The Vortex Dynamics of Tropical Instability Waves and their Impacts on Equatorial Mixing in a Regional Ocean Model

Abstract:

Tropical instability waves (TIWs) and vortices (TIVs) in the equatorial Pacific exhibit energetic horizontal and vertical circulation characterized by regions of high Rossby number and low Richardson number. Their strong anticyclonic vorticity and vertical shear can influence the broader-scale circulation by driving lateral mixing and vertical exchange. Here we present recent modeling work aimed at understanding the vorticity and potential vorticity dynamics of TIWs and TIVs and their implications for mixing at the Equator.

The anticyclonic vorticity of TIVs mixes Equatorial Undercurrent (EUC) water with North Equatorial Counter Current (NECC) water within TIV cores, driving an equatorward heat flux that affects the SST budget in the cold tongue. Using Lagrangian vorticity analyses, we show that part of this anticyclonic vorticity is formed through vortex tilting of the horizontal vorticity associated with the vertical shear in the upper EUC. In turn, recent observations have shown that TIWs modulate vertical mixing in the upper EUC. We examine the vortex dynamics of this process and show that TIW induced strain can drive increases in the EUC vertical shear through horizontal vortex stretching, contributing to an increase in vertical mixing. Calculations using a simple 1D model of mixing in the EUC driven by periodic TIW strain indicate that TIWs may increase the turbulent heat flux by up to 30%, with the magnitude of the increase being sensitive to the parameterization for shear driven turbulent diffusion. These results emphasize the role of submesoscale physics in the equatorial region, which are active when both the Rossby and Richardson numbers are O(1).