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Abyssal Circulation

The basic elements of the deep (thermohaline) ocean circulation were known in Sverdrup's time. (He mapped global volume fluxes in units of million cubic meters per second, now known as sverdrups.) At the time it was believed that deep water known as Montgomery's "common water"3 was formed in a few concentrated areas south of Greenland and along the Antarctic Shelf by top-to-bottom convection. But there is no top-to-bottom convection. The work of V. Worthington, J. Reid, A. Gordon, and D. Roemmich has since shown that the formation of Montgomery's common water requires a complex interplay of water masses. Starting in 1960, Stommel and Arons provided a dynamical (though highly idealized) framework, with deep water transported to lower latitudes along western boundaries and communication between the oceans basins accomplished via the Antarctic Circumpolar Current. A subsequent visualization called "the great global conveyor belt" has enjoyed popular support because of its vividness, and support by chemists because of its simplicity, but it is important to keep in mind that this subject is still under active development.

Hydrography

The fashion at the time was to map the measured scalar fields of temperature and salinity and to infer the current velocities by a joint application of the hydrostatic and geostrophic equations. Since the scalar fields were relatively smooth and steady, the inferred currents were relatively smooth and steady. We had so much confidence in the method that we issued current charts on pocket handkerchiefs to our World War II pilots in the Pacific so that they could navigate the "known" surface currents toward the nearest islands.

There are two shortcomings to the hydrographic method. First, smooth scalar distributions do not necessarily call for smooth, steady current systems, the scalar fields being space and time integrals of the motion field. One has found smooth scalar fields in the presence of extremely complex float trajectories. The downed flyers would have found the current charts useful only if they had been willing to integrate their drifting experience over a year or two.

The second shortcoming is that the hydrographic method gives only relative currents, and much effort has been expended to find the so-called depth of no motion. The problem was treated in the 1970s by Stommel (with Schott, Behringer, and Armi) in the work on the ß-spiral, and similarly by Wunsch in his application of inverse methods. It is ironic that progress on the problem of the depth of no motion came about just as it was becoming clear that ocean currents were seriously time dependent at all depths.

Ekman Spiral

All students of oceanography learn about the Ekman spiral, an elegant early-century mathematical solution to the wind-driven current profile. But it has been very difficult to extract a clear spiral signature from a noisy environment until the work of Price and Weller, and Niiler's recent statistical analysis of 50,000 float observations. In more general terms, "Ekman dynamics" has been observationally confirmed by Davis in the Mixed Layer Experiment (MILE), and by Rudnick in an acoustic Doppler current profiler (ADCP) transect across the Atlantic.

UNDERSTANDING VARIABILITY

Fifty years ago physical oceanographers were deploying around the ocean in a few vessels taking Nansen casts and bathythermographs (BTs). The underlying theology was that of a steady ocean circulation: differences between stations were attributed to the difference in station position, not the difference in station time.4 We now know that more than 99 percent of the kinetic energy of ocean currents is associated with variable currents, the so-called mesoscale of roughly 100 km and 100 days. Incredible as it may seem, for one hundred years this dominant component of ocean circulation had slipped through the coarse grid of traditional sampling. Our concept of ocean currents has changed from something like 10 ± 1 cm/s to 1 ± 10 cm/s. This first century of oceanography, since the days of the Challenger expedition in the 1870s, came to an abrupt end in the 1970s.

The Mesoscale Revolution5

By 1950, the oceanographic community had become aware of the meandering of the Gulf Stream. If there was any doubt, the multiple ship Operation Cabot (the first of its kind), under the leadership of Fritz Fuglister, dramatically demonstrated the shedding of a cold-core eddy. At first it was thought that transients are confined to the regions of the western boundary currents. But the acoustic tracking of neu

3  

 Referring to 9 percent of global ocean volume within the narrow limits of 1.0-1.5°C and 34.7-34.8 ppt salinity.

4  

 But Helland-Hansen and Nansen in their classical 1909 paper on the physical oceanography of the Norwegian Sea were aware of the mesoscale variability.

5  

 By "revolution" I mean that an oceanographer totally familiar with the topic at the beginning of the period, but with no further learning experience, would flunk a freshman exam at the end of the period. Other topics have been remarkably stationary. (See a delightful review of Sverdrup's chapter in the "Ocean Bible" [Sverdrup, H.U., M.W. Johnson, and R.H. Fleming 1942. The Oceans: Their Physics, Chemistry, and General Biology, Prentice-Hall, Inc.] by Bruce Warren. 1992. Physical oceanography in The Oceans. Oceanography 5:157-159).



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