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Spatial variability in boundaries and thickness of oxygen minimum zone (OMZ) is derived based on measured dissolved oxygen data obtained from sensors on board biogeochemical (BGC) Argo floats between 2013 and 2019 in the Bay of Bengal (BoB). Upper and lower boundaries of the OMZ varied from 60 to 200 m and 100 to 800 m respectively with the thickness of 80–650 m in the BoB. Relatively thicker OMZ is noticed in the northern than southern BoB associated with stratification. The oxygen concentrations in the OMZ in the NW was low (<1.5 μM) than NE BoB (2.5 μM) indicating that thick and intense OMZ occurs in the NW region associating with stratification and high primary production. Significant decrease in particle‐back‐scatter signal was observed toward offshore from shelf indicating organic matter from the shelf sediments may be supporting bacterial carbon demand in the OMZ. The particle backscatter signal peaked in the OMZ region with a higher signal in the north than southern BoB and it is consistent with the low oxygen concentration in the former indicating that organic matter from shelf sediments may be supporting carbon needs in the OMZ. In addition to this, the occurrence of eddies significantly controls the intensity of the OMZ in the BoB through mixing at the upper boundary of OMZ. Therefore, this study suggests that spatial variations in intensity of OMZ in the BoB are governed by stratification, primary and export productions, organic matter decomposition, and eddy‐driven mixing. Plain Language Summary: Based on the dissolved oxygen data collected by Biogeochemical (BGC) Argo floats, we have shown for the first time the boundaries and thickness of the OMZ in the BoB. The occurrence of OMZ in the BoB is mainly caused by freshwater discharge from the majors rivers, that do inhibit mixing of atmospheric oxygen, primary production, export of dead organic matter to OMZ, and subsequent decomposition by bacteria, that consumes oxygen in the water, and mixing of oxygen by eddies. We noticed that the thick and intense OMZ exists in the northwestern BoB associating with stratification, high primary production, and slower rate of dead organic matter sinking to OMZ and weaker eddy activity. Key Points: The thick oxygen minimum zone (OMZ) occurred in the northern Bay of Bengal (BoB) associated with strong salinity stratificationParticle back‐scatter signal suggests cross‐shelf transport of organic matter may support heterotrophic carbon demand in the OMZIntense OMZ in the northwestern BoB associated with high primary production, and sinking carbon fluxes [ABSTRACT FROM AUTHOR]

Physical and biogeochemical observations from an autonomous profiling Argo float in the Bay of Bengal show significant changes in upper ocean structure during the passage of tropical cyclone (TC) Hudhud (7–14 October 2014). TC Hudhud mixed water from a depth of about 50 m into the surface layers through a combination of upwelling and turbulent mixing. Mixing was extended into the depth of nutricline, the oxycline, and the subsurface‐chlorophyll‐maximum and thus had a strong impact on the biogeochemistry of the upper ocean. Before the storm, the near‐surface layer was nutrient depleted and was thus oligotrophic with the chlorophyll‐a concentration of less than 0.15 mg/m3. Storm mixing initially increased the chlorophyll by 1.4 mg/m3, increased the surface nitrate concentration to about 6.6 μM/kg, and decreased the subsurface dissolved oxygen (30–35 m) to 31% of saturation (140 μM). These conditions were favorable for phytoplankton growth resulting in an estimated increase in primary productivity averaging 1.5 g C·m−2·day−1 over 15 days. During this bloom, chlorophyll‐a increased by 3.6 mg/m3, and dissolved oxygen increased from 111% to 123% of saturation. Similar observations during TC Vardah (6–12 December 2016) showed much less mixing. Our analysis suggests that relatively small (high) translation speed and the presence of cold (warm) core eddy leads to strong (weak) oceanic response during TC Hudhud (TC Vardah). Thus, although cyclones can cause strong biogeochemical responses in the Bay of Bengal, the strength of response depends on the properties of the storm and the prevailing upper ocean structure such as the presence of mesoscale eddies. Plain Language Summary: It is very difficult to quantify the upper ocean physical and biogeochemical response to extreme weather conditions like tropical cyclones (TCs). In situ vertical profiles of temperature, salinity, chlorophyll fluorescence, dissolved oxygen, nutrients, and optical backscatter from the biogeochemical‐Argo floats can provide unique opportunity to quantify the upper ocean biogeochemical response to tropical cyclone, if cyclones traverse over the floats. In this study, the proximity of a biogeochemical‐Argo float to TC Hudhud and TC Vardah tracks, having similar intensity when it was close to the float location, reveals that oceanic responses are quite higher in the former than the latter. Our results suggest that translation speed of the storm and background oceanic conditions such as the presence of mesoscale eddies determines the ocean biogeochemical evolution in response to TCs. Key Points: Upper ocean biogeochemical response to the tropical cyclone Hudhud and Vardah using BGC‐Argo float in the Bay of BengalPreexisting oceanic conditions and TC translation speed on biogeochemical responses are examinedThe importance of profiling float on the calculation of primary productivity is examined [ABSTRACT FROM AUTHOR]

Upper oceanographic and surface meteorological time-series observations from a moored buoy located at 9.98°N, 88°E in the south-western Bay of Bengal (BoB) were used to quantify variability in upper ocean, forced by a tropical cyclone (TC) Jal during November 2010. Before the passage of TC Jal, salinity and temperature profiles showed a typical BoB post-monsoon structure with relatively warm (30 °C) and low-saline (32.8 psu) waters in the upper 30- to 40-m layer, and relatively cooler and higher salinity (35 psu) waters below. After the passage of cyclone, an abrupt increase of 1 psu (decrease of 1 °C) in salinity (temperature) in the near-surface layers (up to 40-m depth) was observed from buoy measurements, which persisted up to 10-12 days during the relaxation stage of cyclone. Mixed layer heat budget analysis showed that vertical processes are the dominant contributors towards the observed cooling. The net surface heat flux and horizontal advection together contributed approximately 33 % of observed cooling, during TC Jal forced stage. Analysis showed the existence of strong inertial oscillation in the thermocline region and currents with periodicity of ∼2.8 days. During the relaxation stage of the cyclone, upward movement of thermocline in near-inertial frequencies played significant role in mixed layer temperature and salinity variability, by much freer turbulent exchange between the mixed layer and thermocline. [ABSTRACT FROM AUTHOR]