The Merger of Subsurface Isolated Eddies: An Application to Oceanic Lens-like Anticyclones

Abstract:

Vortex merger is a phenomenon characterizing the whole class of geophysical vortices, from atmospheric storms and large oceanic eddies up to small scale turbulence. In the ocean, this process takes place at different depths in the water column, involving both surface and subsurface-intensified eddies. We focused on the merger of subsurface isolated eddies in an idealized, full primitive-equations context. This work has been motivated by the evidence of colliding Mediterranean water EDDIES (MEDDIES) off of Gibraltar Strait, as pointed out by several observational and realistic-modeling studies. The critical conditions for merger, like critical merger distance and time needed for merger, were determined. We found that the predictions of classical two-dimensional merger are not verified for subsurface isolated eddies. For instance, critical merger distances will be reduced because of the eddy potential vorticity structure.
The choice of a full primitive-equations and three-dimensional context, allowed us to investigate in detail the characteristics of the merged eddy (radius, vertical extension and potential vorticity). The merging process yields slightly shallower, more oblate and energetic eddies, when compared to the initial-state structures.
Merger-related effects, like production of peripheral filaments and small-scale eddies, were also investigated. These effects confirmed the role of merger in both direct and inverse energy cascades