Tropical cyclones in the Arabian Sea and the Bay of Bengal: comparison of environmental factors

The Northern Indian Ocean is vulnerable to tropical cyclone (TC) related hazards that adversely affect people, infrastructure, and economies in the region. Considering the region's importance, supportive environmental factors for TC (> 17 knots) activity in the Arabian Sea (AS) and the Bay of Bengal (BoB) have been examined utilizing ocean-atmosphere datasets (1891-2016). The reasons for more TCs during 2011 and 2015 and less during 2013 in the AS than the BoB have been discussed. A decreasing (increasing) trend in the TC frequency is observed in the BoB (AS), which correlates negatively (positively) with sea surface temperature (SST). Though the TC frequency is larger in the BoB, the AS has experienced higher TC frequency on five occasions (1902, 2001, 2004, 2011 and 2015) during the 1891-2016 period. The observed trend in the Indian Ocean Dipole (IOD) index emphasizes a positive impact on the TC genesis in the AS. Results revealed that warmer SSTs supported by the co-occurrence of El Nino/Southern Oscillation (ENSO) and IOD events associated with a relatively deep 26 degrees C isothermal layer (D26) enhanced the TC formation in the AS during 2011 and 2015. The TC genesis is suppressed under the neutral conditions of ENSO and IOD (i.e., 2013) in the AS, and that brings the relative importance of the SST cooling associated with a deeper mixed-layer and shallow D26. Further, the observed differences in parameters between the regions are larger during the primary TC peak season (October - December). Although a recent increment of TCs is noted in the AS (compared to the BoB), the specific roles of the influencing factors on TC activities over the AS remain debatable.