Biogeochemical State of the Indian Ocean (BIO) is a high resolution, coupled physical-biogeochemical modeling system developed at INCOIS to study the evolution of biogeochemical state of the Indian Ocean at both short and long time scales. This system has been developed using the Regional Ocean Modeling System (ROMS).
Ocean models have a unique ability to integrate our empirical and theoretical understanding of the marine environment. Ecosystem Modeling is a key scientific technique by which we can elucidate the mechanism of the marine system and predict its evolution in both short and long term. Coupled physical-biological models, thus, become a useful tool to study biogeochemical and ecological responses to physical forcing and for this reason there has been considerable effort in the recent years to simulate biogeochemical processes of the Northern Indian Ocean using different suites of models. The fundamental modeling technology for operational forecast, climate change science and environmental risk assessment is of high strategic importance. This is particularly important when ocean warming scenarios suggest that consequent upper ocean stratification may lead to ocean deoxygenation (up to ~1-7% decrease) and expansion of sub-surface hypoxia. Such forecasts are supported by recent studies that conclusively report loss of dissolved oxygen in the global ocean within the past few decades.
To address these operational and scientific needs, a suite of high resolution, coupled physical-biogeochemical models have been configured by the scientists of Indian National Centre for Ocean Information Services (INCOIS) under the modeling projects of the Ministry of Earth Sciences which integrates ocean simulation, observation and analysis to study the role ocean physics plays in marine environmental health and ecosystem functioning in the coastal ocean and adjacent deep sea. The modeling framework involves an online coupling of the Regional Ocean Modeling System (ROMS) physics/dynamics integrated with an ecosystem model. The suite of high resolution models include two very high resolution physical-biogeochemical models with horizontal resolution 1/48° (approximately 2.25 km spatially averaged) exclusively for the east and west coast of India, and a high resolution physical-biogeochemical model with horizontal resolution 1/12° (ROMS-1/12°; approximately 9km spatially averaged) for the entire Indian Ocean basin. ROMS-1/48° models viz. the east coast model (77°E to 99°E, 04°N to 23°N; red colour box in the Figure) and the west coast model (ROMS-1/48°; 65°E to 77.5°E, 08°N to 26°N, red colour box in the Figure) are nested with ROMS-1/12° model (30°S to 30°N; 30°E to 120°E). The boundary conditions for ROMS-1/48° models are prescribed from the ROMS-1/12° model. All the regional scale models use 40 vertical levels in a terrain-following s-coordinate system.
The biological component of the high resolution, coupled modeling system consists of the nitrogen cycle model with parameterized sediment denitrification described by Fennel et al. (2006). The nitrogen cycle model includes seven state variables viz. phytoplankton, zooplankton, nitrate, ammonium, large and small detritus classes with nitrogen concentration and phytoplankton chlorophyll. The time rate of change of concentration of each state variable describes the balance of advection-diffusion and source-sink terms among the related state variables of the nitrogen cycle. The biological model also resolves the full carbon cycle. The model carbonate chemistry is described in the coupled set-up following Zeebe and Wolf-Gladrow (2001) and Fennel et al. (2008). The full carbon cycle is represented in the model using four state variables viz. alkalinity, dissolved inorganic carbon, large and small detritus class with carbon concentration. The dynamics of dissolved inorganic carbon includes the primary production and respiration as sinking and source processes respectively following redfield stoichiometry besides gas exchange at the air-sea interface. The biogeochemical processes, such as calcite formation and dissolution, nitrate uptake and regeneration, and sulphate reduction are represented in the dynamics of alkalinity. The partial pressure of carbon dioxide pCO2 is calculated in the surface layer as described in the Fennel et al. (2008). Oxygen is included as model tracer and biogeochemical dynamics of oxygen is described in the model following Fennel et al. (2013). The primary production is estimated by making use of the Vertically Generalized Productivity Model (VGPM; Behrenfeld and Falkowski, 1997). The optimal rate of productivity i.e., optimal water column carbon fixation is modeled as a seventh order polynomial function of SST in VGPM. However, in INCOIS ROMS applications, the relationship between optimal rate of productivity and SST follows the exponential relationship by Eppley (1972), as modified by Antoine et al. (1996).
The National Centre for Medium Range Weather Forecast (NCMRWF) developed a data assimilated Unified Model (NCUM) at a horizontal resolution of 0.125 for generating10-day numerical weather forecasts. NCUM simulated analyses and forecasts are made available to INCOIS to force the ocean model. Three-hourly analyzed atmospheric forcing fields obtained from the NCMRWF are used to force the ocean models configured at INCOIS. Atmospheric forcing component includes air pressure at 2 m, air temperature at 2 m, net shortwave and longwave flux, rainfall rate and wind velocity. Surface heat and momentum fluxes are internally calculated by ROMS using the bulk parameterizations. The model runs for 5 days in hind-cast mode followed by 5 days in forecast mode thereby regularly updating to generate daily analysis of biogeochemical state of the Indian Ocean.
