The ocean is a huge body of water that is constantly in motion. General patterns of ocean flow are called currents. Many geographic globes and world maps show general patterns of currents at the ocean surface. They are given names like the Gulf Stream, which flows northward off the east coast of the United States, or the Eastern Australian Current (EAC), which flows to the south along eastern Australia (Remember Finding Nemo?). Currents are often mentioned in historical accounts of Columbus’ voyages to the New World. These currents are actually based on long term measurements. Ocean motion is actually far more complex and variable than globes and maps imply. In fact, if you were to place a small object in the ocean, say a rubber ducky with a global positioning system (GPS) antenna, and recorded its position, you would find that the rubber ducky often changes its direction. In some cases, the Rubber Ducky would move in nearly circular patterns. Some of these loops would be roughly 100 to 200 km (or about 60 to 120 miles) in diameter. These loops are known as mesoscale eddies. These features are important because they are "hot " spots of intense biological and physical activity.
Figure 1. Two eddies (identified by the blue circles) within the Gulf Stream.
Nope. If the rubber ducky moves in a counterclockwise direction in the northern hemisphere, it 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 ducky is clockwise, the feature is called an anti-cyclonic 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 ocean weather. We often see atmospheric eddies in weather maps. Meteorologists call these features low and high pressure systems. The next time you watch your local weather, check out the circulation patterns. You’ll notice that the air circulation is counterclockwise (cyclonic) for northern hemispheric lows and clockwise (anti-cyclonic) for highs.
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 when clever oceanographers developed and used new instruments. Drifters (remember our rubber ducky?) 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, these methods along with others such as 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).
Earth rotation is very important for understanding eddies. Mesoscale eddies occur when there is a balance of two major forces -- one is a horizontal pressure gradient force arising from differences in water density and the other is an "apparent" force associated with the Earth’s rotation. This is called the Coriolis force. Eddies occur virtually everywhere in the ocean, including Arctic regions and even off the coast of Antarctica in the southern hemisphere. But, there is a twist – the Coriolis effect causes cold-core eddies in the southern hemisphere to rotate clockwise (cyclonic) and warm-core eddies rotate counterclockwise (anti-cyclonic). The size of eddies and rings vary as do their lifetimes - a month or so to more than a year, with an average of a few months. Earth’s rotation and the Coriolis effect play an important role in establishing both.
The formation mechanisms and the life histories of eddies and rings remain the subject of oceanographic research. There are probably several different conditions giving rise to their formation – just as there are for their atmospheric counterparts, the low and high pressure eddies. For example, currents like the Gulf Stream meander in a rather wave-like fashion and become unstable (Figure 2). These flow instabilities lead to pinching off of relatively warm or cool waters that act as seeds for eddies or rings. The warm-core features occur closer to the east coast of the United States and cold-core features are seen generally to the west and/or south of the Gulf Stream. Again, mature phase eddies or rings form nearly circular features of 100 - 200 km in diameter. It is worth noting that energetic rings and eddies are commonly found in the vicinity of faster flowing currents including the Gulf Stream and its North Pacific counterpart off Japan called the Kuroshio. Try the following bath tub or pool experiment. Run your hand through the water or create a jet of water using a hose to create a small scale 'Gulf Stream' flow. You will notice eddies forming to either side of the wake of your hand or the jet . These eddies are a simple model of those eddies found in the Gulf Stream.
Given that eddies seem to form in so many places, other processes must also be capable of producing them. For example, flow over seamounts (and mountains in the atmosphere) can cause eddies. Very long, westward traveling planetary scale waves called Rossby waves also likely play roles in eddy formation (Figure 3). 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.
There are several reasons for caring about eddies. For example, if you were to sail in a race from Boston to Bermuda, you would like to optimize your chances of winning the race! Of course, you would study weather maps and try to keep the wind at your back, but, what about the currents? For sure, you would want to go with the Gulf Stream flow as much as possible. And now from what we have learned about mesoscale eddies, you would certainly want to plot a course based on say a sea surface temperature map showing eddy features to take advantage of mesoscale eddy currents that could reach 100 cm/sec or more or more than a couple of knots (nautical miles per hour). In fact, it is very possible that your knowledge of eddies could be the difference between winning or losing the race. Merchant ships use current information, which includes eddies, to make better time and save considerable fuel and money. In the big picture, physical oceanographers need to include mesoscale eddies in their analyses and models to study and monitor ocean currents on all scales.
But, say you are a biologist. Should you care about these mesoscale eddies? The answer is a resounding yes! It turns out that generally warm-core eddies are relatively poor in plant nutrients (i.e., nitrates, phosphates, and silicates), which algae called phytoplankton need to survive and thrive. On the other hand, cold-core eddies are rich in nutrients. So, freshly upwelled cool, nutrient-rich waters in the center of a cold core eddy are a feast for phytoplankton. Interestingly, cold core eddies are often, but not always, visible in ocean color satellite images (greener waters occur with more phytoplankton, bluer waters indicate fewer phytoplankton, Figure 5) as well as in images of sea surface temperature. A variety of small animal organisms, zooplankton, depend on the phytoplankton as a food source and similarly larger animals including fish graze on the zooplankton – this is the so-called ‘food chain.’ So, because the presence of eddies can make a given part of the ocean to be more or less productive, eddies are indeed extremely important if we are to quantify and understand life in the sea. In fact, local fishermen off of Hawaii have known for years that fishing off the lee of the Hawaiian Islands is best during the eddy formation season, October through March and that the edge of eddies are often the places where the best fishing is found.
Figure 5. Color satellite image of several eddies forming in the lee of the Hawaiian Islands in July 2002.
Want more Information?
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.