Equatorial circulation of a global ocean climate model with anisotropic horizontal viscosity

Horizontal momentum flux in a global ocean climate model is formulated as a n anisotropic viscosity with two spatially varying coefficients. This frict ion can be made purely dissipative, does not produce unphysical torques, an d satisfies the symmetry conditions required of the Reynolds stress tensor. The two primary design criteria are to have viscosity at values appropriat e for the parameterization of missing mesoscale eddies wherever possible an d to use other values only where required by the numerics. These other visc osities control numerical noise from advection and generate western boundar y currents that are wide enough to be resolved by the coarse grid of the mo del. Noise on the model gridscale is tolerated provided its amplitude is le ss than about 0.05 cm s(-1). Parameter tuning is minimized by applying phys ical and numerical principles. The potential value of this line of model de velopment is demonstrated by comparison with equatorial ocean observations. In particular, the goal of producing model equatorial ocean currents compar able to observations was achieved in the Pacific Ocean. The Equatorial Unde rcurrent reaches a maximum magnitude of nearly 100 cm s(-1) in the annual m ean. Also, the spatial distribution of near-surface currents compares favor ably with observations from the Global Drifter Program. The exceptions are off the equator; in the model the North Equatorial Countercurrent is improv ed, but still too weak, and the northward flow along the coast of South Ame rica may be too shallow. Equatorial Pacific upwelling has a realistic patte rn and its magnitude is of the same order as diagnostic model estimates. Th e necessary ingredients to achieve these results are wind forcing based on satellite scatterometry, a background vertical viscosity no greater than ab out 1 cm(2) s(-1), and a mesoscale eddy viscosity of order 1000 m(2) s(-1) acting on meridional shear of zonal momentum. Model resolution is not criti cal, provided these three elements remain unaltered. Thus, if the scatterom eter winds are accurate, the model results are consistent with observationa l estimates of these two coefficients. These winds have larger westward str ess than NCEP reanalysis winds, produce a 14% stronger EUC, more upwelling, but a weaker westward surface flow. In the Indian Ocean the seasonal cycle of equatorial currents does not appe ar to be overly attenuated by the horizontal viscosity, with differences fr om observations attributable to interannual variability. However, in the At lantic, the numerics still require too large a meridional viscosity over to o much of the basin, and a zonal resolution approaching 1 degrees may be ne cessary to match observations. Because of this viscosity, increasing the ba ckground vertical viscosity slowed the westward surface current; opposite t o the response in the Pacific.