Circulation on the Northwestern Iberian Margin: Swoddies

Teles-Machado A, Peliz A, McWilliams JC, Dubert J, Cann BL
Progress in Oceanography 140 (2016) 116133,

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We study the eddies that form by destabilization of the Iberian Poleward Current. Some anticyclones have been identified in satellite Sea Surface Temperature, as they are persistent and remarkably warm, and they contain IPC waters in their core. The number of observed eddies is small, and little is known about their statistics, places of formation, separation processes, and behavior away from the slope. In this study, the output of a 20-year high-resolution numerical simulation is analyzed to study the formation of anticyclonic and cyclonic eddies by destabilization of the Iberian Poleward Current, using an automatic eddy detection algorithm. The model reproduces the formation of some of the eddies at the same time and place, and with similar sizes as observed in satellite SST, although it fails to reproduce their observed trajectories as they propagate away from the slope. We found distinct layers with different relative vorticity distribution. The top 200 m of the water column has an anticyclonic dominance, with stronger anticyclones; in the layer between 200 and 600 m there are more and stronger cyclones; and from 600 to 1000 m there is again an anticyclonic dominance, with more and stronger anticyclones than cyclones. The results show that the cyclones form mainly where topographic contours veer cyclonically in the poleward direction, while the anticyclones tend to form in places where topographic contours veer anticyclonically. We found that there is a relationship between the formation of eddies and the wind variability. A sudden decrease in southerly winds results in the development of instabilities in the IPC and formation of eddies. The shedding of the surface intensified anticyclones is accompanied by the shedding of deeper layer cyclones. In general, the deep cyclones spin down faster than the surface anticyclones; anticyclones are tracked, and conserve their maximum relative vorticity for longer periods. The vertical structure of these surface intensified anticyclones, at the moment of formation, extends to deeper than 1200 m, and this vertical extent decreases with time. They can survive winter deep convection that homogenizes their core.