Changes in the ocean thermohaline circulation

The ocean thermohaline circulation (THC) was introduced in the box on page 120 in Chapter 5, where Figure 5.18 illustrates the deep ocean currents that transport heat and fresh water between all the world's oceans. Also mentioned in the box was the influence on the THC in past epochs of the input of large amounts of fresh water from ice melt to the region in the North Atlantic between Greenland and Scandinavia where the main source region for the THC is located.

With climate change due to increasing greenhouse gases we have seen that both temperature and precipitation will increase substantially especially at high latitudes (Figures 6.6 and 6.7), leading to warmer surface water and additional fresh water input to the oceans. Some increased melting of the Greenland ice cap would add further fresh water. The cold dense salty water in the North Atlantic source region for the THC will become less cold and less salty and therefore less dense. As a result the THC will weaken and less heat will flow northward from tropical regions to the north Atlantic. All coupled ocean-atmosphere GCMs show this occurring during the twenty-first century though to a varying extent from a small amount to over 50% change; a typical example is shown in Figure 6.14, which indicates a weakening of about 20% by 2100. Although there is disagreement between the models as to the extent of weakening, all model projections of the pattern of temperature change under increasing greenhouse gases show less warming in the region of the north Atlantic (Figure 6.6) - but none shows actual cooling occurring in this region during the twenty-first century.

The question is often raised as to whether an abrupt transition could occur in the THC or whether it might actually be cut off as has occurred in the past (see Chapter 4, page 89). From model projections so far it is considered very unlikely that an abrupt transition will occur during the twenty-first century. However, the stability of the THC further into the future is bound to be of concern especially if global warming continues largely unchecked and if the rate of meltdown of the Greenland ice cap accelerates. Intense research is being pursued - both observations and modelling - to elucidate further likely changes in the THC and their possible impact.

Climate Change And Thc

Year

Figure 6.14 Change in the strength of the thermohaline circulation (THC) in the north Atlantic as simulated by the Hadley Centre climate model for four SRES scenarios (A1FI, A2, B2 and B1). The unit of circulation is the Sverdrup, 106 m3 s-1.

Year

Figure 6.14 Change in the strength of the thermohaline circulation (THC) in the north Atlantic as simulated by the Hadley Centre climate model for four SRES scenarios (A1FI, A2, B2 and B1). The unit of circulation is the Sverdrup, 106 m3 s-1.

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  • nereo
    What triggers changes in the ocean thermohaline circulation?
    8 years ago

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