SENSITIVITY OF THE SURFACE EQUATORIAL OCEAN TO THE PARAMETERIZATION OF VERTICAL MIXING

This study investigates the sensitivity of the dynamics of the sur-fac e equatorial ocean to the parameterization of vertical mixing. A new h igh-resolution, numerical model of a zonally independent equatorial ch annel helps to explore this question and includes three parameterizati ons, all of which increase mixing for decreasing Richardson numbers. I t compares the smooth increase of eddy coefficients traditionally used in general circulation models, the dramatic increase of the eddy coef ficients for small Richardson numbers recently observed in the equator ial Pacific, and the combination of a mixing mechanism based on the di agnostic adjustment of the water column to noncritical Richardson numb ers and of a bulk mixed layer model. The meridional and vertical veloc ity fields in the surface layer are very sensitive to the strength of mixing implied by the different parameterizations. For the smooth Rich ardson number dependence of the eddy coefficients, equatorial upwellin g due to easterly winds reaches the surface. The dramatically increasi ng eddy coefficients for small Richardson numbers yield reduced equato rial upwelling rates in the surface layer. The diagnostic adjustment o f the Richardson number shows in the surface layer close to the equato r reversed meridional shear and downwelling in response to easterly wi nds! A simple model for the low-latitude wind current in the presence of horizontal density gradients reproduces this reversal of the meridi onal and vertical flows. If the equatorial Ekman number is large, ther e is a latitude range where within the upper layer the vertically aver aged flow and density are dominated by rotation, while the vertical sh ear of horizontal velocities is strongly influenced by vertical fricti on. In this region vertical shears point downstream of the wind stress and of the pressure forces due to gradients in density. For an easter ly wind the pressure gradient forces surface waters toward the equator and can reverse the vertical shear of meridional velocity and the equ atorial vertical velocity. The critical value of the vertical eddy coe fficient for this reversal to occur is of the order of 5 X 10(-3) m2 s -1. This value is of the same order as measured in the surface equator ial Pacific and used in general circulation models. The physics of thi s reversal are so basic it is likely they are active in the ocean and three-dimensional circulation models.