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Sea Science: Gulf Stream current, eddies keep big river of the Atlantic moving

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Sea Science: by Scott McDowell

The Gulf Stream “system” begins with westward surface flow through the Caribbean, looping through the Gulf of Mexico, then northward as the Florida Current (FC) and into the Atlantic. The beginning of the Gulf Stream (GS) “current” is defined as the location where the FC branches away from the U.S. east coast, south of Cape Hatteras.

Thereafter, the GS acts as a river within the upper layers of the western North Atlantic, extending northeastward for more than 1,000 nautical miles with sinuous meanders and shedding of large eddies.

The GS is prominent in satellite imagery from its contrast with cooler surrounding waters, as well as by satellite altimetry that detects the large vertical bulge of low-density water contained in the warm GS. In practical terms, the sea level in the GS is roughly three feet higher than on the north side of the boundary, where coastal waters are considerably more dense. Fortunately, the horizontal GS boundary spans 5-10 miles so there isn’t a large wall of water for mariners to encounter at the GS edge.

The volume of water transported by the GS increases from 30 million cubic-meters-per-second (30 Sverdrup-units of flow) in the FC at Cape Hatteras to roughly 145 SV at the longitude of Newfoundland and latitude of New Jersey. This influx of water is mostly associated with deep entrainment from both sides of the GS.

The average speed of Gulf Stream surface waters throughout the western North Atlantic (www.oceancurrents.rsmas.miami.edu). Northeastward flow in the GS clearly decreases east of 50 degrees W.

At this location south of Newfoundland, the GS branches with most of the flow contained in a northern leg, thereafter called the North Atlantic Current (NAC), which continues north and northeastward. At mid-ocean, the NAC transitions into a weak eastward flow called the North Atlantic Drift Current that transports relatively warm waters to latitudes higher than in any other ocean. This NADC is responsible for moderating climates in the United Kingdom and western Scandinavia.

The GS is a major current in the North Atlantic with a close equivalent in the North Pacific: the Kuroshio Current off the coast of Japan. Although there is no Caribbean Sea nor Gulf of Mexico analog in the Pacific, the Kuroshio and GS are both Western Boundary Currents that return flow to the northeast and the central ocean. These important currents balance the westward current flow at lower latitudes caused jointly by the eastward rotation of the earth and westward surface winds at subtropical latitudes. Amazing similarities in our two northern oceans.

East of Cape Hatteras, our GS forms large eddies as the surface current meanders north or south from its average direction. As the GS bends northward, the clockwise current can turn sharply to the south and pinch off, creating a warm-core eddy that spins clockwise, thereafter embedded in cooler waters from the north. Warm water from the south side of the GS remains trapped in the spinning eddy.

Warm-core eddies have typical diameters of 50-80 nautical miles and are readily distinguishable by satellite surface thermal imagery due to their contrast with cooler inshore waters. They normally persist for three months to one year while drifting southwestward, eventually coalescing with the GS.

Similarly but opposite, when the GS meanders sharply to the south and turns counter-clockwise, another type of pinch-off eddy is formed containing relatively cool waters in comparison with surrounding waters of the Sargasso Sea, thus the name cold-core eddy. These eddies have typical diameters of 100-150 nautical miles and can exist for one to four years as they drift freely toward the south and southwest. They gradually become indistinguishable from the surrounding waters as their surface temperatures increase.

The physics of GS eddies are well known and Atlantic mariners regularly navigate along their peripheries to gain from favorable currents on one edge or the other. My marinas guidebook provides NOAA websites for near-real-time GS positions and locations of major eddies.

Additionally, websites are given for private marine forecasters of Atlantic surface currents and GS eddy trajectories. Use oceanographic knowledge to increase your speed over the ground.
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. Comment at editor@the-triton.com.

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