Paleoclimatc data were also the first to point to the ability of the Earth's climate system to switch abruptly between significantly different climatic modes. The coupling between the ocean, the atmosphere, and the ice is probably responsible for these variations. Because the atmosphere has a rather short time constant, the oceanic thermo-haline circulation variability is one major process that can be linked to long-term climatic changes, from decades to millennia. Geological data do not suggest that the thermo-haline circulation is a self-oscillating, perhaps self-regulating, mechanism. On the contrary, they show that instabilities of the ice sheets, ice shelves, and sea ice are playing a fundamental role in driving the thermo-haline circulation. They raise the problem of the impact of precipitation changes over the northern North Atlantic on the flux of deep water formed over decades to millennia. The significance and abruptness of past climatic changes are heightened by the fact that they cannot be studied using instrumental data because the instrumental period is too short, and because the origin of the changes is poorly understood. It is quite plausible that the mechanisms responsible for past abrupt changes might act again in response to the trace-gas-induced warming of the next century. In this chapter, I concentrate on the reconstruction of ocean circulation variations during the last glaciation, on evidence of the main mechanisms that are responsible for water mass and heat transport changes, and on their impact on ocean chemistry, atmospheric composition, and climate. Special attention is paid to the coupling between the oceans, the atmosphere, and the cryosphere. Finally, I review the major uncertainties associated with these reconstructions and their consequences on our ability to understand the future behavior of the climate system.
14.2 Physical Properties and Circulation of the Glacial Ocean
The oceanic general circulation is forced by wind stress and exchange of heat and freshwater with the atmosphere, because it is highly sensitive to density differences maintained by salt and heat in the areas of deep and intermediate water formation. Therefore, paleoclimatologists have developed various methods to generate quantitative estimates of past sea surface temperature and salinity and to reconstruct the major features of the global deep water circulation.
14.2.1 Sen Surface Temperature (SST)
An accurate reconstruction of past SS I variations is a prerequisite to paleo-oceanographic studies. During the past decades, two methods have been commonly used to estimate past SSTs based on planktonic foraminifcral composition: the Imbrie and Kipp transfer function (Imbrie and Kipp, 1971) and the modern analog technique (Hutson, 1977). They usually provide simitar estimates within statistical errors. This result gives strong support to the validity of the CI JJVI AP reconstruction of SST during the Last Glacial Maximum (LGM) (CLIMAP, 1981; Prell, 1985). Basically, it shows that high latitudes experienced major ctxiling (up to 10 C) and large sea-ice cover, while tropical areas experienced only minor SST drops (Figure 14.1). The cooling was stronger close to equatorial upwellings, which were more active than today because of enhanced trade winds.
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