Simultaneous Changes of the AMOC and Atlantic Climate in the Past

Especially in the Northern Hemisphere the amount of heat carried poleward by the oceans is very much tied to the strength of the AMOC. Analysis of ice cores from Greenland revealed more than 20 so-called 'Dansgaard-Oeschger events' during the last ice age (100000 10000 BC) over the course of which Greenland temperatures jumped by roughly 10 °C within a few decades subsequently followed by a gradual cooling on a millennium time scale [29,30]. Based on the analysis of ocean sediment cores these fluctuations are thought to be linked to abrupt changes in the deep-ocean circulation in the North Atlantic [31,32]. The observations are in qualitative agreement with numerical model simulations that associate the climate variations with temporal changes in the vigour of the AMOC [33,34]. The North Atlantic cold phases are generally thought to be linked with a very weak (or inactive) state of the AMOC that goes along with a near-cessation of NADW formation and northward heat transport in the North Atlantic. Warm phases are expected to coincide with a strong state of the AMOC. During the last ice age, the reduction in the NADW formation rates are likely to have arisen from events of massive input of freshwater from the Laurentide ice sheet (covering Canada) into the North Atlantic [35]. The subsequently fresher and thus less dense subpolar upper-ocean waters stabilised the vertical stratification of the water column, hence, heavily impeding deep-water formation.

To simulate the effect of freshwater input on deep water formation Vellinga and Wood [36] carried out a 'water hosing' experiment using a numerical model. They added a sufficient quantity of freshwater to the northern North Atlantic to cause the AMOC to switch off. This resulted in a strong cooling over the North Atlantic peaking at a temperature reduction of 8 °C around Greenland, standing out from patterns of moderate cooling over the entire Northern Hemisphere and warming over the Southern Hemisphere (Fig. 3). Thus, besides its strong importance for climate over the North Atlantic section the AMOC may also have a moderate impact on global climate patterns.

FIGURE 3 Change in surface air temperature during the years 1920 1930 after the collapse of the AMOC in a water hosing experiment using the HadCM3 climate model. Areas where the anomaly is not significant have been masked. This figure has been published by Vellinga et al. [36].

Additionally, palaeoclimate records suggest that changes in the global circulation involving the AMOC during the early last deglacial period (19000 14500 a ago) went along with a significant net transfer of CO2 from the ocean to the atmosphere, leading atmospheric CO2 concentration to rise by about 50 ppmv [37].

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