Modeling Anticyclone Formation Above Topographic Depressions

Abstract:

In several different North Atlantic basins, semi-stationary mesoscale anticyclones (ACs) have been repeatedly observed over several decades above bowl-like topographic depressions. These ACs include the Mann Eddy (of the Newfoundland Basin), the Lofoten Basin Eddy, and the Rockall Trough Eddy. Significant water mass transformations or thermohaline fluxes occur in these regions, and the persistent anticyclones were previously suggested to influence these processes.

Motivated by these previous findings, we conduct numerical simulations to study the dynamics of AC formation above topographic depressions in idealized settings. In one or several isopycnal layers experiments, it is found that coherent and long-lived ACs can spontaneously emerge above topographic bowls, either from random initial conditions or under random forcing. The bowl-trapped ACs are characterized by anomalously low PV, and several mechanisms of PV segregation contributing to the formation are contrasted. A regime diagram for AC formation from random initial conditions is presented in terms of a nonlinearity parameter, or in terms of a potential vorticity homogenization parameter.

The aforementioned observed ACs are top-intensified, with a significant barotropic component as well. A top intensification metric is examined in 2-layer experiments. It is found that the emergent bowl-trapped ACs are top or bottom intensified, in case that the domain-mean initial conditions are top or bottom intensified, respectively. That is in contrast to the emergent slope current in the simulations, as well as in contrast to Taylor Caps occurring over seamounts, which are always bottom intensified. Scaling relations are suggested to predict the emergent top-intensification metric based on initial conditions. Finally, results are contrasted with topographic turbulence theories, which predict formation of a slope current, but not of a trapped AC within a topographic depression.