Penetration of surface-generated mesoscale eddies into oceanic abyssal (numerical experiments)

Abstract

Several series of numerical experiments were conducted to generate a mesoscale eddy in a deep rotating basin (4000 m deep) with stratification typical of mid- and low-latitude of the World Ocean, including an upper mixed layer, a density jump layer, and a main pycnocline. A sea level anomaly driven by Ekman transport convergence was created at the basin’s center by specifying a wind stress configuration typical for a stationary atmospheric anticyclone. Due to geostrophic adjustment, the resulting pressure anomaly generated an anticyclonic baroclinic eddy in the basin, the rotation velocity of which decreased with depth. Numerical experiments varying the horizontal scale of the eddy, the Coriolis frequency, and the stratification parameters yielded a universal relationship between the ratio of the rotation velocity in the surface and bottom layers and the ratio of the horizontal scale of the eddy to the first baroclinic Rossby radius of deformation. Comparing this universal relationship with published estimates of the horizontal scale of mesoscale eddies in the World Ocean from altimetry and shipboard measurements revealed that at low- and mid-latitudes, the rotation velocity typically weakens with depth by more than 80 %. In the presence of a bottom topography disturbance in the form of an abyssal channel under a mesoscale baroclinic eddy, a current arises in the channel whose velocity is significantly higher than the bottom velocity in the undisturbed eddy. The velocity disturbance caused by the abyssal channel is not limited to the channel itself, but reaches the surface, provided that the width of the channel exceeds the first baroclinic Rossby radius of deformation.