Evaluation of model-simulated tropical cyclone response on the biogeochemical parameters using profiling float...

In this study, the measurements from a Biogeochemical Argo float (A-float) were used to evaluate the capability of coupled bio-physical model using Regional Ocean Model System (ROMS) in simulating the physical and biogeochemical state during the Tropical Cyclone (TC) Ockhi in the northeastern Arabian Sea (16.36°N and 69.66°E). In response to TC Ockhi, the mixed layer depth deepened upto 49 m depth due to strong winds-induced vertical mixing; however, the mixing influence can be evident upto 95 m depth from the A-float measurements. Due to this strong wind-induced vertical mixing and upwelling, the near-surface chlorophyll from A-float initially increased to 0.3 mg m^-3 and gradually enhanced to a maximum of 3.6 mg m^-3, within seven days after the passage of TC. This increase in chlorophyll leads to the enhancement of the primary productivity within euphotic depth by 1 g C m^-2 day^-1 compared to the pre-TC value. It is also found that, the TC-induced vertical mixing leads to a reduction (11 μM) in dissolved oxygen (DO) initially at the near-surface, followed by an increase of DO by 25 μM due to enhancement of the primary productivity. Although the ROMS model exhibited a similar temporal evolutions as apparent in observations, it consistently overestimated the near-surface chlorophyll and DO throughout the study period, except during the peak enhancement phase due to post-TC impact, where it showed slight underestimation (2 mg m^-3). In response to TC, the ROMS simulated chlorophyll concentration increased to 1.7 mg m^-3 due to enhancement of nitrate concentration (0.5 μM), which increased the estimated average primary productivity to 0.9 g C m^-2 day^-1 at the near-surface. The ROMS estimated chlorophyll attains the maximum peak at the near-surface within four days after the passage of TC, as compared with seven days from observations.