Oceanic circulation should be visualized in three dimensions. We have already mentioned that wind action on the surface sets different layers of water in movement in different directions. Where the wind causes a surface current, the moving water must be replaced by a corresponding inflow from elsewhere. This may be surface water from other regions or deep water rising to the surface, often both. Also, when surface water flows from low to high latitudes, cooling leads eventually to sinking and this causes movement of water at deep levels.
The replacement of the water of the North and South Equatorial Currents is derived partly from surface water from higher latitudes and partly from upwelling deep water. The temperature of the Canaries and Benguela Currents is low compared with other surface water at these latitudes because of mixing with enormous volumes of cold water from below. Similarly, upwelling into the California, Peru and West Australia Currents cools the surface water.
Water movements also arise from density changes due to differences of temperature or salinity. In low latitudes the surface is warm, and has a low density. We have already seen how this water is carried by surface currents into high latitudes, and there it loses heat and increases in density until it eventually becomes heavier than the underlying water. It then sinks and returns towards the Equator at deep levels. However, these density changes occurring as water moves from place to place are often modified by the effects of alterations in salinity. In low latitudes, although warming reduces the density, this is offset to some extent by evaporation raising salinity and thus increasing density. Heavy rainfall in some tropical areas reduces surface density by dilution. In high latitudes water density is increased by cooling and also by the greater salinity which occurs when ice crystals separate in the formation of sea ice, whereas density is lowered by dilution of the water by snow, rainfall, land drainage and melting of ice. The effects on water density of interactions between ocean and atmosphere are therefore extremely complex. It is a generalization to say that the density of surface water at high latitudes increases to the point at which water sinks and subsequently flows to lower latitudes below the surface.
Several factors influence the course of subsurface currents. They are subject to the Coriolis effect and to tide-generating forces. They are deflected or obstructed by submarine ridges. Their direction may be modified by the presence and movements of other water masses. Atmospheric interactions at the surface may have remote effects on the deep levels.
The relationships of these influences are intricate and not fully understood. Evidently water movements below the surface are subject to much variation from time to time associated with deep turbulence and eddies. In many areas it is possible to distinguish three main systems of subsurface water movements, the Bottom Current, the Deep Current and the Intermediate Current. These must not be regarded as steady progressions but rather as representing an overall transport of water within which turbulence, eddies and gyres move different parts in different and changing directions.
In the Atlantic, Indian and Pacific Oceans the Bottom Currents result mainly
from the sinking of cold water around the Antarctic continent. The spread of cold bottom water from the Arctic Ocean is obstructed by the series of submarine ridges between Scotland and Labrador (see Section 1.2.3) and by the shallow Bering Straits. Cold water sinking in the Arctic is therefore trapped in the Arctic Basin. Beneath the Southern Ocean the cold water can escape, and creeps slowly northwards along the bottom, initially at a temperature of about 0oC but gradually becoming warmer as heat is gained by admixture with other warmer water, and perhaps a little by conduction through the sea-bed.
The bottom current in the Atlantic, deriving from the Antarctic and usually termed the Antarctic Bottom Current, flows mainly up the western basin to the west of the mid-Atlantic ridge, being held back from the eastern basin by the Walvis ridge. A corresponding ridge between Tristan da Cunha and the Brazil coast, the Rio Grande ridge, is incomplete and permits the passage of the bottom water. Just south of the Equator the mid-Atlantic ridge is cut by the Romanche Channel (Figure 1.3), through which some bottom water from the Antarctic eventually enters the eastern basin.
The Antarctic Bottom Current flows northwards across the Equator and has been traced to about latitude 40ON. Here it gradually loses its identity as it merges with water flowing in the opposite direction, the North Atlantic Deep Current (Figure 1.7). This water comes mainly from the cooling and sinking of surface water carried into the Arctic by the North Atlantic Drift. We have seen that the coldest water is held back within the Arctic basin by submarine ridges, but a large volume of water spills over the crest and this becomes the North Atlantic Deep Current. It has at first a temperature of 7-8O and is characterized by relatively high salinity and oxygen content. It flows down the southern face of the ridge and along the bottom until it meets the colder Bottom Current moving northwards from Antarctica. It continues its progress southwards getting colder as it goes, flowing above the Antarctic Bottom Current at levels between about 1500 and 3000 m, and is joined by part of the deep water outflow from the Mediterranean.
Near latitude 60°S, the Deep Current rises to the surface, upwelling to replace surface water, some of which is spreading northwards under wind influence as a surface drift while some is sinking due to cooling to become the Antarctic Bottom Current. The cold surface water spreading to the north has a temperature of 0-4°C and the salinity is reduced to about 34%o (see Section 4.3.1) by melting ice. At approximately latitude 50°S it reaches an area of warmer and lighter surface water and sinks below it, continuing to flow northwards as the Antarctic Intermediate Current at depths between about 800 and 1200 m. This water can be traced to about 20°N.
Regions where surface currents meet, and surface water consequently sinks, are termed convergences. The Antarctic Intermediate water sinks at the Antarctic Convergence, and this occurs all round the Southern Ocean, mainly between latitudes 50 and 60°S. Further north at about 40°S is the Subtropical Convergence, another zone where surface water sinks and mixes with the Intermediate water. In the North Atlantic, a southward-flowing Intermediate Current is formed where the Labrador Current dips beneath the Gulf Stream (Figure 1.6).
In the Indian and Pacific Oceans the Intermediate and Bottom Currents both flow in a northerly direction, as in the Atlantic, but are not traceable so far north. The Deep Currents seem to be derived largely from the backflow of the Intermediate and Bottom Currents, and flow southwards until they rise to the surface in the Southern Ocean. All around Antarctica the deep water upwells and the surface water sinks, and at the same time most of the water of the Southern Ocean at all depths is also flowing eastwards as the Antarctic Circumpolar Current. This brings about a continual transport of water from the Atlantic to the Indian Ocean, from the Indian to the Pacific and the Pacific to the Atlantic, whereby the waters of the major ocean basins are intermixed.
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