Eddy-induced Track Reversal and Upper Ocean Physical-Biogeochemical Response of Tropical Cyclone Madi in the Bay of Bengal

Abstract. The life cycle of the tropical cyclone Madi in the southwestern Bay of Bengal (BoB) during 6^th to 12^th December 2013 was studied using a suite of ocean and atmospheric data. Madi formed as a depression on 6^th December and intensified into a very severe cyclonic storm by 8^th December. What was distinct about Madi was its (1) swift weakening from very severe cyclone to a severe cyclone while moving towards north on 9^th, (2) abrupt track reversal close to 180-degree in a southwestward direction on 10^th, and (3) rapid decay in the open ocean by 12^th December while still moving southwestward. Using both in situ and remote sensing data, we show that oceanic cyclonic eddies played a leading role in the ensuing series of events that followed its genesis. The sudden weakening of the cyclone before its track reversal was facilitated by the oceanic cyclonic (cold-core) eddy, which reduced the ocean heat content and cooled the upper ocean through upward eddy-pumping of subsurface waters. When Madi moved over cyclonic eddy-core, its further northward movement was arrested. Subsequently, the prevailing northeasterly winds assisted the slow moving system to change its track to a southwesterly path. While travelling towards southwestward direction, the system rapidly decayed when it passed over the regions of cyclonic eddies located near the western boundary of the BoB. Though Madi was a category-2 cyclone, it had a profound impact on the physical and biogeochemical state of the upper ocean. Cyclone-induced enhancement in the chlorophyll a ranged from 5 to 7-fold, while increase in the net primary productivity ranged from 2.5 to 8-fold. Similarly, the CO₂ out-gassing into the atmosphere showed a 3.7-fold increase compared to the pre-cyclone values. Our study points to the crucial role oceanic eddies play in the life cycle of cyclone in the BoB. Eddies being ubiquitous and tropical cyclones occur twice a year in the BoB, there is an urgent need to incorporate them in the models for the better prediction of the cyclone track and intensity.