An Analysis of the Ocean Coupling Potential Intensity in North Indian Ocean Tropical Cyclones

The North Indian Ocean (NIO) tropical cyclone (TC) basin stretches over two large bodies of water that straddle the Indian subcontinent. Sixteen of the twenty deadliest TCs in recorded history have originated from the NIO (https://www.wunderground.com/hurricane/deadlyworld.asp). While TCs generally occur more frequently in the three other Northern Hemisphere TC basins (North Atlantic (NA), West Pacific (WP) and East Pacific (EP)), TCs originating in the NIO basin pose the greatest human risk, in part because of the location of numerous population centers in coastal regions vulnerable to storm surge flooding, coupled with a relatively high probably of landfall.

In this study the authors conduct an analysis of the maximum potential intensity (MPI) of TCs originating in the NIO basin, which includes both the Bay of Bengal (BoB) and Arabian Sea (AS). The MPI is defined as the theoretical maximum intensity attained by a TC under ideal conditions (Rotunno and Emanuel 1987, Emanuel 1988, 1995). The MPI framework, as developed in Rotunno and Emanuel (1987) and Emanuel (1988), describe TCs as Carnot cycle heat engines in which latent heat energy extracted from the sea surface can be used to generate kinetic energy, which is then balanced by frictional dissipation. The sea surface temperature (SST) determines the efficiency of heat and energy extraction from the sea. The MPI derived using operational or reanalysis derived SST can be used as a relatively good predictor of the intensity in strong TCs. However, the PI theory omits several environment-TC interactions which act to decrease the realized MPI of TCs. One of these interactions is the cooling of the sea surface beneath the storm via entrainment of colder water from below the ocean mixed layer. Several recent studies have augmented the MPI theory by calculating MPI using a depth averaged temperature of the ocean, instead of SST, to attempt to account for TC induced cooling of the sea surface. While this type of study has been conducted in the NA, WP, and EP TC basins (Lin et al. 2013; Balaguru et al. 2015), it has not been conducted in the NIO basin.

The MPI calculated using an ocean mean temperature (OPI) at 50m, 100, and 150m is compared to the MPI calculated using the SST (SST-MPI) for all NIO TCs from 1998-2016. An analysis shows that the OPI is a better predictor of future intensification as well as the storm lifetime maximum intensity, than the SST-MPI. This is in agreement with previous studies in other TC basins. Taking into account the storm translation speed further improves the agreement of OPI with the observed intensity. When partitioning between storms which originated from the Arabian Sea (AS) and Bay of Bengal (BoB) we find large differences in the agreement between OPI, MPI-SST and observed intensity. We speculate the that differing ocean mixed layer depths and conditions in the BoB compared to the AS are a possible reason for this difference.