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.

中文翻译：

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疯狂的朱利安涛动对“旋风孤儿”（2020）和西南季风爆发的影响

Amphan旋风是孟加拉湾异常强的热带气旋，在2020年5月的活跃期内，其最低中心压力达到907 mb。在这项研究中，我们分析了导致旋回发生的海洋和地表大气状况，孟加拉湾的这场风暴，以及导致气旋形成的过程（如Madden-Julian涛动（MJO）和Amphan本身）如何反过来影响了西南季风的预处理和爆发。为此，我们采用了多参数方法，将近实时卫星观测，海洋模型预测和重新分析相结合，以更好地理解所涉及的过程。我们发现，第二条向下的开尔文波到达孟加拉赤道湾，再加上较高的海洋高热量和MJO对流相的位置，有助于为循环形成创造条件，其中MJO向北传播的分支触发了循环形成。这次MJO事件与Amphan一起，为阿拉伯东南部的东南部极大地增加了大气湿度，并建立了低层西风，使西南季风于6月1日开始气候变化。