MERRA-NOBM (NASA Ocean Biogeochemical Model) Reanalysis
Model Description
Ocean biology plays an important role in global ecosystems, fisheries, and climate. Phytoplankton forms the base of the ocean food chain, determines the abundances and health of fisheries, and through the process of photosynthesis, plays an important role in the global carbon cycle. Although they represent only a minor fraction of the global carbon biomass, their uptake and turnover rates are so high that they represent about half the total primary production of the Earth.
The NASA Ocean Biogeochemical Model (NOBM) and the Ocean-Atmosphere Spectral Irradiance Model (OASIM) were developed at GMAO. While the legacy version of NOBM was coupled with the Poseidon ocean general circulation model, the NOBM is now coupled to the complex and interactive GEOS modeling system (Figure 1). The GEOS modeling system is comprised of components representing the atmosphere (AGCM), ocean (MOM), sea-ice (CICE), aerosols and carbon gases (GOCART), and ocean biogeochemistry (NOBM). The Modular Ocean Model (MOM) is a global model with nominal horizontal resolution of 0.25 degrees and 40 vertical levels, extends globally and covers inland and coastal waters to 10 m depth (Griffies, 2012). The NOBM utilizes these circulation fields and associated vertical mixing processes to produce horizontal and vertical distributions of constituents.
Figure 1: The GEOS-NOBM framework allows to directly assess the exchange and feedback between ocean and atmosphere in coupled mode and can also be run using the GEOS data atmosphere onlyand improve our understanding of the complex interactions between the atmosphere and the ocean physic and biogeochemistry.
The NOBM (Gregg and Casey, 2007, Figure 2) includes a coupled biogeochemical/radiative model and carries an explicit representation of most of the major carbon pathways in the oceans. These processes include fluxes at the surface, photosynthetic production and respiration, grazing and associated respiration, formation of detritus and remineralization by bacteria, carbonate chemical exchanges, and fluxes of inorganic and organic components (particulate and dissolved) at all levels in the global ocean. The NOBM contains 6 phytoplankton groups (diatoms, chlorophytes, cyanobacteria, coccolithophores, dinoflagellates, and the high latitude components of Phaeocystis spp.), 4 nutrient groups, a single herbivore group, and 3 detrital pools. The phytoplankton groups differ in maximum growth rates, sinking rates, nutrient requirements, and optical properties. The 4 nutrients are nitrate, regenerated ammonium, silica to regulate diatom growth, and iron. Three detrital pools provide storage of organic material, sinking, and eventual remineralization back to usable nutrients. Carbon cycling involves dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). Sources of DOC include phytoplankton, herbivores, and carbon detritus, and a sink to DIC. Sources of DIC include phytoplankton, herbivores, carbon detritus, and DOC, and exchange with the atmosphere, which can be either a source or sink. The ecosystem sink for DIC is phytoplankton, through the process of photosynthesis.
Figure 2: The NOBM includes the representation of several phytoplankton groups as well as the major carbon pathways in the oceans. It is coupled to the OASIM radiative transfer model and the MOM circulation model.
The NOBM is coupled to the Ocean-Atmosphere Spectral Irradiance Model (OASIM; Gregg and Carder, 1990; Gregg, 2002; Gregg and Casey, 2009) to simulate the propagation of downward spectral irradiance in the atmosphere and oceans and the upwelling irradiance/radiance in the oceans (Figure 3). The atmosphere and ocean portions of the irradiance are implemented at variable spectral resolution over the range 200 nm-4 μm, depending upon the major atmospheric and oceanic absorbing sources. Three irradiance paths are enabled: a downwelling direct path, a downwelling diffuse (scattered) path, and an upwelling diffuse path. There are 33 spectral bands, which account for >99% of the total solar extraterrestrial irradiance. For the Photosynthetically Active Radiation (PAR) spectral region (defined here as 350-700 nm following historical precedent) the model utilizes 25nm spectral resolution. The atmospheric component of OASIM tracks irradiance through cloudy and clear skies, accounting for spectral absorption and scattering of atmospheric gases, clouds, and aerosols. Biases and uncertainties in the atmospheric component of OASIM have been characterized for clear sky high spectral resolution (1nm; Gregg and Carder, 1990) and under mixed cloudy and clear skies for 25 nm spectral resolution (Gregg and Casey,2009). The atmospheric portion of OASIM has been validated and published (Gregg and Casey 2009).
Figure 4: The absorption and scattering properties used by OASIM are derived from a variety of carefully controlled laboratory observations.
Oceanic radiative properties are driven by water absorption and scattering, detrital absorption and scattering, PIC, Colored Dissolved Organic Carbon (CDOC), and the optical properties of the phytoplankton groups (Gregg 2002). All oceanic radiative calculations include the spectral nature of the irradiance. Phytoplankton group-specific optical properties are derived from a variety of carefully controlled laboratory observations (Ahn et al. 1992; Bricaud et al. 1988; Bricaud et al. 1983; Morel 1988; Morel and Bricaud 1981; Sathyendranath et al. 1987). These characterizations are very similar to the compilation by Stramski et al. (2001) with different phytoplankton classification.
See References.
For more information contact: Cecile S. Rousseaux (cecile.s.rousseaux@nasa.gov) and Lionel Arteaga (lionel.arteagaquintero@nasa.gov)
Page author: Watson Gregg & Cecile S. Rousseaux
