Ocean currents in a changing climate

The ocean surface is not flat. Its topography is affected not only by waves, tides, and Earth’s heterogeneous gravitational field, but also by currents. Since 1992, satellites peering down on the globe from criss-crossing tracks have monitored the sea-surface height with centimeter precision—enough to infer the changing current patterns.

Because the ocean is three-dimensional, however, the surface topography can’t tell the whole story. That’s particularly true for so-called western boundary currents, such as the Gulf Stream in the North Atlantic and the Agulhas Current off the east coast of South Africa. Western boundary currents carry warm ocean water from the tropics to the midlatitudes; they have a lot of influence on local and global climate, so it’s important to understand how they evolve over time.

In 2010 Lisa Beal of the University of Miami led a seagoing mission to study the Agulhas Current in situ. Her team deployed an array of current and pressure sensors, as shown in the figure, positioned to align with one of the satellite tracks and to cover all possible paths of the changing current. Although the instruments remained in place for just three years, they allowed the researchers to more accurately interpret satellite data over a 22-year period—just long enough to tease out how the current might be responding to anthropogenic climate change.

What they found was a surprise. The winds that help to drive the current have strengthened over the past decades, so theory predicts that the current should have strengthened too. Analysis by Beal and her Miami colleague Shane Elipot, however, found that the current got no stronger—but it did get wider, probably because the increased wind energy went into producing turbulent eddies tens of kilometers across. Apart from its implications for global heat transport, the result suggests that offshore pollutants and organisms could more readily be swept into the deep ocean. (L. M. Beal, S. Elipot, Nature, in press, doi:10.1038/nature19853 .)