A parent to the

Florida Current The Florida Current is a thermal ocean current that flows from the Straits of Florida around the Florida Peninsula and along the southeastern coast of the United States before joining the Gulf Stream Current near Cape Hatteras. Its contributing ...

, the Loop Current is a warm

ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours ...

that flows northward between

Cuba Cuba ( , ), officially the Republic of Cuba ( es, República de Cuba, links=no ), is an island country comprising the island of Cuba, as well as Isla de la Juventud and several minor archipelagos. Cuba is located where the northern Caribbea ...

and the

Yucatán Peninsula The Yucatán Peninsula (, also , ; es, Península de Yucatán ) is a large peninsula in southeastern Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north ...

, moves north into the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

, loops east and south before exiting to the east through the

Florida Straits The Straits of Florida, Florida Straits, or Florida Strait ( es, Estrecho de Florida) is a strait located south-southeast of the North American mainland, generally accepted to be between the Gulf of Mexico and the Atlantic Ocean, and between th ...

and joining the Gulf Stream. The Loop Current is an extension of the western boundary current of the North Atlantic subtropical gyre. Serving as the dominant circulation feature in the Eastern Gulf of Mexico, the Loop Currents transports between 23 and 27

sverdrup In oceanography, the sverdrup (symbol: Sv) is a non- SI metric unit of volumetric flow rate, with equal to . It is equivalent to the SI derived unit cubic hectometer per second (symbol: hm3/s or hm3⋅s−1): 1 Sv is equal to 1 hm3/s. It is u ...

s and reaches maximum flow speeds of from 1.5 to 1.8 meters/second. A related feature is an area of warm water with an "

eddy Eddie or Eddy may refer to: Science and technology *Eddy (fluid dynamics), the swirling of a fluid and the reverse current created when the fluid flows past an obstacle * Eddie (text editor), a text editor originally for BeOS and now ported to Lin ...

" or "Loop Current ring" that separates from the Loop Current, somewhat randomly every 3 to 17 months. Swirling at 1.8 to 2 meters/second, these rings drift to the west at speeds of 2 to 5 kilometers/day and have a lifespan of up to a year before they bump into the coast of

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. These eddies are composed of warm Caribbean waters and possess physical properties that isolate the masses from surrounding Gulf Common Waters. The rings can measure 200 to 400 kilometers in diameter and extend down to a depth of 1000 meters.

Effect on tropical cyclones

Around 1970, it was believed that the Loop Current exhibited an annual cycle in which the Loop feature extended farther to the north during the summer. Further study over the past few decades, however, has shown that the extension to the north (and the shedding of eddies) does not have a significant annual cycle, but does vacillate in the north-south and east-west directions on an inter-annual basis. The Loop Current and its eddies may be detected by measuring sea surface level. Sea surface level of both the eddies and the Loop on September 21, 2005 was up to 60 cm (24 in) higher than surrounding water, indicating a deep area of warm water beneath them. On that day, Hurricane Rita passed over the Loop current and intensified into a Category 5 storm with the help of the warm water. In the Gulf of Mexico, the deepest areas of warm water are associated with the Loop Current. The rings of current that have separated from the Loop Current are commonly called Loop Current eddies. The warm waters of the Loop Current and its associated eddies provide more energy to hurricanes and allow them to intensify. As hurricanes pass over warm areas of the Gulf of Mexico, they convert the ocean's heat into storm energy. As this energy is removed from the seas, a wake of colder water can be detected along the hurricane's path. This is because heat is withdrawn from the ocean mixed layer in a number of ways. For instance, sensible and latent heat are lost directly to the tropical cyclone across the air-sea interface. Also, the horizontal divergence of wind-driven mixed layer currents results in the upwelling of colder

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

water. Finally, the turbulent entrainment of colder thermocline waters caused by wind stirring also results in the cooling of the surface waters. These are the reasons that the depth of the ocean mixed layer is more important in hurricane deepening than sea surface temperature. A thin veneer of warm surface waters will be more susceptible to hurricane induced cooling than waters with a larger mixed layer and deeper thermocline. Furthermore, models suggest that cyclones are more likely to reach a larger fraction of their maximum potential intensity over warm oceanic features where the 26 °C isotherm extends beyond 100 meters. An example of how deep warm water, including the Loop Current, can allow a hurricane to strengthen, if other conditions are also favorable, is

Hurricane Camille Hurricane Camille was the second most intense tropical cyclone on record to strike the United States, behind the 1935 Labor Day hurricane. The most intense storm of the 1969 Atlantic hurricane season, Camille originated as a tropical depression ...

