Cold-core rings are a type of oceanic eddy, which are characterized as unstable, time-dependent swirling ‘cells’ that separate from their respective ocean current and move into water bodies with different physical, chemical, and biological characteristics. Their size can range from 1 mm to over 10,000 km in diameter with depths over 5 km. Cold-core rings are the product of warm water currents wrapping around a colder water mass as it breaks away from its respective current. The direction an eddy swirls can be categorized as either cyclonic or anticyclonic depending on the hemisphere. A counterclockwise movement of water in the Northern hemispheres is cyclonic, but the same counterclockwise movement is anticyclonic in the Southern hemisphere (Yasuda, 2000). Although eddies have large amounts of kinetic energy, their rotation is relatively quick to diminish in relation to the amount of viscous friction in water. They typically last for a few weeks to a year. The nature of eddies are such that the center of the eddy, the outer swirling ring, and the surrounding waters are well stratified and all maintain their distinct properties throughout the eddy’s short time-scale.
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Formation
The governing forces of an eddy are very dependent on its size. Small eddies are largely governed by viscosity and direction of a water body. However larger eddies are formed from a balance between the horizontal pressure gradient force, arising from differences in densities of the meeting water masses, and the Coriolis force. Due to the turbulent nature of the earth’s oceans, eddies can be found almost everywhere. Mesoscale eddies are usually seen in areas of intense, winding currents such as the Gulf Stream and the Antarctic Circumpolar Current that feeds into the South Pacific and Indian oceans, but in general can be caused by a combination of factors such as cooling of sea surface temperature, convection, direct generation from wind, or water flow past an irregular coastline. Cycolic cold-core eddies are frequently formed at the polar front by the Gulf Stream and Labrador Current. The cold, nutrient-rich waters from the Labrador Sea flow south and get caught in the eastward meandering of the Gulf Stream, traveling east across the Atlantic Ocean.
Physics, Chemistry, and Biology
All eddies are capable of transporting energy, momentum, heat, physical and chemical water properties, and even small organisms across very large distances. Since eddies mix waters with different properties, they act as an exchange of nutrients from say the continental shelf to the deeper ocean as they travel. This makes them ideal hot spots for primary productivity, especially in areas of low nutrients, such as the center of open ocean gyres. The importance of these swirling masses lies in the incredible amount of kinetic energy they are able to transport both horizontally and vertically, their participation in air-sea interaction, and the irreversible mixing of water masses. These processes all contribute to the transfer of nutrients, oxygen, and trace chemicals, ocean stratification and density fields, and patterns of warmth that drive atmospheric and oceanic circulation.
Permanent and Semi-permanent Cold-core Rings
Semi-permanent and permanent eddies are also recognized in relative abundance throughout the world. These are eddies that form in the same location on a regular seasonal or systematic basis, and typically have the same trajectory. Some permanent eddies are regular enough to be given names within the ocean current system such as the warm-core Kuroshio Ring off Japan and the cold-core Agulhas Ring off the tip of South Africa. Western boundary currents like the Agulhas, Brazil, and East Australian Currents are known for shedding eddies downward off their end points. A semi-permanent cold-core eddy is periodically formed by the Loop Current in the eastern Gulf of Mexico, where warm water from the South Equatorial Current travels upwards through the south Caribbean ocean and flows into the Loop Current off the coast of Cancun.