Quantifying Tropical Cyclone's Effect on the Biogeochemical Processes Using Profiling Float Observations in the Bay of Bengal

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 50m 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 0.15mg/m(3). Storm mixing initially increased the chlorophyll by 1.4mg/m(3), increased the surface nitrate concentration to about 6.6M/kg, and decreased the subsurface dissolved oxygen (30-35m) to 31% of saturation (140 mu M). These conditions were favorable for phytoplankton growth resulting in an estimated increase in primary productivity averaging 1.5gC.m(-2).day(-1) over 15days. During this bloom, chlorophyll-a increased by 3.6mg/m(3), 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.