The clockwise flow that extends northward into the Gulf of Mexico and joins the Yucatan Current and the Florida Current is known as the Loop Current (Hofmann and Worley, 1986). Historically, average transport values for these three currents have been around 30 Sv (e.g. Morrison and Nowlin, 1977; Nowlin and McClellan, 1967; Schmitze and Richardson, 1968). Although recent measurements (Sheinbaum et al. 2002) contend that the mean transport is 24 Sv. Near-surface velocities approach 80 cm s-1 in the Yucatan Current (Coats, 1992) and have been reported as high as 150 cm s-1 (at 300 m) (Nowlin and McLellan, 1967). Additionally, the Yucatan and Florida current have been shown to be within 10% of each other's volume at any given time (Molinari and Morrison, 1988). Thus, variability in both the Loop and Yucatan currents would be expected to have a strong impact on the Florida Current as well.
The average ship-drift dervied surface velocities show the well-know omega-shaped flow pattern of the Loop current. The Loop Current (1) feeds the Florida Current that transports significant amounts of heat poleward; (2) transports surface waters of tropical origin into the Gulf of Mexico; and (3) is fed by the Caribbean current and the Yucatan Current. Click here for example plots of seasonal averages.
The Loop Current is variable in position. At one extreme, it has an almost direct path to the Florida Current, causing the shear in the flow to set up a quasi-permanent clockwise recirculation known as the Cuban Vortex. This feature may help initiate Loop Current expansion. (Coats, 1992; Nowlin and McLellan, 1967; Cochrane, 1972; Hoffmann and Worley, 1986). At the other extreme, the Loop Current intrudes into the Gulf of Mexico, forming an intense clockwise flow as far north as 29.1N. Occasionally this loop will reach as high as the Mississippi river delta or the Florida continental shelf (Wiseman and Dinnel, 1988; Molinari and Mayer, 1982; Huh et al., 1981; Vukovich et al., 1979). It was this large loop phase of the current from which Nowlin and McClellan (1967) derived the name Loop Current. The Loop Current returns to its direct configuration by slowly pinching off its extension to form a large, warm-core ring that then propagates westward at speeds of 2-5 km day-1 (Coats, 1992; Elliott, 1982; Shay et al., 1998). The 900 km (Auer, 1987) expanse of Loop Current position is reminiscent of the variability in position at the Gulf Stream Extension region.
Early accounts attempted to identify a spring seasonal signal in the Loop current intrusion (Leipper, 1970; Behringer et al, 1977; Nowlin and Hubertz, 1972; Maul, 1977). However, although the intrusion may tend to form more frequently in the spring, it can occur in any season and has periods varying from 6-17 months (Molinari, 1980). Changes in the Yucatan current position have been correlated with Loop Current position, allowing it to serve as index to the extent of the intrusion (Molinari and Cochrane, 1972). However, the manner in which the two currents affect each other with regards to position is as yet unclear. The Loop Current draws its waters from the Yucatan Current, which is ultimately fed by the Caribbean Current, Guiana Current and North Equatorial Current. This provides a vital link between North Atlantic and South Atlantic waters. However, analysis of 12 years of data indicated no significant correlation between monthly Loop Current position and Florida Current transport (Maul and Vukovich, 1993). Although the frequencies of ring separation vary, Sturges (1992) also was unable to correlate ring separation and changes in transport of the Florida Current. The annual fluctuations in Loop Current flow are apparently due to wind forcing (Sturges and Evans, 1983).
The Deepwater Horizon oil spill in the Gulf of Mexico during 2010 has unfortunately shown how important it is to know and understand not just ocean currents, but all aspects of the ocean environment. The long term effects of the oil spill on the Gulf of Mexico's ecosystem are still unknown but scientists will be studying it for many years (links).
Auer, S.J., 1987: Five-year climatological survey of the Gulf Stream System and its associated rings, Journal of Geophysical Research, 92(11), 11709-11726.
Behringer, D.W., R.L. Molinari, and J.F. Festa, 1977: The variability of anticyclonic current patterns in the Gulf of Mexico, Journal of Geophysical Research, 82(34), 5469-5476.
Coats, D.A., 1992: The Loop Current, J.D. Milliman and E. Imamura (eds.), In the Physical oceanography of the U.S. Atlantic and Eastern Gulf of Mexico, U.S. dept of the Interior, Mineral Management Service, Atlantic OCS Region, Herndon, Va., Chapter 6.
