EDDIES IN THE OCEAN* (with links to Metr 201 material)
*"Stolen" and adapted from several sources on the internet.
Ocean motion is actually far more complex and variable than globes and maps showing the large scale gyres imply. In fact, if you were to place a non-moored buoy in the ocean, with a global positioning system (GPS) antenna on board, and recorded its position, you would find that the buoy often changes its position. In some cases, the buoy would move in nearly circular, loop-like patterns. Some of these loops would be roughly 100 to 200 km in diameter. These loops are known as mesoscale eddies. These features are important because they are "hot " spots of intense biological and physical activity.
Metr 201 Link to Scales of Circulation
Figure 1. Two eddies (identified by the circles, one warm (red) and one cold (blue) within the Gulf Stream.
CYCLONIC AND ANTICYCLONIC EDDIES
If the buoy indicates that the ocean moves in a counterclockwise direction in the northern hemisphere, that loop is called a cyclonic eddy. The center of the eddy is likely cooler and lower in height (by a few tens of centimeters) than the outer lying waters. On the other hand, if the rotation of the buoy is clockwise, the feature is called an anticyclonic eddy and the center is warmer and higher (by a few tens of centimeters) than outer waters. The cyclonic eddy is called a cold-core eddy or ring and the anti-cyclonic eddy is called a warm-core eddy or ring.
You can think of eddies as the oceanographyic equivalent of the atmospheric eddies we've seen on weather maps. Meteorologists call these features low (cyclones) and high pressure (anticyclones) systems.
We cannot see eddies by simply looking at the ocean’s surface. In fact, the existence of mesoscale eddies (or rings) was discovered by oceanographers only in the 1960’s with the development of new instruments. Drifters and floats placed at depth in the ocean as well as satellite images of sea surface temperature and ocean color and even historical tracks of derelict ships provided evidence of mesoscale eddy features. Some of the most obvious eddies occur off the east coast of the United States, in the vicinity of the Gulf Stream (Figure 2). Today, the most used methods is satellite altimetry (measuring sea surface height to within a few centimeters or less), and a special radar (synthetic aperture radar), acoustical (sound propagation) and current meter measurements are used to track and study mesoscale eddies.
Figure 2. A movie of how eddies form in the Gulf Stream -- note the differences in rotation between the warm core (yellow, W) and cold core (blue, C) eddies (from www.oc.nps.navy.mil).
DIVERGENCE AND CONVERGENCE ASSOCIATED WITH EDDIES
Winds produce surface currents that sometimes cause convergence (coming together) or divergence (moving apart) of upper ocean waters over surface areas several kilometers in scale. Under the right divergent conditions, cool, nutrient-rich waters can upwell (move vertically) from deeper waters to act as a seed for the formation of a cold-core eddy (Figure 3). Likewise, warmer, nutrient-poor waters may converge, be downwelled, and a warm-core eddy can form (Figure 3).
Figure 3. A schematic of how different eddies look with depth in the water (from www.oc.nps.navy.mil).
The E-Flux experiment conducted off the Hawaii Islands takes advantage of the atmospheric conditions that often prevail there. As shown in Figure 4, northeasterly Trade Winds often blow strongly between mountains such as Haleakala on Maui and Mauna Loa and Mauna Kea on the Big Island of Hawaii and over the channels separating these two islands. Of course, the winds are weaker directly in the shadows (wakes) of the mountains, so great differences in wind speeds at the ocean surface can lead to localized areas of divergent waters, causing upwelling, and convergent waters, causing downwelling (Figure 4). Eddies may be generated in these areas, especially if some small rotation of ocean water is already in place. Scale is important for mesoscale eddies, so not all ‘eddy seeds’ result in mature eddies, but clearly some wonderful eddies do occur with considerable regularity. To summarize, the magnitude and direction of surface winds as well as surrounding fluid flows play important roles in determining eddy formation, as well as their lifetimes, and strengths. Eddy strength can be characterized by the speed of eddy currents and how close to the surface cool water rises or how deeply warm water descends.
Figure 4. A schematic of how winds flowing through the Hawaiian Islands cause convergence and divergences in the lee of the Hawaiian Islands. Figure from Chavanne et al. 2002.
Bidigare, R. R., C. Benitez-Nelson, C. L. Leonard, P. D. Quay, M. L. Parsons, D. G. Foley and M. P. Seki (2003) Influence of a cyclonic eddy on microheterotroph biomass and carbon export in the lee of Hawaii. Geophys. Res. Letters, 30(6), 13-18.
Chavanne, C., P. Flament, R. Lumpkin, B. Dousset, and A. Bentamy (2002) Scatterometer observations of wind variations induced by oceanic islands: Implications for wind-driven ocean circulation. Can. J. Remote Sensing, 28(3), pp.466-474.
Pond, S., and G. L. Pickard (2003) Introduction to Dynamical Oceanography, 2nd Ed. Elsevier Science, Boston, 329 p.
Simpson, J. J., C. J. Koblinsky, L. R. Haury and T. D. Dickey (1984) An offshore eddy in the California current system. Progress in Oceanography, 13(1), 112 p.