The surface ocean circulation is mainly driven by the winds. At mid-latitudes, the atmospheric westerlies induce eastward currents in the ocean while the trade winds are responsible for westward currents in the tropics (Fig. 1.8). Because of the presence of continental barriers, those currents form loops called the subtropical gyres. The surface currents in those gyres are intensified along the western boundaries of the oceans (the east coasts of continents) inducing well-known strong currents such as the Gulf Stream off the east coast of the USA and the Kuroshio off Japan. At higher latitudes in the Northern Hemisphere, the easterlies allow the formation of weaker subpolar gyres. In the Southern Ocean, because of the absence of continental barriers, a current that connects all the ocean basins cab be maintained: the Antarctic Circumpolar Current (ACC). This is one of the strongest currents on Earth, which transports about 130 Sv (1 Sverdrup = 106 m3s-1). All these currents run basically parallel to the surface winds. By contrast, the equatorial counter-currents, which are present at or just below the surface in all the ocean basins, run in the direction opposite to the trade winds.
|
Because of the Earth's rotation, the ocean transport induced by the wind is perpendicular to the wind stress (to the right in the Northern Hemisphere, to the left in the Southern Hemisphere). This transport, known as the Ekman transport, plays an important role in explaining the path of the wind-driven surface currents (Fig. 1.8). Furthermore, along a coastline or if the transport has horizontal variations, this can lead to surface convergence/divergence that has to be compensated by vertical movements in the ocean. An important example is the equatorial upwelling (Fig. 1.9 ). In the Northern Hemisphere, the Ekman transport is directed to the right of the easterly wind stress and is thus northward. By contrast, it is southward in the Southern Hemisphere. This results in a divergence at the surface at the equator that has to be compensated by an upwelling there. In coastal upwelling, the wind stress has to be parallel to coast, with the coast on the left when looking in the wind direction in the northern hemisphere (for instance, northerly winds along a coast oriented north-south). This causes an offshore transport and an upwelling to compensate for this transport.
|
At high latitudes, because of its low temperature and relatively high salinity, surface water can be dense enough to sink to great depths. This process, often referred to as deep oceanic convection, is only possible in a few places of the world, mainly in the North Atlantic and in the Southern Ocean. In the North Atlantic, the Labrador and Greenland-Norwegian Seas are the main sources of the North Atlantic Deep Water (NADW) which flows southward along the western boundary of the Atlantic towards the Southern Ocean. There, it is transported to the other oceanic basins after some mixing with ambient water masses. This deep water then slowly upwells towards the surface in the different oceanic basins. This is very schematically represented on Fig. 1.10 by upward fluxes in the North Indian and North Pacific Oceans. However, while sinking occurs in very small regions, the upwelling is broadly distributed throughout the ocean. The return flow to the sinking regions is achieved through surface and intermediate depth circulation. In the Southern Ocean, Antarctic Bottom Water (AABW) is mainly produced in the Weddell and Ross Seas. This water mass is colder and denser than NADW and thus flows below it. Note that, because of the mixing of water masses of different origins in the Southern Ocean, the water that enters the Pacific and Indian basins is generally called Circumpolar Deep Water (CDW).
|
This large-scale circulation (Fig. 1.10 ), which is associated with currents at all depths, is often called the oceanic thermohaline circulation as it is driven by temperature and salinity (and thus density) contrasts. However, winds also play a significant role in this circulation. First, they influence the surface circulation and thus the upper branch of the thermohaline circulation which feeds the regions where sinking occurs with dense enough surface waters. Secondly, because of the divergence of the Ekman transport, the winds influence the upwelling of deep water masses towards the surface in some regions. This plays a particularly important role in the Southern Ocean. Winds could also act as a local/regional preconditioning factor that favours deep convection.
The thermohaline circulation is quite slow. The time needed for water masses formed in the North Atlantic to reach the Southern Ocean is of the order of a century. If the whole cycle is taken into account, the timescale is estimated as several centuries to a few millennia, depending of the exact location and mechanism studied. On the other hand, this circulation transports huge amounts of water, salts and energy. In particular, the rate of NADW formation is estimated to be around 15 Sv. Uncertainties are larger for the Southern Ocean, but the production rate of AABW is likely quite close to that of NADW. As a consequence, the thermohaline circulation has a important role in oceanographic as well as in climatology (see section 2.2).