Altimetry and marine productivity

A most simplistic way to describe the vertical structure of the ocean is to consider that the ocean is made of a deep layer of cold water topped by a warmer surface layer. The thermocline is the boundary between these two layers.

Warm eddies, seen as positive sea level anomalies (highs) in altimetric maps, are often associated with surface water convergence. This induces local accumulation of warm surface water, down-welling of this surface water and hence a deeper thermocline.

On the opposite, cold eddies seen as negative sea level anomalies (lows) in altimetric maps, are associated with the surface divergence. In such regions, upwelling from the deeper layer occurs to compensate for the surface water loss and the thermocline moves up. With this upwelling, nutrients and phytoplankton are brought to the surface where they are exposed to solar radiation. This activates photosynthesis. Cold eddies are thus favorable locations for increased primary production.

Sea level anomalies and tuna behavior:
As an example, one tuna feeding behavior can be summarized as follows. Tuna prefer warm surface waters, mostly to help them maintain their relatively warm internal temperature. Still they need to occasionally visit the colder enriched upwelling areas in order to feed. The fronts between warm and cold structures (i.e. between positive and negative sea level anomalies) are thus naturally areas with a higher probability of fish presence. Source : http://www.catsat.com