[A-CG36] Extratropical oceans and atmosphere

[ACG36-P09] Importance of geostrophic shear on eddy-induced Ekman pumping and associated vertical heat transport in the Kuroshio Extension region

*Kento Usui¹, Tomoki Tozuka¹ (1.Graduate School of Science, University of Tokyo)

Keywords:Kuroshio Extension region, Ekman pumping, Mesoscale eddies

The Kuroshio Extension (KE) region is characterized by a large amount of heat release from the ocean, which enhances the baroclinicity of the overlying atmosphere and contributes to storm track formation. Previous studies highlighted the importance of mesoscale eddy activity in the KE region in vertically transporting heat from the subsurface and maintaining the thermal structure in the upper ocean despite substantial heat loss. Thus, the accurate estimation of the vertical velocity as well as vertical heat transport associated with them has been a major research focus. Previous studies have shown that the classical Ekman pumping velocity, which is proportional to wind stress curl, is modified by fine-scale ocean structures such as vorticity and geostrophic shear associated with horizontal temperature gradients. However, a formulation incorporating both effects has yet to be established even though both relative vorticity and horizontal temperature gradients are present in the interior of mesoscale eddies. In this study, a new formulation of Ekman pumping velocity that accounts for oceanic relative vorticity and geostrophic shear is derived and validated with the vertical velocity obtained from an eddy-resolving ocean model in the KE region. The vertical velocity within mesoscale eddies consists of two components, with one diagnosed by the quasi-geostrophic omega equation and the other a residual frictional component. It is found that the frictional component is better explained by the new formulation compared to the previous formulation that only considers vorticity. Furthermore, the effect of geostrophic shear accounts for a large portion of vertical eddy heat transport from the subsurface in eddy interiors. Therefore, the new formulation, which incorporates both relative vorticity and geostrophic shear, may give a more accurate estimation of wind-driven upper ocean vertical velocity and vertical heat transport associated with mesoscale eddies.