, which made landfall on the

Mississippi Gulf Coast The Mississippi Gulf Coast, also known as Mississippi Coast, Mississippi Gulf Coast region, Coastal Mississippi, and The Coast, is the area of Mississippi along the Mississippi Sound at the northern extreme of the Gulf of Mexico. Geography At th ...

in August 1969. Camille formed in the deep warm waters of the Caribbean, which enabled it to rapidly intensify into a category 3 hurricane in one day. It rounded the western tip of Cuba, and its path took it directly over the Loop Current, all the way north towards the coast, during which time the

rapid intensification In meteorology, rapid intensification is a situation where a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained wi ...

continued. Camille became a Category 5 hurricane, with an intensity rarely seen, and extremely high winds that were maintained until landfall ( sustained winds were estimated to have occurred in a very small area to the right of the eye). In 1980,

Hurricane Allen Hurricane Allen was a rare and extremely powerful Cape Verde hurricane that affected the Caribbean, eastern and northern Mexico, and southern Texas in August 1980. The first named storm and second tropical cyclone of the 1980 Atlantic hurricane ...

strengthened to a Category 5 hurricane while moving over the Loop Current, but it weakened before its landfall in

Hurricane Ivan Hurricane Ivan was a large, long-lived, Cape Verde hurricane that caused widespread damage in the Caribbean and United States. The cyclone was the ninth named storm, the sixth hurricane and the fourth major hurricane of the active 2004 Atlant ...

rode the Loop Current twice in 2004. In 2005, Hurricane Katrina and Hurricane Rita both greatly increased in strength when they passed over the warmer waters of the Loop Current.

Hurricane Wilma Hurricane Wilma was an extremely intense and destructive Atlantic hurricane which was the most intense storm of its kind and the second-most intense tropical cyclone recorded in the Western Hemisphere, after Hurricane Patricia in 2015. Part o ...

of 2005 was expected to make its Florida landfall as a Category 2 hurricane, but after encountering the southeastern portion of the Loop Current, it reached the Florida coast as a category 3 instead. While not as infamous as Katrina,

Hurricane Opal Hurricane Opal was a large and powerful Category 4 hurricane that caused severe and extensive damage along the northern Gulf Coast of the United States in October 1995. The fifteenth named storm, ninth hurricane and strongest tropical cyclon ...

most definitively illustrates the deepening abilities of a warm core ring. After crossing the

, Opal reentered the Gulf of Mexico and passed over an eddy shed by the Loop Current. Within a fourteen-hour period, sea surface pressure dropped from 965 to 916 hectopascals, surface winds increased from 35 to 60 meters/second, and the storm condensed from a radius of 40 kilometers to 25 kilometers. Prior to the storm, the 20 °C isotherm was located at a depth between 175 and 200 meters, but was found 50 meters shallower after the storm had passed. While the majority of this hurricane induced cooling of the mixed layer was attributed to

upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...

(due to Ekman divergence), another 2000 to 3000 watts/meter squared were estimated to be lost through heat flux at the air-water interface of the storm's core. Furthermore, buoy-derived sea surface temperature readings recorded temperature dropping 2° to 3 °C as Opal passed over Gulf Common Waters, but only 0.5° to 1 °C as the storm encountered the more massive ocean mixed layer associated with the warm core eddy. In 2008,

Hurricane Gustav Hurricane Gustav () was the second most destructive hurricane of the 2008 Atlantic hurricane season. The seventh tropical cyclone, third hurricane, and second major hurricane of the season, Gustav caused serious damage and casualties in Haiti, ...

transited the Loop Current, but due to the current's temperature (then only in the high 80's-degrees-F) and truncated size (extending only halfway from Cuba to Louisiana, with cooler water in-between its tip and the Louisiana coast) the storm remained a category 3 hurricane instead of increasing strength as it passed over the current. In August 2021,

Hurricane Ida Hurricane Ida was a deadly and extremely destructive Category 4 Atlantic hurricane in 2021 that became the second-most damaging and intense hurricane to make landfall in the U.S. state of Louisiana on record, behind Hurricane Katrina in 200 ...

moved over the Loop Current, allowing it to rapidly intensify from a Category 2 into a Category 4 hurricane over just one night.

