an anomalous upper‐level convergence over the Mediterranean region. This wave train propagates along the subtropical westerly jet from the east Atlantic/Mediterranean region and reaches the Indian subcontinent in 4 days. It induces an anomalous cyclonic circulation over the 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‐northwestward moving cyclones, there is no Rossby wave intrusion over the Indian subcontinent, hence the cyclones move in a west‐northwestward direction assisted by the beta effect and climatological winds which are from east to west over the south and central Bay of Bengal. This shows that the track of cyclones in the north Indian Ocean can be modulated by atmospheric changes in the extratropics and can act as a precursor for the prediction of the track of cyclones in this region. [ABSTRACT FROM AUTHOR]

Generally, monsoon conditions are unfavourable for cyclogenesis in the Arabian Sea due to the strong vertical wind shear present over the basin which prevents the vertical development of cyclones. However, there are instances where tropical cyclones form over the basin during the onset phase of the Indian summer monsoon (ISM). The current study examines the evolution of the synoptic conditions in the years during 1979–2019 in which a cyclone forms during the onset phase of the ISM. The effect of various atmospheric and oceanic factors, the modulation of wind circulation over the Indian subcontinent and the north Indian Ocean region due to the possible influence of tropical and extratropical weather systems has been taken into consideration. Our analysis indicates that a weak mean monsoonal wind circulation caused by the southward intrusion of subtropical westerlies (at 200 hPa) is conducive for cyclogenesis over the Arabian Sea during the monsoon onset phase. Such a weak onset phase of the monsoon, coupled with the seasonal warm SSTs, may lead to cyclone formation in the Arabian Sea. [ABSTRACT FROM AUTHOR]