Warm-core eddies are a famous feature of the Gulf Stream and Sargasso Sea region. They have been extensively studied by shipboard research, and they are highly discernible features in both sea surface temperature (SST) and ocean color (chlorophyll) data. The "CZCS Classic Scenes" chapter entitled "Gulf Stream Rings" (Chapter 7) provides a general overview of these types of eddies and how they form.
When a Gulf stream warm-core eddy forms, the water in the warm core is usually quite warm, because it was extracted from the surface waters of the Sargasso Sea. The Sargasso Sea is a tropical water body that is very warm year-round, in the range of 27-30 °C. So it isn't difficult to call the core of a Gulf Stream warm core eddy "warm"—or even hot.
However, such eddies do not always form where the water is tropical. The example examined in this Science Focus! article is decidedly non-tropical—it is found in the chilling waters of the Southern Ocean. And this example also illustrates how a few fortuitous observations can provide significant information about the physical and biological dynamics of this region.
First of all, some context. The region being examined is the Southern Ocean in the vicinity of South Georgia Island. (For more on this region, read "South Georgia: A View Through the Clouds". Clouds will be an important aspect of this article, too.)
Here's a diagram of the South Atlantic and Antarctic Ocean, showing the various oceanic fronts in the region. A front is a region where two currents, or two distinct water bodies, meet. NSTF: North Subtropical Front; SSTF: South Subtropical Front; SAF: Subantarctic Front; APFZ: Antarctic Polar Frontal Zone; PF: Polar Front; SF: Scotia Front. In the map below, South Georgia Island is the crescent-shaped island between the "PF" and "SF" labels on the fronts.
Permission to use this image obtained from Igor Belkin, URI/GSO, image author.
What this diagram doesn't show is the distribution of sea surface temperature (SST) and phytoplankton chlorophyll. Of course, these are constantly changing, but the particular month shown below, October 2003, was very interesting. This image was generated with the Giovanni Ocean Color Multi-dataset Intercomparison Interface. The color scale is for chlorophyll concentration; the numbers are for contour lines of SST. The Polar Front follows the 3 °C contour fairly closely; moving northward, the proximity of the 9 °C and 12 °C contours indicates the approximate position of the Subantarctic front. The occurrence of higher chlorophyll concentrations is also an indicator. (Click the image to see it larger; note that the file name of the larger image indicates an incorrect month.)
The most prominent feature in this image is a circular eddy northeast of South Georgia Island, approximately 25 °W, 51 °S. By its circular appearance, we would presume that this is an eddy. But we can confirm the diagnosis using Giovanni.
First we zoom in on the eddy, still using the Multi-dataset Intercomparison Interface. Right in the middle of the eddy, we can see that the low chlorophyll concentration center coincides with water temperatures that are above 3 °C. Because warm water is buoyant, if we had sea surface height data from an altimeter like Jason or TOPEX/Poseidon, the center of the eddy would also be an area where the sea surface is elevated.
To ultimately confirm our diagnosis that this is a warm-core eddy, even though warm is only 3 degrees above the freezing point of pure water, we can now generate a scatter plot of the SST and chlorophyll concentrations for this image.
This plot demonstrates that the circular feature certainly is a warm-core eddy. The "warmest" water temperatures, in the range of 2.5 to 3 °C, are associated with the lowest chlorophyll concentrations, less than 0.2 milligrams per cubic meter. And the lowest temperatures, sometimes even below 0 °C (seawater freezes at a lower temperature than pure water because of its salt content), are associated with the higher chlorophyll concentrations around the eddy.
It's very unusual to see such a well defined eddy in an image of monthly averaged chlorophyll concentrations, because eddies, along with the currents and fronts that create them, are constantly moving. Because a monthly average plot takes all of the observations for a given month, if a lot of observations are obtained, this will "blur" or "smear" a distinct feature (like an eddy) that moves. So examination of this image made us suspect that the eddy was easy to see because there were probably very few observations of it in October 2003. As noted earlier, clouds are an important part of this article, because the Southern Ocean tends to be VERY cloudy.
Now go to Part 2.