OOEAN waters move in a global circulation system, transporting huge amounts of heat around the globe. The thermal capacity of the ocean is very large compared to that of the atmosphere, and through its circulation patterns, it can absorb heat in one region and return it to the atmosphere (often decades or centuries later) at an entirely different place. Ocean circulation is, thus, a key determinant of regional and global climate.
Ignoring tidal forces, the long-term mean oceanic circulation is driven by three external influences: wind stress, heating and cooling, and evaporation and precipitation; all of which, in turn, are ultimately driven by solar radiation. Ocean currents are either wind-driven (surface currents) or due to changes in the density of sea water (deep currents). The circulation of the atmosphere is, in turn, influenced by the distribution of oceanic properties, such as sea-surface temperature and the distribution of sea ice. In particular, the amount of evaporation from the ocean depends strongly on the sea-surface temperature; and when the evaporated water is returned as rain, it releases its latent heat into the surrounding air. This heating is probably the strongest driving force for atmospheric winds.
To understand the oceanic and atmospheric circulation fully, they are now treated as a single system of two interacting components, coupled at the air-sea interface through the fluxes of momentum, heat, and mass. However, early ideas on the nature of oceanic circulation pictured the motion in equilibrium or perpetual steady state. This view was formed from limited observations, often based on tracking floating objects released into the ocean. The classic picture of steady-state circulation was laid out in Harald Sver-drup's definitive textbook of 1942, The Oceans: Their Physics, Chemistry and General Biology, and although it included a description of the deep thermohaline circulation (now recognized as critical for climate), Sverdrup paid most attention to surface currents like the Gulf Stream. Through the 1950s, oceanographers concentrated on surface winds and currents, far easier to measure and far more important for mariners than the slow, deep circulation. How much of the overall general circulation was driven by the winds, and how much by density changes related to temperature and salinity, was still unknown.
Seminal efforts to numerically-model oceanic circulation began in the late 1960s, led by Kirk Bryan, a Woods Hole oceanographer, who with his collaborator Michael Cox built a model for a highly-simplified ocean basin with five depth levels. Their model produced a picture that looked roughly like the North Atlantic Ocean's Gulf Stream and equatorial flow. Despite deep skepticism from empirical oceanog-raphers who were still mapping the basic patterns of motion in the ocean, Bryan extended his work, motivated by "the pressing need for a more quantitative understanding of climate." Recognizing that surface winds helped drive the ocean currents that transported heat from the tropics poleward, Bryan coupled his ocean basin model to Syukuro Manabe's model of atmospheric circulation. Published in 1969, this coupled ocean-atmosphere circulation model developed by Bryan and Manabe was a forerunner of modern general circulation models (GCMs).
Paralleling the increase in available computing power, the first global ocean model was developed by Cox in 1975, using nine depth levels and a horizontal grid resolution of 2 degrees latitude by 2 degrees longitude. Cox found he could simulate the ocean circulation for only a few years in its centuries-long progress, but overall the "in silico" ocean behaved somewhat like the real one. In 1988, S. Manabe and R. Stouffer developed a coupled atmosphere-ocean model that incorporated a realistic land and ocean topography. Starting from two different initial conditions, asynchronous time integrations of the coupled model, under identical boundary conditions, led to two stable equilibria. In one equilibrium, the North Atlantic has a vigorous thermohaline circulation and relatively saline and warm surface water. In the other equilibrium, there was no thermohaline circulation, with an intense halocline in the surface layer at high latitudes. These results suggested that the real ocean system might have at least two equilibria, echoing the result obtained by Henry Stommel 25 years previously, with a very simple pen-and-paper box model. The simulation raised the "intriguing possibility," as Manabe and Stouffer put it, that global warming might shut down the North Atlantic thermohaline circulation within the next century or so. Halting the steady flow of warm water into the North Atlantic would bring devastating climate changes in Europe and perhaps beyond.
With the advent of high-performance super-computers, coupled ocean-atmosphere models improved rapidly through the 1990s, and a number of studies confirmed the possibility that changes in ocean climate from global warming could eventually shut down the North Atlantic thermohaline circulation, however, the timescale of such a change is still far from certain. In the last decade, the availability of several polar orbiting satellites, that measure winds, waves, sea-surface temperatures, and ocean color, at high spatial and temporal resolution, means much more data are now available for the calibration and validation of numerical circulation models. Also recently launched is the Argo program—an international program to seed the global ocean with an armada of some 3,000 free-floating buoys that measure vertical profiles of ocean temperature and salinity, thus complimenting the surface data from satellites.
sEE also: Bryan, Kirk; Climate Models; Current; Gulf Stream; Manabe, Syukuro; Stommel, Henry; Sverdrup, Harald; Thermohaline Circulation; Woods Hole Oceano-graphic Institution.
BIBLIOGRAPHY. G.L. Pickard and W.J. Emery, Descriptive Physical Oceanography (Pergamon Press, 1985); H. Sverdrup, M.W. Johnson, and R.H. Fleming, The Oceans: Their Physics, Chemistry and General Biology (Prentice-Hall, 1942).
Albert Gabric Griffith University
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