Mesoscale eddies — swirling bodies of water up to 200 km across. In the North Atlantic Ocean, eddies spinning off the meandering Gulf Stream are among the most energetic observed anywhere. Eddies can trap and transport ecosystems great distances across the open ocean as well as pump nutrients from the depths to the sunlit surface.

Oceanographer PETER GAUBE is collaborating in NASA-sponsored research to describe biological responses to and interactions with these oceanic weather systems. Research vessel surveys, and measurements by aircraft, satellites, and autonomous floats are being used to study the problem. There is another surprising oceanographic research platform in this mix: white sharks.

Satellite tracking shows that the sharks swim more often inside eddies than outside, and they prefer a specific type — anticyclones. Generally, anticyclones are associated with downwelling, are warmer, and are often lower in nutrients and phytoplankton. Why would a top predator leave coastal waters with relatively high prey abundance and choose to spend so much time in these oceanic deserts? "The observations from white sharks really piqued my interest," says Gaube, and motivated him to reconcile the apparent contradiction.

Echosounder data show that shark prey may be more abundant in anticyclones. The measurements reveal a lot of biomasss composed in layers deep below the surface. Backscatter strength was a factor of five higher in an anticyclone than in an adjacent cyclone. Data from the sharks' transmitters and autonomous profiling floats show that anticyclones are also warmer at depth than cyclones because anticyclonic downwelling circulation presses the warm surface layers deeper into the water column.

Gaube hypothesizes that sharks expend less energy on deep foraging dives in the warmer anticyclones, and may be able to remain at depth to forage for longer periods.