Abstract
The northern Indian Ocean, comprising of two marginal seas, the Arabian Sea (AS) and the Bay of Bengal (BoB), is known for the occurrence of tropical cyclones. The simultaneous occurrence of the cyclones Luban in the AS and Titli in the BoB is a rare phenomenon, and, in the present study, we examined their contrasting upper ocean responses and what led to their formation in October 2018. Being a category-2 cyclone, the maximum cooling of sea surface temperature associated with Titli was 1°C higher than that of Luban, a category-1 cyclone. The higher tropical cyclone heat potential in the BoB compared with the AS was one of the reasons why Titli was more intense than Luban. The enhancement of chlorophylla(Chl-a) and net primary productivity (NPP) by Luban was 2- and 3.7-fold, respectively, while that by Titli was 3- and 5-fold, respectively. Despite this, the magnitudes of both Chl-aand NPP were higher in the AS compared with the BoB. Consistent with physical and biological responses, the CO2outgassing flux associated with Titli was 12-fold higher in comparison to the pre-cyclone value, while that associated with Luban was 10-fold higher. Unlike the Chl-aand NPP, the magnitude of CO2flux in the BoB was higher than that in the AS. Although the cyclones Luban and Titli originated simultaneously, their generating mechanisms were quite different. What was common for the genesis of both cyclones was the pre-conditioning of the upper ocean in 2018 by the co-occurrence of El Niño and the positive phase of Indian Ocean dipole along with the cold phase of the Pacific decadal oscillation, all of which worked in tandem and warmed the AS and parts of the BoB. What triggered the genesis of Luban in the AS was the arrival of the Madden–Julian oscillation (MJO) and the mixed Rossby-gravity wave during the first week of October. The genesis of Titli in the BoB was triggered by the eastward propagation of the MJO and the associated enhanced convection from the AS into the region of origin of Titli along with the arrival of the downwelling oceanic Rossby wave.
Highlights
The North Indian Ocean (NIO), consisting of the Arabian Sea (AS) and the Bay of Bengal (BoB), is one of the warmest tropical oceans and accounts for ∼7% of the global tropical cyclones that occur annually (Gray, 1985)
The simultaneous occurrence of cyclones over the AS and the BoB in the NIO is an unusual phenomenon that happened in October 2018 for the first time since the reliable record became available in 1960
The higher TCHP in the BoB compared with the AS was one of the reasons why Titli was more intense than Luban
Summary
The North Indian Ocean (NIO), consisting of the Arabian Sea (AS) and the Bay of Bengal (BoB), is one of the warmest tropical oceans and accounts for ∼7% of the global tropical cyclones that occur annually (Gray, 1985). To understand the contribution of each term on the mixed layer heat budget, we have analyzed the simplified version of the mixed layer heat balance followed by Vialard et al (2008) and Foltz and McPhaden (2009), as shown in the following equation For this calculation, we have taken temperature, salinity, and zonal and meridional water velocity data from HYCOM. T is the vertically averaged mixed layer temperature, ρ is the density of the seawater with a value of 1,026 kg/m3, Cp is the specific heat capacity of seawater at constant pressure, t is the time, h is the mixed layer depth, Qnet is the net heat flux (W/m2) at the surface, and Qpen is the penetrating shortwave radiation below the mixed layer and is calculated as follows: Qpen = Qsw×0.47×exp(−z/hν). Box A in the AS (11–16.5◦N; 52.2–67.2◦E) and Box B in the BoB (13.7–18.7◦N; 84.4–88.9◦E) encompassed the tracks of cyclones Luban and Titli, respectively (see Figure 1)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
