Vertical Eddy Mixing in the Tropical Upper Ocean: Its Influence on Zonal Currents

In this study, the authors explore how vertical-mixing parameterizations influence the structure of zonal currents in the eastern equatorial Pacific using an isopycnal ocean model that contains an explicit surface mixed layer. The mixing parameterizations considered are the schemes that depend on the Richardson number (Ri). One of the schemes (the Step scheme) consists of high (nc) and low (nb) values of mixing coefficients, depending on whether Ri is less than or greater than a critical value. In simulations using the Step scheme, there is a region of large vertical shear just beneath the mixed layer where Ri is low and the mixing coefficient is nc; this high mixing controls the depth and strength of the westward surface drift. Near the undercurrent core, Ri is high and the mixing coefficient is nb; this low mixing is nevertheless dynamically important in that it affects the strength of the undercurrent. For the Ri-dependent schemes investigated, it is demonstrated that the extrema attained by mixing coefficients at low and high Ri are the crucial factor rather than the detailed structure of the Ri-dependent functions.