The thermohaline circulation

The main outline of the thermohaline circulation was seen in Chapter 1 (§1.3.2, Figs. 1.14 and 1.16). In regions where surface water is made denser through evaporative salinification, winter cooling, salt rejection during sea-ice formation or sub-ice shelf freezing, sufficiently extreme conditions can result in convection occurring to considerable depths, even to the bottom of the ocean. Along continental shelves and under ice shelves these processes may be widespread. In the open ocean deep convection to > 1000 m tends to occur in limited regions (Fig. 2.14). Such convection occurs in a number of places globally: through salinification in the Red Sea and Persian Gulf; through cooling in the Norwegian Sea, the northern Pacific, polar latitudes of the Southern Ocean, the Gulf of Lyon in the western Mediterranean, the Adriatic and the Levantine Sea north of Cyprus; through cooling supplemented by salt rejection in the Labrador and Greenland Seas, the Sea of Okhotsk and the Bering Sea in the northwest Pacific;

Fig. 2.39. A snapshot of the velocity field at a depth of 120 m off South Africa from the Fine Resolution Antarctic Model. Note the strong Agulhas Current flowing south along the east coast of Africa and the eddies of approximately 200 km diameter spun off into the Atlantic. [Picture courtesy of David Stevens.]

2.12

and through salt rejection in the Arctic, and around Antarctica, principally near ice shelves in the Weddell and Ross Seas.

The products of convection form the intermediate and deep waters of the global ocean, making up most of the water column below 1000 m. These water masses spread out from their source region and depth, driven by density differences, and mixing with water originating from other locations. This mixing modifies the original temperature and salinity, and hence density, of the water. Vertical mixing is accentuated in regions of rough bathymetry, such as mid-ocean ridges, by tidally-driven mixing. Eventually this mixing, or entrainment in convection occurring perhaps thousands of kilometres from the water's origin, allows the sub-surface water to return to the ocean surface and be exposed to the atmosphere once more. Herein lies one of the mechanisms by which the ocean can affect the atmosphere on long timescales. This upwelling water may have different temperature, salinity and nutrient properties to the water it is displacing and hence modify the air-sea fluxes of heat, moisture and gases.

Of the large number of regions where convection to 1000 m or below can occur, very few actually contribute significant proportions of the global ocean's water. Some basins have sufficiently shallow sills that the products of local deep convection cannot (currently) escape the basin. This is so in the Arctic and the eastern Mediterranean. Most of the intermediate levels in the ocean are thus made up of Antarctic Intermediate Water (AAIW), formed all around the Southern Ocean south of 50° S. This competes with various products from Northern Hemisphere sources, but the latter have smaller fluxes and occupy regional seas. AAIW occupies roughly 75% of 1000-2500 m depths in the Atlantic, Indian and Pacific Oceans. The deepest waters in most of the oceans are made up of very cold water from the Antarctic coastal waters (Antarctic Bottom Water - AABW). This can be recognized up to 40-50°N in the Atlantic.

The deepest waters further north in the Atlantic are formed from a mixture of Labrador Sea Water with water that has crossed the Greenland-Iceland-Scotland Ridge at depths around 1000 m, so-called Iceland-Scotland Overflow Water (ISOW). This mixture - North Atlantic Deep Water (NADW) - flows south between the AAIW and AABW and then mixes into the deep waters of the Southern Ocean, eventually flowing north into other ocean basins as this new mixture. Remnants of NADW can be traced into the abyssal Indian and Pacific Oceans through its nutrient and radiocarbon signature. NADW is relatively nutrient poor in its source region, but throughout its journey the rain of nutrient material from surface productivity progressively enriches its nitrate content and erodes its oxygen levels. The deep Pacific has high and low values respectively of these two constituents, as it lies furthest from theNADW's source region. NADW's radiocarbon levels also act as a tracer, and provide evidence that it takes roughly 1000 years for NADW to reach the North Pacific (§3.7, Fig. 3.20).

The origin of ISOW is still under debate. The conventional view is that winter cooling in the Greenland and Norwegian Seas, supplemented by salt rejection during sea-ice formation east of Greenland, leads to deep convection and supply of the water that overflows into the North Atlantic. However, in the mid-1990s Mauritzen hypothesized that the water actually originated from subduction of the warm and salty Norwegian coastal Current into the Arctic, and recirculation

Fig. 2.40. Schematic of Mauritzen's circulation pattern for ISOW formation. Warm water flows in the Norwegian Coastal Current into the Norwegian Sea, then partly cooling and sinking and partly cooling and freshening. Two water masses result, one leaving the Norwegian Sea through the Faeroe Channel at depth (ISOW) and the other constituting the, less dense, East Greenland Current. [Adapted from Fig. 2 of Mauritzen (1996). Copyright (1996), with permission from Elsevier Science.]

Railway Map Derbyshire

at intermediate level through the Fram Strait into the Greenland Sea (Fig. 2.40). With some additional mixing, and deepening, with convected water within the Greenland Sea this then provided the outflow water. It is certainly true that Atlantic water is subducted into the Arctic below the surface fresh layer, at depths of 300-800 m, and that the Fram Strait is deep enough to allow this water to exit the Arctic basin. The real source of ISOW is probably some combination of the two mechanisms.

Convection and outflow of its products is only one arm of the thermohaline circulation. Fig. 1.14 shows that part of the upper ocean supply route leading to the northern Atlantic. This contains both a warm water path and a cold water path. Significant upwelling occurs in the Antarctic Circumpolar Current. This water can mix into the Atlantic, as the ACC is deflected north upon passing through Drake Passage at the southern end of South America to form the southern part of the South Atlantic sub-tropical gyre. Similarly, some of this water will be entrained southwards into the sub-polar gyre of the Weddell Sea and so enter a region of AABW formation. A warm water path, that transports the products of slow mid-ocean upwelling in the Pacific, Indian and Atlantic Oceans back to the North Atlantic, also exists. To what degree these routes represent real physical pathways for water as opposed to mean energy transports is not known. Nevertheless, timescales of the order of 1000 years are found in both the natural world and climate models (§5.3), supporting the importance of these mechanisms. Both models and palaeoclimate data support the possibility of changes of state and abrupt flips in this seemingly slow circulation, with severe climatic impacts. This possibility for disruption to the state of the ocean's circulation will be considered in more detail in §5.3.

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