Stability and Variability of the Thermohaline Circulation in the Past and Future: a Study with a Coupled Model of Intermediate Complexity

Abstract

We present the results of a study of the stability properties of the thermohaline ocean circulation (THC) and the possible mechanisms of rapid climate changes using the climate system model of intermediate complexity, CLIMBER-2. We consider two climate states, the warm modern climate and the cold glacial climate, and compare the stability properties of the THC in both cases. While for modern climate there are two stable circulation modes: the "warm" and the "off" mode, under glacial conditions we find only one ("cold") stable mode of Atlantic ocean circulation. This mode is characterized by deep water formation south of Iceland and has a shallower overturning with weaker outflow to the southern Atlantic. Another mode of the THC similar to modern "warm" mode is marginally convectively unstable under glacial conditions and a relatively small negative perturbation in the freshwater flux to the Nordic Seas can trigger rapid transitions between "cold" and "warm" modes. After being excited, the "warm" mode is metastable on the time scale of a few hundred years. We speculate that the stable "cold" mode corresponds to stadial periods with cold climate over the Northern Atlantic, while temporal transitions between "cold" and "warm" modes could explain the observed Dansgaard-Oeschger oscillations. The warm modern climate does not possess this type of instability, but rapid global warming can trigger a complete collapse of the THC. We discuss the mechanisms and uncertainties of future changes in the THC.

Insights from a synthesis of old and new climate-proxy data from the Pyramid and Winnemucca lake basins for the period 48 to 11.5 cal ka

“…Some authors have suggested that during the last glacial period, North Atlantic air temperatures oscillated in response to changes in strength of the Atlantic meridional overturning circulation (AMOC) when the production of North Atlantic Deep Water (NADW) decreased and(or) shifted to more southerly latitudes causing a decrease in heat exported by surface currents to the North Atlantic region (Ganopolski and Rahmstorf, 2001;Sarnthein et al, 2001;Elliot et al, 2002). Such changes in air temperature would have propagated downwind, influencing the pattern of sea-surface temperature anomalies in the North Pacific Ocean, which, in turn, affected the water-vapor flux to the overlying atmosphere as well as the trajectory of storm systems passing over the Pacific Ocean bringing precipitation to the Great Basin.…”