Modelling Studies of Arabian Sea Physics

Shipboard and moored measurements can only sample the ocean at fixed times or locations. A numerical model that is adequately verified by in situ measurements can extend, interpolate, and extrapolate these measurements in time and space. Moreover, numerical models can be used to plan in situ measurement strategies to ensure that the relevant features of the ocean are adequately sampled. A coupled physical-biological model provides a mechanism for integrating satellite altimeter, SST and ocean color data with in situ measurements into a common coherent framework for analysis and interpretation. This will become particularly useful with the launches of the Topex-Poseidon altimeter and the SeaWiFS ocean color sensor. A coupled model will allow the maximum exploitation of the satellite ocean color data and other remotely sensed variables.

Numerous modelling efforts have sought to explain the observed flows in the tropical and subtropical northern Indian Ocean, with particular attention given to the semi-annual reversals in the Somali Current along the east coast of Africa (e.g., Cox, 1970, 1976, 1979; Hurlburt and Thompson, 1976; Lin and Hurlburt, 1981; Luther and O'Brien, 1985; Luther et al., 1985; McCreary and Kundu, 1988, 1989; Kindle and Thompson, 1989; see Luther (1987) or Knox (1987) for a review).

Luther et al. (1990), Prell et al. (1990), and Brock et al. (1991), using a reduced-gravity ocean circulation model of the Arabian Sea, were able to qualitatively reproduce much of the spatial and temporal variability observed in upwelling/downwelling and primary production. Reduced gravity models are efficient in that they retain the physics necessary to reproduce many of the observed features of ocean circulation while remaining relatively simple to analyze. In these simulations, nutrient injection into the photic zone and biological activity are inferred from upward movements of the model pycnocline, since the model does not include active thermodynamics or mixed-layer processes. Biological activity, particularly primary production, will be better quantified by the addition of mixed-layer physical dynamics and biological processes to the numerical models.

The ocean circulation model developed by Luther and O'Brien and collaborators (Luther and O'Brien, 1985; Luther et al., 1985; Simmons et al., 1988; Woodberry et al., 1989; Luther and O'Brien, 1989; Jensen, 1991; Potemra et al., 1991) is being coupled with the photosynthesis-irradiance (P-I) model of Platt and Sathyendranath (Platt and Sathyendranath, 1988; Sathyendranath and Platt, 1989; Sathyendranath et al., 1989; Platt and Sathyendranath, 1991). John Brock, Trevor Platt, and Shuba Sathyendranath of the Bedford Institute of Oceanography are collaborating with Mark Luther in this effort. A mixed layer parameterization with active thermodynamic forcing as in McCreary and Kundu (1989) is being added to the ocean circulation model in cooperation with J. P. McCreary at Nova University. The inclusion of mixing and thermodynamic processes in the ocean circulation model is essential for the accurate determination of sea surface temperature and for the determination of the parameters needed by the P-I model.

Modelling efforts for the Indian Ocean are also underway by J. P. McCreary and P. Kundu at Nova University (McCreary and Kundu, 1988, 1989), by J. C. Kindle at the Naval Research Laboratory (Kindle and Thompson, 1989), and by D. L. T. Anderson and collaborators at Oxford University, and G. R. Bigg and collaborators at East Anglia University (Anderson et al., 1991; Bigg et al., 1992). The Nova and NRL models are very similar to the Luther and O'Brien model. All three models use a layered, reduced-gravity formulation with very high horizontal resolution (order 10 km). The major differences among the models are in the mixing parameterizations and the boundary conditions. Luther, Kindle, and McCreary are collaborating to evaluate the effects of these differences and to improve the mixed-layer formulations in the models. Kindle and McCreary are also incorporating active biological processes in their models. The Anderson and Bigg model is a variation of the Bryan-Cox-Semtner model developed at the Geophysical Fluid Dynamics Laboratory. It has high vertical but coarse horizontal resolution (order 30 to 100 km). Therefore, the model does not adequately resolve the sharp gradient in wind stress curl found in the Findlater Jet that drives the corresponding gradient in open-ocean upwelling in the Arabian Sea.