The Global Thermohaline Conveyor

In the 1980s, Wallace Broecker suggested that the fluxes of heat and freshwater around the globe in ocean currents and water masses could be viewed as a kind of 'thermohaline conveyor belt' (Figure 6.41 ). This was not intended to be a realistic picture of warm and cold currents (although it is sometimes wrongly interpreted as such), but is a representation of the overall effect of warm and cold currents on the vertical circulation within the ocean. Its usefulness lies in the fact that because it reduces the oceanic circulation to its essentials, it allows us to think more easily about oceanographic and climatic problems. In particular, viewing the

Figure 6.41 The thermohaline conveyor, first envisaged by Wallace Broecker. The red/orange part of the conveyor represents the net transport of warm water in the uppermost 1000 m or so, the blue part the net transport of cold water below the permanent thermocline. You may see other versions of this diagram elsewhere, because the current pattern in the Indian Ocean and the Pacific Ocean, and the seas in between, is not as well known as that in the Atlantic. This version assumes that there is a strong throughflow of warm water westward between the islands of the Indonesian archipelago.

thermohaline circulation as a 'conveyor' has attracted attention to the possible consequences of global warming in response to increased concentrations of atmospheric CO: produced by the burning of fossil fuels and deforestation - the so-called enhanced greenhouse effect.

Before addressing this question in particular, let's look at another related matter. The conveyor 'cartoon' graphically illustrates the idea that the driving mechanism for the global thermohaline circulation - sometimes referred to as the meridional overturning circulation - is the formation of North Atlantic Deep Water.

So, why is no Deep Water formed in the North Pacific?

It is sometimes said that the reason for there being no 'North Pacific Deep Water' is that the topography of the Pacific basin at northern high latitudes is so different from that of the Atlantic. It is true that there are no semi-enclosed seas like the Norwegian and Greenland Seas, where water that has acquired characteristic temperature and salinity values can accumulate at depth behind a sill. However, there is a well-developed subpolar gyral circulation (Figure 3.1 ). Might we not therefore expect some cold deep water masses to form and then spread out at depth, rather in the manner of Labrador Sea Water/North West Atlantic Deep Water (cf. Figures 6.20 and 6.36)?

The formation of deep water masses depends on the production of relatively dense surface water, through cooling and/or increase in salinity: North Atlantic Deep Water is both cold and relatively saline (5 - 35.0). We have already noted that surface salinities in the Pacific are significantly lower than those in the Atlantic, particularly in the northernmost part of the basin, where values may be 32.0 or less (Figure 6.11(a)). The input of freshwater to the North Atlantic, through precipitation, rivers and melting ice. is about 104cmyr'. while that to the North Pacific is about 91 cmyr1. Evaporation rates are about 103 cm yr_l for the Atlantic and 55 cm yr1 for the Pacific.

What does this suggest aboul the cause of the relatively low surface salinities in ihe North Pacific, in comparison with those in the North Atlantic - is it low evaporation or high precipitation?

warm upper waters cold deep and bottom waters

The important difference is in the evaporation rates - compare the differing values of Qe in the two areas (Figure 6.8). The resulting E-P values (-36cmyr' for the North Pacific and-1 cmyr"1 for the North Atlantic) reflect a net transfer of freshwater from the sea-surface of the Atlantic to the sea-surface of the Pacific, in the form of water vapour carried in the atmosphere.

But why the low rates of evaporation in the North Pacific? As graphically illustrated by the 'conveyor belt' image in Figure 6.41, in contrast to the North Atlantic which is supplied with warm water from the South Atlantic, and is a region where cooled surface water sinks, the North Pacific is continually supplied by cool water from below (cf. end of Section 6.5) and, as a result, has relatively low sea-surface temperatures (Figure 6.5). As discussed in Section 6.1.2, a cool sea-surface cools the overlying air, thereby reducing its ability to hold moisture and so reducing the evaporation rate. What is more, the effect of a low evaporation rate is to limit the extent to which the density of surface water may be increased through increase in salinity. It has been calculated that even if the surface waters of the North Pacific were cooled to freezing point, because of their low salinity they would still not be dense enough to sink and initiate deep convection. Paradoxically, therefore, in the North Pacific, a cool sea-surface prevents the surface layers from becoming sufficiently dense to sink, and there can be no North Pacific Deep Water.

Concerns about the effect of global warming on the thermohaline circulation are focused on whether such warming could result in North Atlantic Deep Water no longer being formed. The observed fluctuations in the rates of production of deep water in the Greenland Sea and the Labrador Sea (Section 6.3.2) seem to confirm that whether or not deep water production occurs may be very finely balanced, and it is possible that an increased production of fresh meltwater from glaciers and sea-ice will in effect 'turn off' the production of deep water in the Greenland Sea, by preventing surface layers from becoming sufficiently dense to sink, and hence inhibiting convection (cf. Question 6.8). Many (but not all) climate researchers believe that the episode of cooling known as the Younger Dryas, which occurred after the end of the last glacial period, was caused by a layer of fresh meltwater (from the large North American ice-sheet) spreading across the North Atlantic, so preventing the production of North Atlantic Deep Water.

QUESTION 6.13 What effect might a reduction in the rate of formation of deep water in the Greenland and Norwegian Seas have on the surface circulation of the North Atlantic in general isee Section 4,3,1 k and why would this have consequences for the climate of north-west Europe'.'

On a more general note, at present, any increase in heat content caused by global warming is being distributed through the atmosphere and the body of the ocean. As a result, any rise in temperature at the surface of the sea (and the land) will be relatively slow. If formation of deep water masses ceased completely, and there were no sinking of surface water, any increase in the heat content of the atmosphere and ocean would be shared between the atmosphere and the uppermost layers of the ocean, and the rise in sea-surface temperature, and in the temperature of the atmosphere, would be relatively fast.

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