The Gulf Stream System

The Gulf Stream proper may be considered to extend from the Straits of Florida to the Grand Banks off Newfoundland. It does, however, have two fairly distinct sections, upstream and downstream of Cape Hatteras (at ~ 35° N).

Between the Straits of Florida and Cape Hatteras, the current flows along the Blake Plateau, following the continental slope (Figure 4.20(b)), so that its depth is limited to about 800 m. In this region, the flow remains narrow and well defined. Its temperature and salinity characteristics show that it is supplemented by water from: (1) the Antilles Current (which includes deep water from the South Atlantic, kept out of the Caribbean Sea by the shallowness of the passages between the Lesser Antilles); and (2) water that has recirculated in the Sargasso Sea.

Figure 4.20 (a) The Gulf Stream in relation to the surface circulation of the Atlantic. The Stream consists of water from the equatorial current system (much of which comes via the Caribbean/Gulf of Mexico) and water that has recirculated in the North Atlantic subtropical gyre. The broken lines represent cold currents.

(b) Map to show the sea-floor topography off the east coast of the United States, and geographical locations of places mentioned in the text.

7 subpolar I gyre /

subpolar gyre subtropical* gyre inrnr /

Tropic of Cancer

Case Hatieras

Bfinin Isiarxls

As the Gulf Stream continues beyond Cape Hatteras. it leaves the continental slope and moves into considerably deeper water (4000-5000 m). While the current was following the continental slope, any fluctuations in its course had been limited and meanders had not exceeded about 55 km in amplitude. Beyond Cape Hatteras there are no topographic constraints, the flow becomes more complex, and meanders with amplitudes in excess of 350 km are common. These meanders often give rise to the Gulf Stream 'rings' or eddies, mentioned in Section 3.5 (and discussed again shortly).

By the time it has reached the Grand Banks off Newfoundland (Figure 4.20(b)). the Gulf Stream has broadened considerably and become more diffuse. Beyond this area it is more correctly referred to as the North Atlantic Current (or, in older literature, the North Atlantic Drift). Much of the water in the North Atlantic Current turns south-eastwards to contribute to the Canary Current and circulate again in the subtropical gyre (Figures 3.1 and 4.20(a)); other flows become part of the subpolar gyre, or continue north-eastwards between Britain and Iceland.

Continuity and recent ideas about how the Gulf Stream is driven

Because of the contributions from the recirculatory flow and the Antilles Current, the volume transport of the Gulf Stream increases as it flows northwards (cf. Question 4.1(b)). The average transport in the Florida Straits is about 30 x IO6 m3 s~'; by the time the Gulf Stream leaves the shelf off Cape Hatteras this has been increased to (70-100) x K^m3 s_1. The maximum transport of about 150 x 106m1s_l is reached at about 65° W. after which the transport begins to decrease again because of loss of water to the Azores Current and other branches of the recirculatory flow (Figure 4.20(a)). We should note here that volume transport values are often quoted in 'sverdrups' (after the distinguished oceanographer). where 1 sverdrup (Sv) = 106m3s_l.

Perhaps not surprisingly, these volume transports are much greater than the values that are obtained using Sverdrup's relationship to wind stress curl (Section 4.2.2). which cannot take account of any recirculatory flow, either at the surface or at depth. Another reason for the discrepancy is that, in winter, Gulf Stream/North Atlantic Current water sinks at subpolar latitudes, forming dense deep water which then flows equatorwards. Not only does the dense deep water contribute to the deep recirculatory flow but, for reasons of continuity (Section 4.2.3), the sinking of surface water 'draws' more Gulf Stream water polewards to take its place. In other words, the Gulf Stream is driven not only by the wind, but also by the deep thermohaline circulation. (Do not worry if you do not fully understand this - we will be considering the thermohaline circulation and formation of deep water masses in Chapter 6.)

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