Role of Multi-Scale Atmospheric Interactions in governing the Tropical Cyclone Movement over the Bay of Bengal

Abstract

Large-scale atmospheric variability, operating on sub-seasonal (10-90 days) to inter-annual (≥91 days) timescales, is known to influence the formation and intensity of tropical cyclones (TC) over the Bay of Bengal (BoB) region. Nevertheless, their role as individually or synergistically governing TC movement over the BoB region is less known. This study, using the Weather Research and Forecasting (WRF-ARW) model, investigates how the interaction of these modes of variability with the mean background environment modulates the movement of two pre-monsoon TCs, namely Fani (2019) and Amphan (2020). Our analysis based on streamline flows and vorticity budget processes reveals that sub-seasonal atmospheric variability, ranging from quasi-biweekly (10–20 days; QBWOs) to intra-seasonal (20–90 days; ISOs) timescales, plays a critical role in determining the two cyclone tracks. The spatiotemporal evolution of large-scale streamline flows in the model sensitivity experiments highlights that a peculiar variation in the establishment of midlatitude troughs and anticyclonic circulation arose by an interaction of sub-seasonal flow fields with the mean background environment and eventually leads to cyclone track deflections. Further, vorticity budget analysis suggests that, beyond horizontal advection, tilting processes induced by sub-seasonal variability also significantly contribute to the vortex development, ultimately influencing the observed cyclone paths. Our findings suggest that pre-monsoon tropical cyclone track prediction on sub-seasonal timescales can be better achieved by accurately incorporating subseasonal flows and their variability into the numerical models.