Jayu Narvekar

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°C and 1.0°C, respectively, which was also captured by Bio‐Argo float in the vicinity of their tracks. The cyclone‐induced chlorophyll a enhancement was 6 and 2 times respectively from their precyclone value of 0.36 and 0.30 mg/m³, 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/m²/day after the passage of Chapala and Megh. The CO₂ flux showed a sixfold and twofold increase respectively compared to the precyclone value of 2.69–3.58 and 6.78 mmol/m²/day. We show that the anomalous co‐occurrence of the positive phase of Indian Ocean Dipole and the strong El Niño 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°C isotherm in the vicinity of the origin of cyclones.

The eastern Arabian Sea (EAS) is a region of complex interplay between several spatially and temporally varying oceanographic processes. Using two sets of in situ data collected from the same stations, separated by a decade, we show that warming and cooling in the EAS were linked to the long‐term variability of this region. Though the warming in the southern part of EAS was consistent with the basin‐wide rise in sea surface temperature in the Arabian Sea (AS), the enhanced freshening was a remote response to episodic events mediated by the strengthening of interbasin exchange between Bay of Bengal and AS through East India Coastal Current and West India Coastal Current. The increased stratification in the southern part of EAS led to the decline in the chlorophyll over the decade. In contrast, enhanced chlorophyll in the northern part of the EAS over the decade was linked to the increased wind speed.

The mixed layer is the most variable and dynamically active part of the marine environment that couples the underlying ocean to the atmosphere and plays an important role in determining the oceanic primary productivity. We examined the basin-scale processes controlling the seasonal variability of mixed layer depth in the Bay of Bengal and its association with chlorophyll using a suite of in situ as well as remote sensing data. A coupling between mixed layer depth and chlorophyll was seen during spring intermonsoon and summer monsoon, but for different reasons. In spring intermonsoon the temperature-dominated stratification and associated shallow mixed layer makes the upper waters of the Bay of Bengal nutrient depleted and oligotrophic. In summer, although the salinity-dominated stratification in the northern Bay of Bengal shallows the mixed layer, the nutrient input from adjoining rivers enhance the surface chlorophyll. This enhancement is confined only to the surface layer and with increase in depth, the chlorophyll biomass decreases rapidly due to reduction in sunlight by suspended sediment. In the south, advection of high salinity waters from the Arabian Sea and westward propagating Rossby waves from the eastern Bay of Bengal led to the formation of deep mixed layer. In contrast, in the Indo–Sri Lanka region, the shallow mixed layer and nutrient enrichment driven by upwelling and Ekman pumping resulted in chlorophyll enhancement. The mismatch between the nitrate and chlorophyll indicated the inadequacy of present data to fully unravel its coupling to mixed layer processes.

The high‐frequency time series data collected during 28th October to 21st November 2006 onboard R/V Mirai provided the first evidence of short‐term bio‐physical coupling in the central equatorial Indian Ocean (EIO). The predominant feature of the thermal structure was the down‐sloping of 28°C‐isotherm and up‐sloping of 14°C‐isotherm indicating deepening of surface‐layer and shoaling of lower‐thermocline respectively. Concomitant with the deepening of surface‐layer, nitracline and subsurface chlorophyll maximum (SCM) also deepened. From the prominent bi‐weekly periodicity in the 14°C‐isotherm and the upward phase propagation in the meridional current velocity, we infer that shoaling was the response of mixed Rossby‐Gravity (MRG) waves. Based on temperature and currents we propose that deepening of surface‐layer was caused by the eastward advection of warmer waters associated with the weakening of Indian Ocean Dipole (IOD). This made the upper ocean more nutrient‐depleted and biologically less‐productive, an indication that the central EIO is returning to its pre‐IOD condition.