Warm core ring
A warm core ring is a type of mesoscale eddy which breaks off from an ocean current, e.g. the Gulf Stream or the Kuroshio Current. The ring is an independent circulatory system of warm water which can persist for several months. The rest of this article will use the Gulf Stream by way of an example but these mesoscale eddies also form in most powerful ocean currents, such as the Kuroshio or Agulhas currents.
Such rings can be detected using infrared satellites or sea height anomalies and are easily identifiable against the surrounding colder waters. These systems will drift west from their origin at the Gulf Stream until they break apart on the coastal shelf or are reabsorbed by the Gulf Stream.
This type of system is theorized to have helped develop several hurricanes, most notably Hurricane Katrina, into significantly stronger storms due to the abundance of warmer ocean water reaching down to a significant depth. In addition, these eddies can damage offshore drilling equipment due to their currents.
Warm core rings are also known for affecting wildlife, bringing warm-water creatures to unusual areas.
Formation and Movement
As the Gulf Stream flows and moves over time, it can develop large loops which can eventually be pinched off from the current, forming an independent eddy of warmer Sargasso Sea water circulating clockwise similar to the direction of the Gulf Stream. While many warm core rings are created in the Gulf of Mexico, they can develop anywhere along the eastern coast of the United States north of the Gulf Stream. Rings are anywhere from 100–200 km across and can include warmer waters as deep as 1500 meters. These rings develop every 6–11 months on average. However, rings in the Gulf of Mexico can last much longer and are formed at much more irregular intervals than along the eastern U.S. coast. In an active year, up to 15 warm core rings can form due to the Gulf Stream.
Rings will drift to the west-southwest at 3–5 km/day for several months up to a year. The rings always rotate clockwise due to the direction of the Gulf Stream and can reach rotational velocities of up to 1 m/s. Usually warm core rings cannot move onto the continental shelf because they reach deeper than the seafloor on the shelf by over 1000 meters, though they can near the shelf.
Warm core rings are often reabsorbed by the Gulf Stream, but they can break apart on their own as well if they move onto the continental shelf.
Detection and Tracking
Warm core rings are easily observed in the Gulf of Mexico or elsewhere through the use of infrared imagery by weather satellites. Since the ocean water temperature of the ring is significantly higher than the surrounding waters, these rings show up easily in infrared images. This, coupled with models of ring movement, allow well-developed tracking of the rings. Because warm core rings include warm water to a significant depth, infrared satellites can differentiate the temperature, unlike cold core rings, which cannot be easily detected.
Warm core rings are also detected by sea surface height anomalies. Since warm water takes up more space as it expands than cold water, the large amount of warm water causes an upwelling in sea height which can be detected by buoys.
Intensification of Hurricanes
Warm core rings have been linked to the intensification of several hurricanes passing over their location. Because high sea surface temperature as well as warmer water at greater depth is the primary intensifier of a hurricane, warm core rings account for tremendous strengthening of these storms.
Notably, Hurricane Opal passed over a ring and had sudden increases of wind speed from 110 miles per hour to 135 miles per hour shortly before landfall, a trend also seen in Hurricane Allen and Hurricane Camille. There is evidence that Hurricane Katrina and Hurricane Rita, both notable storms which reached Category 5 intensity, as well as Hurricane Ivan, were also strengthened by warm core rings.
Effects on Wildlife
Warm core rings typically include far less biological specimens than the surrounding ocean. When the rings approach continental shelves, coastal currents are affected, which can cause organisms to drift onto the shelf that ordinarily would not be there. In fact, there are human accounts of sea turtles and tropical fish which normally live in much warmer waters coming near the coastal shelf due to the deep, warm waters of a warm core ring.
Damages to Offshore Drilling
Due to currents around warm core rings of up to nearly 5 miles per hour, this phenomenon can damage offshore oil platforms and increase the risk of accidents.
Many fish species’ life cycle involves two distinct habitats. The adults live in warmer temperate waters south of Cape Hatteras, NC while the juveniles are found in estuaries of cooler waters north of Cape Hatteras. Warm Core Rings play an important role in the transport of larvae between the two habitats. Species like the bluefish (Pomatomus saltatrix) and pearly razorfish (Xyrichtys novacula) spawn near the western edge of the Gulf Stream just south of Cape Hatteras. Because of the convergence of the Gulf Stream from the south and cooler coastal water current from the north, most water around Cape Hatteras flows into the Gulf Stream. The larvae released near this convergence is swept into the Gulf Stream and flows north. Since the larvae are planktonic, they don’t swim into the center of the Gulf Stream but stay near the western edge. This is beneficial for when warm core rings form. Warm core rings are formed when the crest of a meander breaks off from the Gulf Stream. Any larvae in the crest of the meanders are then entrapped in the warm core ring. Once the warm core ring breaks way, it takes a southwesternly path towards the coast. The interaction between warm core rings and the continental shelf creates a weakening of the ring and enables the larvae to escape and continue their journey to nearby estuaries. The warm core rings formed along the northeastern states can last between 4 and 5 months. During this time the larvae grow so that by the time they reach the estuaries, they are able to swim away from the warm core ring into the estuaries.
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