Simulation of the Atlantic meridional overturning circulation in an atmosphere-ocean global coupled model. Part I: a mechanism governing the variability of ocean convection in a preindustrial experiment.

International audienceThe thermohaline circulation (THC) is a large scale oceanic circulation driven by density gradients. Its Atlantic component is responsible for a significant part of the northward heat transport of the climate system (Broecker, 1991; Lavin et al., 2003), roughly 1PW over the 6PW total energy transported by the entire system (Ganachaud and Wunsch, 2000, 2003; Trenberth and Caron, 2001). Although some results question the impact of THC on climate (Seager et al., 2002), a number of studies showed that its variability can cause large changes in regional surface temperatures and precipitation (Manabe and Stouffer, 1999, 2000; Dong and Sutton, 2002; Vellinga and Wood, 2002; Swingedouw et al., 2006). It is thought to interact with the main atmospheric modes of variability, namely the El Niño Southern Oscillation (ENSO, Dong and Sutton, 2002) and the North Atlantic Oscillation (NAO, Wu and Gordon, 2002). Some variations in its strength may have played an important role in paleoclimate fluctuations (Clark et al., 2002; Rahmstorf, 2002). Shaffrey and Sutton (2006) have also suggested, following Bjerknes' hypothesis (1964), that on decadal timescales, an increase in THC heat transport in the northern extratropics (20°N-70°N latitude band) is compensated by a decrease in midlatitude heat and moisture transport by the storm tracks and vice-versa. When the ocean heat transport rises, the induced decrease in the equator-to-pole temperature gradient weakens the atmospheric baroclinicity which causes a reduction in the atmospheric transient energy transport. Given this potential climatic role of the THC, there is an obvious need to better understand this component of the climate system