Jo 60 80 100 120 140 160 180 200 220 240 260

distance I Km)

Towards the east. In the Gulf Stream, the flow is to the north (or strictly, north-east), and the Coriolis force acting to the right of the flow, i.e. towards the east, balances the horizontal pressure gradient acting to the left (west). The density distribution is such that at the depth corresponding to the zero-velocity contour, the horizontal pressure gradient has become zero because the effects of the sea-surface slope and the density distribution have balanced out with depth. Below the zero-velocity contour, the horizontal pressure gradient reverses. Flow is to the south (or strictly south-west), and the horizontal pressure gradient force to the left of the flow (i.e. to the east) is balanced by the Coriolis force to the right of the flow (i.e. to the west).

The velocity section shown in Figure 4.22 is one of many that show a deep counter-current flowing south-westwards beneath the Gulf Stream. However, until the 1960s many oceanographers found the idea of a significant current close to the deep sea-bed. at depths of 3000-5000 m. hard to believe. The determination of distributions of temperature, salinity and velocity is fraught with difficulty, especially if the T, S and direct current measurements are widely spaced, so the sceptics could reasonably argue that other interpretations of the data, not involving counter-currents, were equally valid.

In 1965. Stommel developed a theory of the global thermohaline circulation that supported the idea of such equatorward deep currents. However, it was freely drifting floats that in the 1950s and 1960s first provided direct evidence of Gulf Stream counter-currents. We will consider these direct current measurements in Section 4.3.3.

First, however, look at Figure 4.23 which shows 7 and S sections across the Gulf Stream between Chesapeake Bay and Bermuda, i.e. downstream of Cape Hatteras. The horizontal scale is much smaller than those of Figures 4.21 and 4.22. and the oceanographic stations were too far apart to allow any filaments or counter-currents to be resolved.

Where i> the Gulf Stream on Figure 4.23?

The Gulf Stream is the region where isotherms and isohalines are close together and slope steeply down to the east, about 300 km offshore; in Figure 4.23(a) its "warm core' (20-22 °C) may be clearly seen extending to depths of 200-300 m.

Wh> docs Figure 4.23 show the Gulf Stream io be flow nig along a frontal boundary?

The zone of steeply sloping isotherms and isohalines indicates a boundary between two different water masses - cooler coastal water to landward and warmer Sargasso Sea water on the seaward side. As discussed in Section 3.5.2, fronts like this, with large lateral variations in density, have intense geostrophic currents flowing along them.

Note that the Sargasso Sea water is not only warmer but also more saline than the water on the coastal side of the Stream, which is influenced by cool freshwater input from land. As in the Straits of Florida (Figure 4.21), the salinity distribution on its own would result in an unstable situation. However, as is common in the oceans, the temperature distribution here has by far the greater effect on the density distribution and the slopes of the isopycnals.

Figure 4.23 (a) Temperature (°C) and (b) salinity sections across the Gulf Stream between Chesapeake Bay and Bermuda, based on measurements made between 17 and 23 April, 1932. These cross-sections, like those in Figures 4.21 and 4.22, were plotted using Fand S measurements of water collected at widely spaced hydrographie stations (shown as crosses along the top), and at specific depths: the contours are interpolations based on the spot measurements. Note that as it was expected that there would be more variability in the western part of the section, the hydrographie stations were positioned closer together there.

200 400 600 800

distance (km)

distance (km)

200 400 600 800

distance (km)

Figure 4.23 (a) Temperature (°C) and (b) salinity sections across the Gulf Stream between Chesapeake Bay and Bermuda, based on measurements made between 17 and 23 April, 1932. These cross-sections, like those in Figures 4.21 and 4.22, were plotted using Fand S measurements of water collected at widely spaced hydrographie stations (shown as crosses along the top), and at specific depths: the contours are interpolations based on the spot measurements. Note that as it was expected that there would be more variability in the western part of the section, the hydrographie stations were positioned closer together there.

Was this article helpful?

0 0

Post a comment