Sensitivity of the Surface Equatorial Ocean to the Parameterization of Vertical Mixing

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  • 1 School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii
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Abstract

This study investigates the sensitivity of the dynamics of the surface equatorial ocean to the parameterization of vertical mixing. A new high-resolution, numerical model of a zonally independent equatorial channel helps to explore this question and includes three parameterizations, all of which increase mixing for decreasing Richardson numbers. It compares the smooth increase of eddy coefficients traditionally used in general circulation models, the dramatic increase of the eddy coefficients for small Richardson numbers recently observed in the equatorial Pacific, and the combination of a mixing mechanism based on the diagnostic adjustment of the water column to noncritical Richardson numbers and of a bulk mixed layer model.

The meridional and vertical velocity fields in the surface layer are very sensitive to the strength of mixing implied by the different parameterizations. For the smooth Richardson number dependence of the eddy coefficients, equatorial upwelling due to easterly winds reaches the surface. The dramatically increasing eddy coefficients for small Richardson numbers yield reduced equatorial upwelling rates in the surface layer. The diagnostic adjustment of the Richardson number shows in the surface layer close to the equator reversed meridional shear and downwelling in response to easterly winds!

A simple model for the low-latitude wind current in the presence of horizontal density gradients reproduces this reversal of the meridional and vertical flows. If the equatorial Ekman number is large, there is a latitude range where within the upper layer the vertically averaged flow and density are dominated by rotation, while the vertical shear of horizontal velocities is strongly influenced by vertical friction. In this region vertical shears point downstream of the wind stress and of the pressure forces due to gradients in density. For an easterly wind the pressure gradient forces surface waters toward the equator and can reverse the vertical shear of meridional velocity and the equatorial vertical velocity. The critical value of the vertical eddy coefficient for this reversal to occur is of the order of 5 × 10−3 m2 s−1. This value is of the same order as measured in the surface equatorial Pacific and used in general circulation models. The physics of this reversal are so basic it is likely they are active in the ocean and three-dimensional circulation models.

Abstract

This study investigates the sensitivity of the dynamics of the surface equatorial ocean to the parameterization of vertical mixing. A new high-resolution, numerical model of a zonally independent equatorial channel helps to explore this question and includes three parameterizations, all of which increase mixing for decreasing Richardson numbers. It compares the smooth increase of eddy coefficients traditionally used in general circulation models, the dramatic increase of the eddy coefficients for small Richardson numbers recently observed in the equatorial Pacific, and the combination of a mixing mechanism based on the diagnostic adjustment of the water column to noncritical Richardson numbers and of a bulk mixed layer model.

The meridional and vertical velocity fields in the surface layer are very sensitive to the strength of mixing implied by the different parameterizations. For the smooth Richardson number dependence of the eddy coefficients, equatorial upwelling due to easterly winds reaches the surface. The dramatically increasing eddy coefficients for small Richardson numbers yield reduced equatorial upwelling rates in the surface layer. The diagnostic adjustment of the Richardson number shows in the surface layer close to the equator reversed meridional shear and downwelling in response to easterly winds!

A simple model for the low-latitude wind current in the presence of horizontal density gradients reproduces this reversal of the meridional and vertical flows. If the equatorial Ekman number is large, there is a latitude range where within the upper layer the vertically averaged flow and density are dominated by rotation, while the vertical shear of horizontal velocities is strongly influenced by vertical friction. In this region vertical shears point downstream of the wind stress and of the pressure forces due to gradients in density. For an easterly wind the pressure gradient forces surface waters toward the equator and can reverse the vertical shear of meridional velocity and the equatorial vertical velocity. The critical value of the vertical eddy coefficient for this reversal to occur is of the order of 5 × 10−3 m2 s−1. This value is of the same order as measured in the surface equatorial Pacific and used in general circulation models. The physics of this reversal are so basic it is likely they are active in the ocean and three-dimensional circulation models.

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