Cochrane, J.D., 1972: Separation of an anticyclone and subsequent developments in the Loop Current (1969), L.R.A. Capurro and J.L. Reid (eds.), In Contributions on the physical oceanography of the Gulf of Mexico, Gulf Publishing Co., Houston, Texas, 91-106.
Elliott, B.A., 1982: Anticyclonic rings in the Gulf of Mexico. Journal of Physical Oceanography, 12, 1292-1309.
Hofmann, E.E., and S.J. Worley, 1986: An investigation of the circulation of the Gulf of Mexico, Journal of Geophysical Research, 91(C12), 14221-14236 .
Huh, O.K., W.J. Wiseman, and L.J. Rouse, 1981: Intrusion of Loop Current waters onto the West Florida continental shelf, Journal of Geophysical Research, 86(C5), 4186-4192.
Leipper, D.F., 1970: A sequence of current patterns in the Gulf of Mexico, Journal of Geophysical Research, 75(3), 637-657.
Maul, G.A., 1977: The annual cycle of the Gulf Loop Current, Part I: Observations during a one-year time series, Journal of Marine Research, 35, 29-47.
Maul, G.A., and F.M. Vukovich, 1993: The relationship between variations in the Gulf of Mexico Loop Current and Straits of Florida volume transport, Journal of Physical Oceanography, 23, 785-796.
Molinari, R.L., 1980: Current variability and its relation to sea-surface topography in the Caribbean Sea and the Gulf of Mexico, Marine Geodesy, 3, 409-436.
Molinari, R.L., and J.D. Cochrane, 1972: The effect of topography on the Yucatan Current, L.R.A. Capurro and J.L. Reid (eds.), In Contributions on the physical oceanography of the Gulf of Mexico, Gulf Publishing Co., Houston, Texas, pp 91-106.
Molinari, R.L., and J.D. Mayer, 1982: Current meter observations on the continental slope at two sites in the eastern Gulf of Mexico, Journal of Physical Oceanography, 12, 1480-1492.
Molinari, R.L., and J. Morrison, 1988: The separation of the Yucatan Current from the Campeche Bank and the intrusion of the Loop Current into the Gulf of Mexico, Journal of Geophysical Research, 93, 10645-10654.
Morrison, and W.D. Nowlin, 1977: Repeated nutrient, oxygen, and density sections through the Loop Current, Journal of Marine Research, 35(1), 105-128.
Nowlin, W.D., and J.M. Hubertz, 1972: Contrasting summer circulation patterns for the eastern Gulf Loop Current versus anticyclonic ring, L.R.A. Capurro and J.L. Reid (eds.), In Contributions on the physical oceanography of the Gulf of Mexico, Gulf Publishing Co., Houston, Texas, pp 91-106.
Nowlin, W.D. Jr., and H.J. McLellan, 1967: A Characterization of the Gulf of Mexico Waters in Winter, Journal of Marine Research, 25, 29-59.
Schmitz, W.J., Jr., and W.S. Richardson, 1968: On the transport of the Florida Current, Deep-Sea Research, 15, 679-693.
Shay, L.K., A.J. Mariano, S.D. Jacob, and E.H. Ryan, 1998. Mean and Near-Inertial Ocean Current Response to Hurricane Gilbert. Journal of Physical Oceanography, 28(5), 858-889.
Sheinbaum, J., J. Candela, A. Badan and J. Ochoa, 2002. Flow structure and transport in the Yucatan Channel. Geophysical Research Letters, 29(3), 10.1029/2001GL013990.
Sturges, W., 1992: The spectrum of Loop Current variability from gappy data, Journal of Physical Oceanography, 22, 1245-1256.
Sturges, W., and J.C. Evans, 1983: On the variability of the Loop Current in the Gulf of Mexico, Journal of Marine Research, 41, 639-653.
Vukovich, F.M., B.M. Crissman, M. Bushnell, and W.J. King, 1979: Some aspects of the oceanography of the Gulf of Mexico using satellite and in situ data, Journal of Geophysical Research, 84(C12), 7749-7768.
Wiseman, Wm, J., Jr., and S.P. Dinnel, 1988: Shelf Current Near the Mouth of the Mississippi River, Journal of Physical Oceanography, 18(9), 1287-1291.