Back-to-Back Occurrence of Tropical Cyclones in the Arabian Sea During October-November 2015: Causes and Responses

In the Arabian Sea, two extremely severe cyclonic storms occurred back to back during October-November 2015. Using a suite of ocean and atmospheric data, we examined the upper ocean responses of tropical cyclones Chapala and Megh, the latter originated immediately after the dissipation of the former. Cyclones Chapala and Megh cooled the sea surface by 1.5 degrees C and 1.0 degrees C, respectively, which was also captured by Bio-Argo float in the vicinity of their tracks. The cyclone-induced chlorophyllaenhancement was 6 and 2 times respectively from their precyclone value of 0.36 and 0.30 mg/m(3), while the net primary productivity showed an increase of 5.8 and 1.7 times respectively from the precyclone values of 496.26 and 518.63 mg C/m(2)/day after the passage of Chapala and Megh. The CO(2)flux showed a sixfold and twofold increase respectively compared to the precyclone value of 2.69-3.58 and 6.78 mmol/m(2)/day. We show that the anomalous co-occurrence of the positive phase of Indian Ocean Dipole and the strong El Nino supported large-scale warming in the western Arabian Sea. Subsequent altered Walker circulation played a decisive role in the preconditioning of both atmosphere and ocean, which led to a situation congenial for cyclogenesis. What triggered the formation of the cyclone Chapala was the arrival of eastward-propagating atmospheric convection associated with the Madden-Julian oscillation. The generation of the second cyclone Megh within 8 days appears to be associated with the oceanic Rossby wave that led to an increase in the upper ocean heat content by deepening the 26 degrees C isotherm in the vicinity of the origin of cyclones.