The impact of vertical resolution in reducing biases in sea surface temperature in a tropical Pacific Ocean model

Abstract

Motivated by observations of fine scale vertical shear and its contribution to mixing in the tropical ocean, this study explores the impact of vertical resolution in an ocean model on sea surface temperature in the tropical Pacific Ocean. We conduct two model experiments that differ in the vertical discretization only, with the grid spacing in one being significantly smaller than the other in the upper ocean. We examine the temperature difference between the high and low vertical resolution experiments. We find that the difference in the upper-most layer is positive in the equatorial cold tongue and negative along the South American coast, thus reducing the commonly seen cool and warm biases in the two regions, respectively. The change in the structure of the vertical diffusivity, as determined by the K-profile parameterization, is identified as the primary cause in reducing the biases. In the central equatorial Pacific, the change in the vertical diffusivity from low to high vertical resolution in the upper pycnocline results in a positive temperature difference that propagates eastward as an equatorial Kelvin wave, rising to the sea surface in the central and eastern regions to increase the sea surface temperature there. In the far eastern equatorial Pacific, the change in the vertical diffusivity in the lower pycnocline produces a negative temperature difference that propagates poleward as coastal Kelvin waves along the west coast of the American continent, outcropping along the South American coast to reduce the sea surface temperature there. The high vertical resolution experiment captures much of the small-scale vertical velocity shear and resolves the fine details of the stratification in the upper ocean. Our analysis suggests that the shear-generated turbulence is the primary contributor to the change in the vertical diffusivity in the central region whereas stratification is the dominant factor in the far eastern region.