The Gulf Stream is the name we give to the stream of warm water emanating from Gulf of Mexico through the Florida St and continuing north and east past Cape Hatteras. Starting with the Ben Franklin map and continuing into the present this perception of the Gulf Stream as a open ocean river is firmly etched in our minds. We also know this warm water continues east and north across the Atlantic contributing in a major role in the mild climate of Europe. Accordingly, you might imagine that anything floating in the center of the current will be swept east and north toward Europe. So why is it that surface drifters positioned in the center of the Gulf Stream at Cape Hatteras, say, never stay in the current but escape to the south into the Sargasso Sea? There are two parts to this story, the first one about surface drifters and the other one about drifters at depth, for they too, will leave the current to the south.
The warm surface water of the Gulf Stream is of course advected by the underlying current, but it is also subject to the action of winds. Waves kicked up by the winds will stir the waters creating a well-mixed surface layer that is shallow in summer and deeper in winter when the winds are stronger. The winds cause this surface layer to slide over the underlying ocean. Due to the rotation of earth this layer slides to the right of the wind which means that the westerlies blowing over the Gulf Stream will push the surface waters toward the south. This wind-driven slippage isn’t large, in fact it is barely measurable. But the cumulative effect is striking: surface drifters placed in the center of the Gulf Stream at Cape Hatteras will sooner or later wind up to its south in the Sargasso Sea. Curiously, even though the winds are weaker in summer, a drifter will exit the stream sooner because the mixed layer is so shallow that its wind-driven velocity is greater. You can see the effect of earth’s rotation at any science museum that sports a Foucault pendulum. You can also imagine the effect: Consider a flat and frictionless bowling alley and send a ball down the lane at a very slow speed. The ball goes in a straight line since it is rolling ahead frictionlessly, no forces are acting on it. But the alley, on our rotating planet, is slowly turning to the left. But the frictionless ball doesn’t know this so relative to the alley it will drift to the right. The slower the ball’s motion, the more earth will turn before the ball reaches the end of the lane and thus the larger the deflection. This drift of surface water to the right is known as Ekman drift after the Swedish oceanographer V. W. Ekman.
Interestingly, drifters deployed in the Gulf Stream below the surface will also exit the stream to the south, but for an entirely different reason. Between the Gulf Stream and the US-Canada slope cold water from the Labrador Sea flows west in what is sometimes referred to as the Slope Sea. But it is closed in the west where the Gulf Stream passes Cape Hatteras. As a result the Labrador Sea water is forced into the east-flowing Gulf Stream. This is a gradual or weak process all along the Gulf Stream, but the effect is cumulative such that any float below the surface in the stream or in the Slope Sea will at some point be expelled to the south of the stream. Due to the increasingly large meandering of the stream east of Cape Hatteras water is not only expelled from the Gulf Stream, but considerable mixing across the current also occurs such a subsurface float can readily move from one side of the current to the other. But on average a float will exit the stream to the south. A consequence of this is that water from the Labrador Sea will mix with Gulf Stream water and eventually wind up on the southern side all along the current from Cape Hatteras to south of Newfoundland! A rather porous river if you will. The flow of warm water is vastly greater than that of cold Labrador Sea water so while the two will mix, the impact of the cold water on the Gulf Stream in terms of temperature is minor.
Southeast of Newfoundland the warm Gulf Stream branches out in three directions. One turns northeast as the well-defined North Atlantic Current, another continues east as a broad drift, and the third turns south and west. As the North Atlantic current flows northeast past the Grand Banks, peels of warm water break off to the east ensuring a warm ocean at these latitudes. At 50-52°N at what is called the Northwest Corner the North Atlantic Current turns sharply to the east. It crosses the Atlantic into and across the Iceland Basin west of the British Isles. It is this continual supply of warm water from the North Atlantic Current branch (compared to the same latitudes of the Pacific) that heats the westerlies that bring warm moist air toward central Europe and Nordic countries. So yes, the Gulf Stream plays a fundamental role in our climate, but except in the Gulf of Mexico and along the US east coast and a bit beyond where its flow is well-defined, one might think of the Gulf Stream as a boundary between the warm North Atlantic and the cold waters from the Labrador Sea, a boundary stretches from Cape Hatteras in the west up past the Grand Banks toward Iceland in the north.