Sensitivity of the thermohaline circulation for different climates - Investigations with a simple atmosphere-ocean model

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Palaeoclimatic records indicate several abrupt changes in North Atlantic climate which are assumed to be caused by disturbances of the thermohaline circulation (THC). By means of an idealized ocean box model we investigate the sensitivity of the THC with respect to a high-latitudinal salinity reduction, simulating a sudden meltwater release of glaciers or icebergs. We study the influence of various surface heat and freshwater flux parameterization schemes. By coupling an atmospheric energy balance model to the ocean model the importance of atmospheric heat and moisture transports in destabilizing the THC is demonstrated.Furthermore, the THC and its sensitivity under different climatic conditions is investigated. Due to the temperature-dependence of the thermal expansion coefficient the oceanic circulation weakens and becomes more vulnerable with decreasing global temperatures. The results indicate that during Ice Ages even relatively weak freshwater invasions might have caused considerable variations in the intensity of the THC, accompanied by severe cold snaps in high latitudes due to the weakened oceanic heat transport.Greenland ice cores and other climate records clearly indicate that various abrupt changes in the North Atlantic climate occured in the past. A well- known example is the Younger Dryas cold event, around 11,000 years BP. It is presumed that these changes are connected with freshwater invasions into the deep water formation areas of the northern North Atlantic, resulting in a weakening of the thermohaline-driven ocean circulation (THC) and consequently in a decreased oceanic poleward heat transport (Broecker, 1991). Paleoclimatic studys actually show relations between meltwater events, owing to retreating glaciers (Keigwin et al., 1991) or massive discharges of icebergs launched from Canada (Bond, 1995; Bond et al., 1992) and cold snaps in the North Atlantic region. Obviously, the sensitivity of the THC plays a key role for the climate variability and is the subject matter of the present paper.Several feedback mechanisms influence the sensitivity of the THC and either amplify (positive feedbacks) or weaken (negative feedbacks) an initial perturbation. Some of these feedback mechanisms, which are associated with heat and salt/freshwater transports in ocean and atmosphere and with outgoing longwave radiation from atmosphere to space, will be discussed in detail in this paper.

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Prange, M. , Lohmann, G. and Gerdes, R. (1997): Sensitivity of the thermohaline circulation for different climates - Investigations with a simple atmosphere-ocean model , Palaeoclimates 2, 2 , pp. 71-99 .

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