In Part 1, we examined an image of the Southern Ocean in October 2003, observed an eddy, and determined that this was indeed a warm-core (where 3 °C is warm compared to the surrounding waters). We also suspected that the reason this eddy was so easy to see in the monthly data plot was due to a limited number of observations in this particularly cloudy oceanic region. Now we'll check to see if our suspicion is correct.
The first thing we will do is to look at the four 8-day SeaWiFS images of this region for the month of October 2003. (You can click on all of these images to see them full-size.)
SeaWiFS 8-day chlorophyll images for October 2003. Top left: September 30 to October 7. Top right: October 8 to October 15. Bottom left: October 16 to October 23. Bottom right: October 24 to October 31. (Happy Halloween!)
The images above demonstrate, clearly, that the only period when the eddy to the northeast of South Georgia Island was visible was October 16 to October 23. Even in the other 8-day images, the cloud cover was so pervasive that there are hardly any glimpses of the sea surface in the eddy location. However, the 8-day images also revealed another eddy, almost directly north of South Georgia. But as the spring bloom blossoms north of the island, it appears that the eddy gets torn apart by the shifting currents and the low productivity water in the center of that eddy is stirred into the higher productivity waters around it.
We then used Giovanni to browse the available high-resolution data for October 2003—fortunately, a ground station on the Palmer Peninsula of Antarctica acquired the high resolution data from SeaWiFS. This search revealed that there was only one day, October 19, when the eddy was clearly visible—and it is the data from this day that makes the eddy so prominent in the 8-day and monthly images. The image shown below was processed with SeaDAS to show how fortunate the observation of this eddy was. It's hard to see the color scale in the reduced-size image (click on it to see the stunning full-size version). The highest chlorophyll concentrations (orange to red) range from about 5 to 10 mg m-3, and the lowest concentrations (dark blue to purple) are 0.05 to 0.1 mg m-3.
Now, there is a way to look under the clouds—in a sense. Giovanni also features data from the NASA Ocean Biogeochemical Model, which is explained in more detail in LOCUS Tutorial 8. NOBM uses a model of ocean circulation and biology coupled to an "assimilation" of the available SeaWiFS data, to calculate chlorophyll concentrations and also the distributions of various kinds of phytoplankton, and it does this for every day of the year. The calculated output from the model, even though it is lower resolution than the SeaWiFS images, allows an observation of the daily changes happening in the ocean, even in cloudy areas like the Southern Ocean.
Below are seven images from the NOBM output data for October 2003. The positions of the two eddies are labeled in each one; Number 1 is the eddy north of South Georgia Island, number 2 is the eddy to the northeast of South Georgia, the eddy that was examined in Part 1. The development of eddy number 2 and the disappearance of eddy number 2 can be observed in these images. Also, observe how the spring bloom in the Southern Ocean slowly strengthens during this period, particularly north of South Georgia Island and along the Scotia Front south of the island. (Click any image to see it larger.)
So now we've seen how the cold waters of the Southern Ocean can still produce a "warm" core eddy, and we have also seen how one break in the clouds can provide us with significant insight into the interaction of physical and biological factors in this dynamic oceanic region.
References for further information:
"Mesoscale eddies in the Subantarctic Front—Southwest Atlantic", Glorioso, Piola, and Leben, Scientia Marina, 69 (Suppl-2), pages 7-15, 2005. (PDF can be downloaded from this page.)
"Southern elephant seal trajectories, fronts and eddies in the Brazil/Malvinas Confluence", Campagna, Piola, Marin, Lewis, and Fernandez, Deep-Sea Research, 53(12), 1907-1924, 2006. (Abstract available in ScienceDirect.)