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.
During some years, a synoptic scale vortex called the Monsoon Onset
Vortex (MOV) forms within the northward advancing zone of precipitating
convection over the Arabian Sea. The MOV does not form each year and the
reason is unclear. Since the Madden-Julian Oscillation (MJO) is known to
modulate convection and tropical cyclones in the tropics, we examined
its role in the formation of the MOV. While the convective and
transition phases of the MJO do not always lead to MOV formation, the
suppressed phase of the MJO hinders the formation of the MOV more
consistently. This non-linear relationship between the MJO and MOV can
be partially explained by the modulation of the large-scale environment,
measured by a tropical cyclone genesis index. It also suggests that the
Arabian Sea is generally near a critical state that is favorable for MOV
formation during the monsoon onset period.
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