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Longitude

FIGURE 9.15. Mean surface currents in the Atlantic Ocean from surface drifters with annual-mean SST superimposed. The vector at the bottom right represents a current of 10 cm s-1. Data courtesy of Maximenko and Niiler (personal communication, 2003).

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Longitude

FIGURE 9.15. Mean surface currents in the Atlantic Ocean from surface drifters with annual-mean SST superimposed. The vector at the bottom right represents a current of 10 cm s-1. Data courtesy of Maximenko and Niiler (personal communication, 2003).

El Niño-Southern Oscillation (ENSO) phenomenon, will be addressed in Section 12.2.

In the southern hemisphere, subtropical gyres are also evident.7 However, on the poleward flanks of the subtropical gyres of the Southern Ocean, the strong zonal flow of the Antarctic Circumpolar Current (ACC) predominates. Fluid in the ACC can circumnavigate the Southern Ocean unimpeded by coasts. This is perhaps the ocean current that is most closely analogous to the atmospheric jet stream.

The pattern of mean currents discussed above extend downward in the water column, but the currents become weaker. For example, Fig. 9.17 (top) shows currents at a

George Deacon, (1906-1984), doyen of British oceanography and father of the Institute of Oceanographic Sciences, elucidated the main water masses of the Southern Ocean and their circulation, using classical water mass analysis. Deacon's Hydrology of the Southern Ocean, published in 1937, summarized information on the circulation of the southern Atlantic and Southern Ocean and set the standard for future physical oceanographic work.

Longitude

FIGURE 9.16. Mean surface currents observed in the Indian Ocean from surface drifters with annual-mean SST superimposed. The vector at the bottom right represents a current of 10 cms-1. Data courtesy of Maximenko and Niiler (personal communication, 2003).

Longitude

FIGURE 9.16. Mean surface currents observed in the Indian Ocean from surface drifters with annual-mean SST superimposed. The vector at the bottom right represents a current of 10 cms-1. Data courtesy of Maximenko and Niiler (personal communication, 2003).

depth of 700 m observed by neutrally buoyant floats8 in the North Atlantic. We observe a general pattern of currents similar to that at the surface (cf. Fig. 9.15) but of reduced magnitude. Mean currents in the abyssal ocean are very weak, except in regions of western boundary currents and where flow is channeled by topography. There is significant stirring of the abyss by ocean eddies, however, because the ocean's eddy field typically decays less rapidly with depth than the mean flow.

Before begining our dynamical discussion, we estimate typical timescales associated with the horizontal circulation described above. Tropical surface waters move rather swiftly, reaching mean zonal speeds of 20 cms-1 or more. The Pacific basin is some 14,000 km wide at the equator, yielding a transit time of 2 years or so.

8 The subsurface currents shown in Fig. 9.17 were obtained by Profiling Autonomous Lagrangian Current Explorer (PALACE) floats. These are designed to drift at a pre-determined depth (700 m in this case) for 10 days or so and then rise to the surface, measuring water properties to obtain a temperature and salinity profile. While on the surface, data is transmitted to satellite and the geographic position of the float determined before it returns to depth to repeat the cycle. Each float has battery power for 100 cycles or so. For more details see Davis et al (2001).

Currents And Pressure at 700m in The Atlantic

Currents And Pressure at 700m in The Atlantic

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FIGURE 9.17. Top: Currents at a depth of 700m in the Atlantic: the arrow at bottom right corresponds to a current of 10 cms-1. Bottom: The associated geostrophic pressure field, expressed as a head of water in cm. Data courtesy of Steve Jayne, WHOI.

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FIGURE 9.17. Top: Currents at a depth of 700m in the Atlantic: the arrow at bottom right corresponds to a current of 10 cms-1. Bottom: The associated geostrophic pressure field, expressed as a head of water in cm. Data courtesy of Steve Jayne, WHOI.

Typical zonal surface currents in the ACC are about 30 cm s-1, and parcels thus circumnavigate the globe, a distance of some 21,000 km at 55° S, in about 2 years (see Problem 6 at the end of this chapter). It takes perhaps 5 years or more for a parcel of water to circulate around the subtropical gyre of the Atlantic Ocean. Timescales of the horizontal circulation increase with depth as the flow decreases in amplitude.

Finally it is important to emphasize that Figs. 9.14 to 9.17 are time-mean circulation patterns. But the ocean is full of time-dependent motions (due to hydrodynamical instabilities and flows driven by variable forcing) so that at any instant the circulation often looks quite different from these mean patterns, as will be discussed in Sections 9.4 and 10.5.

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