Process

Hurricane strengthening and weakening is the product of extensive thermodynamic interactions between the atmosphere and the ocean. Generally speaking, the evolution of a hurricane's intensity is determined by three factors. First, the initial intensity of a tropical cyclone is a predominant factor and its strength will be reflected throughout the storm's life. Second, the thermodynamic state of the atmosphere through which the cyclone moves will affects its ability to strengthen, as strong horizontal winds will disperse internal circulation and prevent the vertical stacking of energy within the storm. The third component affecting hurricane intensity is the heat exchange between the upper layer of ocean waters and the core of the storm. For this reason, a major focus of hurricane research has been sea surface temperature prior to a storm. However, recent studies have revealed that surface temperature is less important in hurricane deepening than the depth of the ocean mixed layer. In fact, a hurricane's sea level pressure has been shown to be more closely correlated with the 26 °C isotherm depth (and oceanic heat content) than the sea surface temperature. Storms passing over the Loop Current or warm core eddies have access to more tepid water, and therefore the higher energy content of the heated molecules. Once Hurricane Rita left the Loop Current and passed over cooler water, it declined in strength, but the main factor in this weakening was an

eyewall replacement cycle In meteorology, eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than , or major hurricanes ( Category 3 or above). When tropical cyclones reach this int ...

(ERC) occurring at that time. The ERC and other atmospheric factors are why Rita did not reintensify when subsequently passing over the eddy vortex. Also of note: tropical depressions, tropical storms, and hurricanes gain strength from, but are not steered by, the temperature of the water. They are steered by the atmosphere, and the atmospheric level involved in steering a hurricane is different at different intensities (i.e., it relates to the minimum pressure of the hurricane).

Sea level and sea temperature

Sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardise ...

is relatively easy to measure accurately using radars from satellites. Sea temperature below the surface is not as easy to measure widely, but can be inferred from the sea level since warmer water expands and thus (all other factors, such as water depth, being equal) a vertical column of water will rise slightly higher when warmed. Thus sea level is often used as a proxy for deep sea temperatures. NOAA's National Data Buoy Center maintains a large number of data buoys in the Gulf of Mexico, some of which measure sea temperature one meter below the surface.

Biology

The Loop Current and Loop Current Eddies affect biological communities within the Gulf of Mexico. In general, however, it is not the warm-core Loop Current and eddies themselves that affect these communities. Instead, it is the smaller cold-core features known as Frontal Eddies that form around the boundary of the Loop Current and Loop Current Eddies, which affect biological communities in the Gulf. Loop Current Frontal Eddies are cold-core, counter-clockwise rotating (cyclonic) eddies that form on or near the boundary of the Loop Current. LCFEs range from about 80 km to 120 km in diameter. These cold features are smaller than the warm-core eddies shed from the Loop Current. Multiple studies have shown differences in biological communities inside versus outside of the various features in the Gulf of Mexico. Higher standing stocks of

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

and micronekton were found in cold-core features than in both the Loop Current and the Loop Current Eddies. However, no difference in the abundance of

euphausiids Krill are small crustaceans of the order Euphausiacea, and are found in all the world's oceans. The name "krill" comes from the Norwegian word ', meaning "small fry of fish", which is also often attributed to species of fish. Krill are consid ...

, planktonic shrimp-like marine crustaceans, was found between areas of upwelling and warm-core eddies, but in 2004 the hyperiid abundance was found to be lower within Loop Current Eddies as opposed to outside. Concurrently, it was found that nutrient (nitrate) levels were low above 100 meters within warm-core eddies, while nitrate levels were high within cold features. Low standing stock of chlorophyll, primary production, and zooplankton biomass was found to be low in LCEs. Low chlorophyll concentrations and primary production are likely a result of low nutrients levels, as many planktonic species require nitrate and other nutrients to survive. In turn, low primary production could be one cause of heterotrophic (organism-eating, as opposed to photosynthetic) species abundances being low inside the Loop Current and Loop Current Eddies. Alternatively, temperature may play a role for low abundances of both communities: Atlantic Blue Fin Tuna have developed behavioral patterns of avoiding the high temperatures associated with warm-core features, such as the Loop Current and Loop Current Eddies, in the Gulf of Mexico. It is possible, also, that planktonic species likewise avoid the higher temperatures in these features.

References

External links