Sea Science: by Scott McDowell
The strong northward current between the eastern shore of Florida and the Bahamas is officially named the Florida Current (FC), as clarified by Dr. Arthur Mariano of the University of Miami. Mariano has created an excellent website illustrating all surface currents in the world’s oceans (http://oceancurrents.rsmas.miami.edu).
Floridians often call their local current the Gulf Stream (GS), but the FC is actually the initial, southern leg of the GS System, which extends along the eastern shore of the United States and into high latitudes of the central North Atlantic.
Although South Florida’s indigenous populations, which date back 12,000 years, had likely encountered the strong current, Ponce de Leon was the first European to encounter it in 1513 during his search for the Fountain of Youth. He named the landmass “La Florida” as the discovery was made during the Easter season, called Pascua Florida (Festival of Flowers).
Numerous oceanographic studies have been conducted to determine the FC’s spatial characteristics as well as the physics that drive this intense current. East of southern Florida, the FC has northward surface-currents with maximum speeds of 5 knots, which vary with the season due to a variety of oceanographic factors. Its width near Miami is roughly 30 miles. Speeds of the FC decrease with depth, penetrating to a maximum of about 3,000 feet.
The volume of water transported by the FC is huge, estimated at 30 million cubic meters per second. This flow rate is roughly 1,700 times that of the Mississippi River. To better appreciate the volume flow of the FC, imagine a wall of water 30 miles wide, 3 feet thick and 2,000 feet high. This much water flows north in the FC each second.
The temperature of the FC water closely matches that of the surface water in the Gulf of Mexico, it’s source. In winter, warm waters of the FC contrast sharply with the relatively cool, nearshore waters along the Atlantic coast of Florida. Boaters easily recognize the western edge of the FC by the sharp surface temperature gradient. During summer, however, the 85-degree water in the core of the FC is minimally warmer than the local waters offshore Florida and surface temperature gradients are often non-existent.
Flow within the FC varies by season, with strongest flow in July and weakest in October; fluctuations can achieve 30 percent of the average flow. Research has shown that flow variability is associated with atmospheric processes, namely the build-up of the Bermuda High and strengthening of the westward-blowing trade winds that peak in summer. Other flow variability in the FC occurs on timescales of days to weeks due to flow turbulence.
Note that trade winds from the east are perpendicular to the Florida coast, which raises the question of how they can impact the northward flow of the FC. Because the broad trades also drive westward surface currents in the Caribbean, this increases flow into the Gulf of Mexico through the Yucatan Straits between Mexico and the western tip of Cuba. In order for the Gulf to maintain a constant level (a physical necessity), outflow also must increase, causing intensification of the FC through the Florida Straits.
Within the Gulf of Mexico, the majority of the entering Caribbean water remains as a coherent flow (like another river) that heads north into the central Gulf. This flow has been named the Loop Current because it turns clockwise, first to the east, then southward along the western shore of Florida, eventually turning eastward to exit the Gulf between northern Cuba and the southern tip of Florida. The flow hugs the left shore and turns counter-clockwise to the north, as the beginning of the FC.
The majority of the water in the FC consists of Gulf water via the Loop Current but a lesser amount of flow joins the FC from westward flow through various passages in the Bahamas.
My next article will describe the northern elements of the mighty Gulf Stream, whose volume flow increases by a factor of five as it extends thousands of miles to the northeast, nearly across the North Atlantic.
Scott E. McDowell has a doctorate in ocean physics, is a licensed captain and author of Marinas: a Complete Guide, available at www.scottemcdowell.com.