Dynamics of the equatorial undercurrent in a high-resolution ocean model

In this study, we investigate the sensitivity of the equatorial circulation simulated with the high-resolution World Ocean Circulation Experiment (WOCE) Community Modeling Effort (CME) model to the specification of eddy viscosity. In one of two simulations the coefficient of vertical viscosity is tripled. We focus on the dynamics associated with the anomalous eastward surface flow found in the simulation results. We find, in agreement with other studies, that close to the western boundary, the equatorial undercurrent is dominated by a large positive pressure gradient. In the top layer, for the low-viscosity case, the balance in the western part of the basin is everywhere between the horizontal pressure gradient and vertical viscosity with the advective terms playing a secondary role. In the high-viscosity zonal momentum budget analysis the advective terms are less significant in the surface layers in the west. Toward the east the advective terms become more important in the low-viscosity experiment, whereas in the high-viscosity case the balance is still predominantly between the horizontal pressure gradient and vertical diffusion terms. It is found that in the CME, horizontal viscosity plays a very minor role in the zonal momentum balance. Changes in the vertical viscosity lead to changes in the nonlinear advective terms. These, in turn, contribute to the unusual eastward surface flow.