8.10: Eddies

Chapter 7 describes how winds are arranged in global patterns that determine climate. It also explains that winds are highly variable in any given location. The local variations are part of weather systems, which we readily associate with the swirling patterns of clouds and precipitation. The global ocean current systems are analogous to the climatic winds, and the oceans have their own “weather systems,” which includes variable swirling motions and meandering fronts, just like atmospheric weather. These swirling motions are called eddies.

The principal difference between atmospheric weather and ocean current “weather” is the scale of the eddy motions. Ocean currents generally move more slowly than most atmospheric winds. Consequently, the Coriolis effect tends to make ocean currents flow in curving paths that have a smaller radius than comparable atmospheric eddies (CC12). In other words, ocean eddies are generally much smaller than atmospheric eddies. Because ocean eddies are smaller, they are more numerous than atmospheric eddies.

Satellites can detect many atmospheric circulations and patterns that contribute to the weather. However, such synoptic measurements cannot be made so easily within the body of the oceans. Therefore, we do not have a detailed understanding of deep ocean current variability. 

Satellite Observations of Eddies

Satellite infrared sensors detect ocean surface temperature by measuring the sea surface heat radiation. Satellite optical sensors tuned to different wavelengths of light can detect ocean color by measuring back-scattered sunlight, creating images that look similar to photographs. Such satellite observations are most effective for observing ocean surface currents where temperature and/or color differ markedly between water masses. Unfortunately, in most areas, the differences are very small. The easiest surface currents to observe by satellite are western boundary currents because the water they carry is warmer than the adjacent coastal waters. Areas near river discharges are also good for satellite observations of currents because the suspended particles in river outflow alter ocean color.

Even in areas where satellite observations are most effective, color or temperature variations of the sea surface are so small that they must be computer-enhanced to be seen by researchers. Computer enhancement of satellite images generally assigns bright false colors to areas that are slightly different in temperature and true color, thus making the demarcations between currents or water masses more prominent. Figure 8-16 is a color-enhanced infrared satellite image of the Gulf Stream region of the Atlantic Ocean.

Figure 8-16. This image of the northwestern Atlantic Ocean shows the sea surface temperature derived from NOAA satellite sensors. Temperatures are depicted by the false colors in this image. Shades of red denote the areas of highest temperature, and progressively lower temperatures are depicted by orange, yellow, green, blue, and violet shades. The image shows the Gulf Stream separating the coastal water mass from the Sargasso Sea water mass. The complex meanders of the Gulf Stream and both cold-core and warm-core eddies are easy to see. For example, the isolated blue area surrounded by yellow and green in the offshore water is a cold core eddy, and the green areas isolated in the light blue coastal water are warm core eddies.

Mesoscale Eddies

The image in Figure 8-16 shows the warm Gulf Stream flowing northward from the tip of Florida along the edge of the continental shelf. The front between this water and the colder coastal water is very sharp (Fig 8-14). A similar front can be seen between Gulf Stream water and colder Sargasso Sea water in the interior of the North Atlantic subtropical gyre. The infrared satellite image shows that there is great complexity along these fronts. We can see numerous secondary fronts and meanders of the Gulf Stream. We also see isolated, almost circular areas of warm water on the coastal-water side of the Gulf Stream and similar isolated areas of cold water on the Sargasso Sea side called warm-core and cold-core rings.

These Gulf Stream meanders and rings are eddies that continuously form and change shape and location. Meanders travel slowly northward along with the Gulf Stream and can become larger as they move. If a meander becomes tight enough, it can break off the Gulf Stream entirely and form a spinning ring of water. If the meander is to the coastal-water side of the Gulf Stream, a warm-core ring that spins clockwise is formed and isolated (Fig. 8-17). If the meander is to the Sargasso Sea side of the Gulf Stream, a cold-core ring that spins counterclockwise is formed as colder coastal water is pinched off inside the meander (Fig. 8-17).

Figure 8-17. Gulf Stream rings. As the Gulf Stream passes north of Cape Hatteras and no longer flows along the edge of the continental shelf, meanders form. The meanders become more extreme as they move north and may eventually be pinched off to form warm- or cold-core rings. The rings generally drift south and dissipate or are reabsorbed by the Gulf Stream. Meander and ring formation is variable and complex, as shown in Figure 8-16.

The rings are from 100 to 300 km across and can extend to the seafloor. They are encircled by swiftly flowing currents that move at approximately 90 cm•s^–1. Both cold- and warm-core rings drift southeastward and slowly disappear as the core water temperature changes to match that of the surrounding water. Many rings are reattached to, and reabsorbed by, the Gulf Stream. Cold-core rings can maintain their integrity for up to several months depending on their direction of travel, but warm-core rings do not generally last as long, because of the shallower water and narrow shelf they travel over.

Gulf Stream rings transport heat, dissolved substances (including nutrients), and marine organisms that are weak swimmers or that prefer the warmer or colder water. Specific locations within the complex frontal system, with its associated warm- and cold-water rings, are better than others for certain fish species. Hence, the location of the Gulf Stream and its rings is important to fishers. This information is also important to ships if they are to minimize fuel costs by taking advantage of, or not fighting, ocean currents. The position of the Gulf Stream front and its rings is now monitored and forecasted. Many oceangoing fishing and other vessels obtain frequent up-to-date satellite images of the Gulf Stream region while at sea, just as they receive weather forecast maps.

Swiftly flowing meanders and rings are also present in other western boundary currents, such as the Kuroshio Current and eddies similar to Gulf Stream rings are present throughout the oceans. The general term for these eddies, including Gulf Stream Rings is “mesoscale eddies.” Most mesoscale eddies are less well defined than Gulf Stream rings, and their current speeds are generally lower. Mesoscale eddies, which in some cases extend all the way to the deep-sea floor, are the ocean equivalent of atmospheric high- and low-pressure zones. They range in diameter from 25 to 200 km, drift a few kilometers per day, and generally have rotating currents of between about 10 cm•s^–1 and the 90 cm•s^–1 of the Gulf Stream ring currents. In comparison, atmospheric depressions are about 1000 km across, travel about 1000 km per day, and sustain rotating winds of up to approximately 20 m•s^–1.

Mesoscale eddies transport and distribute heat and dissolved substances within the oceans. One area off the tip of South Africa has been well studied because numerous eddies are created at the location where the south-moving Agulhas Current meets the subtropical convergence and turns eastward in the Indian Ocean subtropical gyre. In this region, numerous large eddies are formed (Fig 8-18), and some of the eddies form warm core rings that spin off into the South Atlantic Ocean, transferring heat and water from the Indian Ocean into the Atlantic Ocean. These eddies travel across the South Atlantic Ocean and to the north and can persist for 3 to 4 years. This transfer of water and heat between oceans is an essential part of the global ocean circulation called the Meridional Overturning Circulation (MOC), which is discussed later in this chapter. 

Figure 8-18. Meanders and mesoscale eddies formed at the southern end of the Aghulhas Current. Eddies spin off to the west into the Atlantic Ocean and transport water and heat from the Indian Ocean surface waters to the Atlantic Ocean surface waters. These eddies, similar to Gulf Stream rings, travel to the northwest and persist for several years before being fully mixed with Atlantic Ocean water.

Mesoscale eddies are the subject of considerable ongoing research efforts. However, mesoscale eddies are smaller and slower-moving than atmospheric eddies, and ocean currents are difficult to measure, especially below the surface layer. As a result, the tracking, understanding, and forecasting of ocean “weather,” particularly below the surface layers, will likely always be much more difficult than the observation and forecasting of atmospheric weather.