Gulf Stream

Gulf Stream The Gulf Stream is a western boundary current and is part of the North Atlantic Gyre (Fig. 1). The name ‘Gulf Stream’ is commonly applied to the entire northern branch of the gyre, but the current is in fact made up of at least three individual parts: the Florida Current, which flows from the Caribbean to approximately 37° N, the Gulf Stream proper between 37° N to approximately 40° W, and then the extension of the current towards Europe, which is the North Atlantic Drift. The velocity of the Gulf Stream is strongest just south of Cape Hatteras and can reach 2.5 m s⁻¹. At this point the current is constrained to the continental shelf and limited to approximately 800 m in depth. Meanders in the current are common but only of approximately 50 km in amplitude. Just north of Cape Hatteras the current leaves the shelf and moves over the deeper water of the North Atlantic, slowing to velocities that are typically 0.7 m s⁻¹. The northern boundary of the current usually corresponds to a sharp gradient in temperature with cold water on the northern side. The southern boundary is typically more diffuse.

The slowing down of the current as it moves away from Cape Hatteras and the lack of topographic steering allows meanders of up to 350 km in amplitude to develop. These large meanders can break from the main current to form eddies called Gulf Stream rings, which can typically be 200 km in diameter. Because of the temperature gradient on either side of the current, the meanders can form rings with either cold or warm water in their centres (‘cold-core’ or ‘warm-core’) and can extend to full ocean depth. The eddies can be re-absorbed by the current or can exist independently for up to a year, and at any one time there may be up to ten rings in existence. Similar rings have been observed forming in other western boundary currents such as the Agulhas and the Kuroshio (see ocean currents).

The reason for the existence of the Gulf Stream and the North Atlantic Gyre is principally Ekman drift (see ocean currents) coupled with the latitudinal variation of the Coriolis force. The prevailing wind field across the North Atlantic is zonal (see Walker circulation), being from the west above approximately 30° N and to the east south of this towards the Equator. These prevailing winds generate currents which are deflected to the right by the rotation of the Earth and result in a general inflow of water into the subtropic convergence (that is the centre of the gyre). The pool of water which develops in the centre causes a decrease in vorticity, and because of the conservation of angular momentum an anticyclonic gyre develops across the basin. The intensification of the western boundary of the gyre (i.e. the formation of the Gulf Stream) results from the fact that the Coriolis force changes with latitude, coupled with the conservation of vorticity across the whole gyre.

Mark A. Brandon

Bibliography

Apel, J. R. (1987) Principles of ocean physics. International Geophysics Series, Vol. 38. Academic Press, London.