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The Florida current is a well-defined component of the Gulf Stream system. On the average, the inner edge is within 10 miles of Miami and Ft. Lauderdale, FL, and at times there is a 2 m/s flow within a few miles of the coast. Note that part of the flow recirculates in an elongated counter-clockwise flowing cell in the South Atlantic Bight. Click here for example plots of seasonal averages. | |
The Florida Current has been shown to have a mean transport of about 30 Sv in historical literature (Schmitz and Richardson, 1968; Niiler and Richardson, 1973). This value has been confirmed in numerous studies and has stood up to modern scrutiny. More recently the STACS study confirmed this value with the finding a mean transport of 31.5 Sv at 27°N in the straits of Florida (Molinari et al., 1985; Leaman et al., 1987; Schott et al., 1988; Lee et al 1985; Larsen and Sanford, 1985). Researchers used undersea cables, current meter moorings and a Pegasus profiler, and all three methods produced transports that were within 1-2 Sv of each other. There has however, been demonstrated that this current is subject to both seasonal and interannual variability. These changes are significant and can amount to as much as a 10 Sv difference between high and low values along the eastern Florida coast (Schott et al. 1988). Most of this water appears to originate in the Gulf of Mexico. Early estimates of inflow through island passages in the Florida Straits are only about 3.5 Sv. (Schmitz and Richardson, 1968) Later estimates are much larger with Schmitz and Richardson (1990) reporting a total of 28.8 Sv for five key passages, Grenada, St. Vincent, St. Lucia, Dominica and Windward. Wilson and Johns (1996) found an influx of 17.5 Sv and note the presence of strong outflows in these passages as well. Flow through these passages is highly variable and may in part account for the considerable variability of the Florida Current.
The transport of the Florida current has been shown to increase substantially between the Straits of Florida and Cape Hatteras. According to Leaman et al. (1989) there is only a relatively small transport increase of 3 Sv just outside the Straits of Florida (33.2 Sv at 29°N). However they found that transports increased 3 fold from the Straits of Florida (29 Sv at 27°N) to Cape Hatteras (93.7 Sv to bottom at 73°W and 86.8Sv to 2000m ). Interestingly they found that the Florida Current doubled it's width at 29°N and that 45.9 Sv of the total transport at Cape Hatteras is baroclinic. Richardson et al. (1969) found a transport og 29.6 Sv off the Florida Keys, increasing to 53 Sv off Cape Fear. They found the increase constant with depth and thus concluded that it was essentially baroclinic. This transport increases downstream to a maximum of about 85 Sv near Cape Hatteras (Worthington and Kawai, 1972) as a result of input from recirculating gyres. The width of the Florida Current is approximately 80 km at 27°N, 120 km at 29°N (Figure 8 of Leaman et al., 1989) and slowly increases to a width of 145 km for the Gulf Stream at 73°W ( Please see Figure 7 of Gulf Stream Example Plots).
The dominant meanders, determined from current meter data in the Florida Current, have wavelengths of 340 km and 170 km, periods of 12 days and five days, and propagate at 28 km/d and 36 km/d, respectively (Johns and Schott, 1987). The amplitude of the meanders increase outside of the constraint of the Straits of Florida. Meanders and the eddies they generate serve as the principal form of mesoscale variability along the path of the Florida Current within the Mid Atlantic Bight (between Cape Canaveral, Florida and Cape Hatteras, North Carolina). The Florida Current is deflected offshore near 32°N and it's eddy variability decreases downstream of this deflection (Olson et al., 1983; Vukovich and Crissman, 1978). The deflection of the Florida Current is caused by the presence of a topographic irregularity known as the Charleston Bump near 31°N. This deflection in the path of the current has been shown to be bimodal in character with the Florida Current assuming either a weakly or strongly deflected state. Bane and Dewar (1988) observed that the transition between weakly and strongly deflected modes can occur rapidly, within a few days. The Florida current can remain in the strongly deflected mode for at least several months. They also note that the differing modes are associated with different types of low frequency variation downstream.
The seasonal signal in the Florida Current was discovered in tide gauge measurements by Montgomery (1938) who found evidence for a seasonal maximum in July and minimum in October with secondary maximum and minimum in January and April respectively. Other early reports of this signal include Iselin (1940), Fuglister (1951), Patullo et al. (1955), Wunsch et al. (1969), Schmitze and Richardson (1968) and Niiler and Richardson (1973). Niiler and Richardson (1973) found a winter transport of 25.4 Sv and summer transport of 33.6 and that seasonal changes accounted for about 45% of the variability between historical transport estimates. This supports Wuncsch et al. (1969) who, based on sea level height differences, concluded that the seasonal transport differences were approximately 10% of the annual mean signal. STACS results indicated that transports can vary between 20Sv and 40 Sv (Molinari et al. 1985; Leaman et al. 1987; Schott et al. 1988; Lee et al 1985; Larsen and Sanford, 1985). Recently the annual signal has been attributed to a build up and breakdown of the Bermuda High as it relates to the seasonal cycle with the trade winds. Lee et al. (1996) found a correlation between the observed wind field and observed transport variability confirming the predictions in the numerical model of Boning et al (1991). In addition to the annual signal, there is also a semiannual summer maximum tied to an additional strengthening of the trade winds and increase in local winds (Lee et al. 1996). Historical records have shown that monthly variability within the Florida current can be as high as the seasonal variability There are also short term variations with periods of 2-20 days. These fluctuations are correlated to local winds and are much stronger in summer than in winter (Lee and Williams, 1988).