Patterns of Vertical Velocity Induced by Eddy Distortion in an Ocean Model

Vertical motions within eddies play an important role in the exchange of properties and energy between the upper ocean and the ocean interior. Here we analyze alternating upward and downward cells in anticyclonic eddies in the East Australian Current region using a global eddy-resolving model. The cells explain over 50% of the variance of vertical velocity within these eddies. We show that the upward and downward cells relate to eddy distortion, defined as the change in eddy shape over time. In anticyclonic eddies in the Southern Hemisphere, an inward distortion is associated with upward motion and an outward distortion is associated with downward motion. We discuss two mechanisms that link eddy distortion to vertical velocity. One mechanism relates to changes in stratification and relative vorticity in the eddy interior. The other mechanism relates to divergence of the horizontal flow in different quadrants of the eddy. We show that mesoscale changes in sea level anomaly can be used to infer the vertical motion within eddies.