Abstract
Cyclone Amphan was an exceptionally strong tropical cyclone in the Bay of Bengal that achieved a minimum central pressure of 907 mb during its active period in May 2020. In this study, we analyzed the oceanic and surface atmospheric conditions leading up to cyclogenesis, the impact of this storm on the Bay of Bengal, and how the processes that led to cyclogenesis, such as the Madden–Julian Oscillation (MJO) and Amphan itself, in turn impacted southwest monsoon preconditioning and onset. To accomplish this, we took a multiparameter approach using a combination of near real time satellite observations, ocean model forecasts, and reanalysis to better understand the processes involved. We found that the arrival of a second downwelling Kelvin wave in the equatorial Bay of Bengal, coupled with elevated upper ocean heat content and the positioning of the convective phase of the MJO, helped to create the conditions necessary for cyclogenesis, where the northward-propagating branch of the MJO acted as a trigger for cyclogenesis. This same MJO event, in conjunction with Amphan, heavily contributed atmospheric moisture to the southeastern Arabian Sea and established low-level westerlies that allowed for the southwest monsoon to climatologically onset on June 1.
Highlights
Tropical cyclones are among the most powerful atmospheric systems on the planet, playing a critical role in heat and energy transport, impacting mixed layer dynamics and aquatic ecosystems, and having devastating effects on coastal populations [1,2,3,4]
While there have been several studies that have analyzed the impact of atmospheric Kelvin waves on cyclogenesis [5,14,17,19], finding them to have the greatest impact on cyclogenesis in Northern Hemisphere in spring [5], fewer have focused on the impact oceanic planetary waves have on cyclogenesis [4]
The Madden–Julian Oscillation (MJO) is the dominant form of intraseasonal variability in the tropics that impacts tropical cyclogenesis and informs monsoon strength and onset
Summary
Tropical cyclones are among the most powerful atmospheric systems on the planet, playing a critical role in heat and energy transport, impacting mixed layer dynamics and aquatic ecosystems, and having devastating effects on coastal populations [1,2,3,4]. The most severe cyclones in the Bay of Bengal occur during positive IOD and La Niña years [8], and this was certainly the case with Cyclone Amphan, which reached a minimum central pressure of 907 mb after rapid intensification and was categorized as a Very Severe Cyclonic Storm (equivalent of category 5 on the Saffir–Simpson scale). The timing of this storm was unique because it occurred just prior to a southwest monsoon onset on June 1 and simultaneously with an equatorially downwelling Kelvin wave in the Bay of Bengal and the Madden–Julian Oscillation (MJO)
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