Origin of the Gulf Stream
We have seen that the Gulf Stream plays a major role in determining the climate in the northern hemisphere, by taking heat up to high latitudes. This is especially true for the climate of northern and western Europe. But what mechanism drives the Gulf Stream? This question has given rise to much discussion. Some early writers thought the mighty Mississippi River had something to do with. The Mississippi empties in the Gulf of Mexico, and from there the water has to go somewhere. Could this escape motion set up the Florida Current? And from there become the Gulf Stream? An ocean captain, named Captain Livingston, pointed out that the proposed cause was not equal to the observed effect, with the Mississippi's flow being smaller than that of the Gulf Stream by more than a factor of three thousand, a reasoning that makes good sense.
Cartoon demonstrating the Coriolis force
on an object in the Northern Hemisphere. Can you envision the complementary diagram for the Southern Hemisphere?
Later a man named Benjamin Franklin (yes, that Ben Franklin; 1706-1790) thought that the trade winds piled up surface waters in the Caribbean Sea, and they had to go someplace, so they escaped to the north. Franklin understood that currents under the trade winds move west (in tropical waters) and that the currents under the westerly winds move east (at temperate latitudes). He was the first to make a map of the Gulf Stream, and his insights, gleaned in part from Boston whalers, helped American captains beat the British competition (at least until everyone caught on). Like Franklin, Sir John Herschel, a leading 19th century European scientist, also thought that winds were the supreme current-producing power in the sea.
Commander M. F. Maury, who in the middle of the 19th century provided the first text on the "Physical Geography of the Sea, and its Meteorology," took issue with the wind hypothesis of Franklin and Herschel. Maury knew, as a captain and as one who obtained information from many captains at sea, that the currents do not invariably follow the winds, but run at right angles, or even against the wind. The problem is in fact difficult because currents respond to the wind in a complicated fashion. It took until the middle of the 20th century to sort out why there is a Gulf Stream.
Diagram illustrating the Atlantic gyres and their relation to the Westerlies
(orange) and the Trade Winds (green). Can you trace in the trajectories of the Westerlies and the Trade Winds in the Southern Hemisphere?
The Coriolis Force and other Ingredients for a Gulf Stream
The important ingredients in trying to understand the connection between the winds and the surface currents are the effects of unequal solar heating (i.e. heat excess at the equator and heat deficiency at the poles) and the rotation of the Earth through a process known as the Coriolis Force. The Coriolis force comes into play whenever considering a rotating sphere (such as the Earth) as the frame of reference to describe motion on it. The first, unequal heating, sets up convection currents in the air while the second, the rotation of the Earth, modifies all motions on the planet. Every moving object responds to the "Coriolis force." In the northern hemisphere the Coriolis force always deflects objects to the right - if the object is moving poleward it veers eastward, if itís heading towards the equator it veers westward - but the direction is always to the right. In the southern hemisphere the Coriolis force deflect objects to the left.
The trade winds and westerlies together set up a large-scale gyre motion of surface waters in each ocean basin, centered roughly on the tropics of Cancer and of Capricorn, the latitudes furthest from the equator where the Sun is directly overhead at least once in a year. Because of the Coriolis force, surface water is deflected toward the interior of the gyres (to the right in the north, to the left in the south). The gyre motion piles up water in its center as a fat lense (it can be up to 2 m higher than average sea level), and cold water is preferentially displaced downward. Now, if the wind should cease, the water will attempt to run downhill from the center of the gyre, and be deflected by the Coriolis force (to the right in the north, to the left in the south). The gyre keeps turning, while balancing the forces of gravity (downhill motion of the central water) with those of the Coriolis force (deflection back toward the center). When calculating currents using the assumption of balances of forces we refer to what are called "geostrophic currents." Thus, the Gulf Stream is just a portion of the circle of the North Atlantic Current that runs from east to west along the Equator before changing directions north and becoming the Gulf stream. The presence of geostrophic currents also explain the enigma pointed out by Commander Maury that the Gulf Stream keeps going even though the wind changes. The system can run "by itself" for a while, until the energy contained within it (which was put there by winds) is used up by friction.