Diving in an eddy

Image of the Month - April 2008

Sea level anomalies on February 19 and April 20, 2005 (left and middle), and geostrophic velocities on April 20, 2005,  in the Southern Ocean South of Africa. A cyclonic (cold) eddy marked by the black arrows is clearly visible in these altimetric maps. (Credits University of Cape Town)

 

The path of the eddy between January and August 2005 (blue circles), superimposed on bathymetry. The blue circles mark the -20 cm sea-level anomaly associated with the cold eddy. In this region, about 3 eddies are generated per year, and their paths are strongly influenced by the local topography. An example of this is provided by the red and black solid lines which represent the paths of selected eddies generated between 1993 and 1998 (Credits University of Cape Town).

The Antarctic Circumpolar Current (ACC) is one of the most important -- if not the best-known -- current on Earth. It sheds eddies all along its path, but more are shed in certain areas than in others, due in particular to the presence of seafloor ridges. Once shed, the paths of these eddies are strongly controlled by the local bathymetry, which results in them being channeled through gaps in the ridges (see Image of the Month, July 2003: Currents steered by gaps in the ridges). Such eddies represent a major heat transport mechanism in the region, and they play a key role in local ecosystems. For example, the edges of these eddies are the prefered feeding ground for albatross, amoung other animals.

Altimetry enables us to spot such features, and to monitor them. However, for a better understanding, having a closer look -- by automated instruments, or during an oceanographic cruise -- can help. Indeed eddiesave three dimensions, and the rounded shape we see in sea surface height measurements is a sign of what's happening beneath.

A vertical cross-section of temperature (left), salinity (center) and geotrophic velocities of the cold eddy shown above in the altimetry. The section is along an east-west line at about 48.9°S. The cold core of the eddy is visible below the surface between 200 and 400 m, and extends down to at least 1000 m. In salinity, the eddy is visible as a dome with higher salinity. The geostrophic velocities show a southward jet on the eastern edge of the eddy (negative velocities; the velocity scale is made so that velocities are positive when going South, negative when going North) and a northward jet on the western edge of the eddy (positive velocities), with a calm center, thus showing that water circulates in a clockwise direction (cyclonic rotation in the Southern hemisphere). (Credits University of Cape Town)

See also:

References:

  • Swart, N. C., I. J. Ansorge, and J. R. E. Lutjeharms (2008), Detailed characterization of a cold Antarctic eddy, J. Geophys. Res., 113, C01009, doi:10.1029/2007JC004190