Concurrent cyclogenesis in the Northern Indian Ocean and Monsoon Onset over Kerala in response to different MJO phases

Madden Julian Oscillation (MJO), one of the dominant intra-seasonal oscillations over the tropics, plays a major role in preconditioning the atmosphere and ocean prior to the monsoon onset over Kerala (MOK). However, the onset process exhibits large variability even when MJO exists in the same phase. This study attempts to un-derstand the characteristics of MOK during different MJO phases and the cyclogenesis process in the north Indian Ocean in concurrence with MOK. During the study period from 1980 to 2020, almost 80% of MOK have occurred in MJO phases I, II and III. The remaining 20% MOK occurred in all other phases of MJO except in phase VIII. The onset days are categorized as early (before 30th May), normal (30th May to 03rd June) and delayed (after 03rd June), and their occurrences under different MJO phases are examined. Various environmental parameters associated with the MJO phases were analysed to understand the cyclogenesis process close to the time of the MOK. Phase I is conducive to cyclogenesis over the Arabian Sea, whereas phase III favours that in the Arabian Sea and the Bay of Bengal. MJO phase II does not support cyclogenesis during the MOK. The decrease in the positive anomaly of lower-level westerlies towards the north of the equator gives rise to cyclonic shear vorticity, which supports the formation of cyclones during phases I and III. Associated with MOK, the total precipitable water shows a negative anomaly in phases I and II of MJO and a positive anomaly in phase III. The convective activity in the north Indian Ocean remains enhanced in phase III, where the amplitude of MJO is also found to be maximum.