An eddy-permitting coupled physical-biological model of the North Atlantic- 1. Sensitivity to advection numerics and mixed layer physics

Physical influences on biological primary production in the North Atlantic are investigated by coupling a four-component pelagic ecosystem model with a high-resolution numerical circulation model. A series of sensitivity expe riments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusi vity implicit in upstream advection approximately doubles primary productio n when compared with a less diffusive, higher-order, positive-definite adve ction scheme. This is of particular concern in the equatorial upwelling reg ion where upstream advection leads to a considerable increase of upper ocea n nitrate concentrations. Counteracting this effect of unrealistically larg e implicit diffusion by changes in the biological model could easily lead t o misconceptions in the interpretation of ecosystem dynamics. Subgrid-scale diapycnal diffusion strongly controls biological production in the subtrop ical gyre where winter mixing does not reach the nutricline. The parameteri zation of vertical viscosity is important mainly in the equatorial region w here friction becomes an important agent in the momentum balance.