Upper ocean biophysical budget analysis during a cyclone using Regional Ocean Modeling System

Abstract

The biophysical response of upper ocean during the passage of the very severe cyclonic storm ‘Titli’ over Bay of Bengal (BoB) during October 2018 is studied using the Regional Ocean Modeling System coupled with nutrient-phytoplankton-zooplankton-detritus (ROMS-NPZD) framework. Assessments with satellite and Argo observations show that the model forced by Scatsat-I winds and reanalysis fluxes simulates the changes in the upper ocean reasonably well. The biophysical variability is significantly realized in a cyclone-induced open-ocean upwelling region underlying the peak intensity of the cyclone. After the passage of the cyclone, surface chlorophyll-a concentration increases tenfold in the upwelling region. This increase of simulated surface phytoplankton further enhances the surface dissolved oxygen concentration by ∼10μM from an initial value of 200μM. Strong temporal correspondences between the depth of the thermocline ($D_{{23}^{\circ }C}$) and the oxycline (r = 0.97) and between nutricline and the mixed-layer depth (MLD) (r = 0.52) are also observed. A detailed term-by-term biophysical budget analysis is carried out with temporal tendency of temperature (and other biophysical parameters) subdivided into various components: (i) net surface heat (or buoyancy) flux; (ii) horizontal advection; (iii) vertical entrainment; (iv) vertical mixing; and (v) source and sink terms as necessary. Results show the cooling of upper layer temperature during cyclone is due to vertical processes (entrainment and mixing), which is recovered in the post-cyclone period due to MLD advection affecting entrainment and vertical mixing, followed by restoration of net surface heat flux. The increase in salinity during the cyclone is due to MLD tendency dominating vertical entrainment process, and the post-cyclone recovery is due to the horizontal advection and net surface freshwater flux. The shoaling of the nutricline allows for the transport of the nutrients from the deeper layers to the euphotic zone and enhancement of phytoplankton concentration at the surface about five days after the cyclone interaction. Budget analysis for biological processes showed the changes in the phytoplankton concentration are associated with (i) nitrate uptake by phytoplankton and (ii) zooplankton grazing on phytoplankton. This assessment of the state-of-the-art coupled ROMS-NPZD model emphasizes its applicability for various other applications on marine ecosystem modeling.