Sharp Cooling of the Northern Hemisphere in the Early Subatlantic Age ( 650-280 BC )

a 19-kyr meltwater pulse originating in the Northern Hemisphere contributed to early deglacial warming in the Southern Hemisphere, while maintaining a cold Northern Hemisphere through its effect on the Atlantic THC and ocean heat transport (Clark et al., 2004). A similar temporal shape to the B/A onset (Fig. 1a) is detected at other abrupt stadial (cold) to interstadial (warm) transitions, named Dansgaard-Oeschger (DO) events, during the last glacial period (Dansgaard et al., 1984). These millennial-time scale variations have been linked to various mechanisms such as a salt oscillator (Broecker et al., 1990), deep-decoupling oscillations (Winton, 1993; Schulz et al., 2002), latitudinal shifts in convection sites associated with THC changes induced by freshwater fl ux perturbations in the North Atlantic (Ganopolski and Rahmstorf, 2001), and a stochastic resonance phenomenon (Ganopolski and Rahmstorf, 2002; Rahmstorf and Alley, 2002). The trigger of these millennial-time scale variations is unknown and there is debate as to whether these fl uctuations are regular or stochastic (Wunsch, 2000; Schulz, 2002; Rahmstorf, 2003). Here we argue that our mechanism for the B/A transition might be similar to other DO events during the glacial phase. However, the warm and cold water routes of the oceanic conveyor belt only temporarily gained in strength, and returned to the glacial mode with reduced strength of the ACC and a relatively northward circumAntarctic frontal system compared to its present day position. In contrast, the southward migration of these fronts prior to the B/A transition is accompanied by deglacial warming, which activates the “fl ywheel” of the oceanic conveyor belt (Fig. 1, 2). Like the “conveyor belt” metaphor (Broecker, 1991; Brüning and Lohmann, 1999) the fl ywheel metaphor illuminates a basic idea. In our case the Southern Ocean can be understood as the engine for the oceanic transport system on glacialinterglacial time scales. Our study suggests that the Achilles Heel (Broecker, 1991) and the fl ywheel of the Atlantic overturning circulation on paleoclimate timescales are located in the North Atlantic and the Southern Ocean, respectively (Fig. 2). A zone of special interest is the area around the Cape of Good Hope because it represents an import route of relatively warm and saline water from the Indian Ocean (Fig. 2c) that is thought to precondition NADW formation (Gordon et al., 1992). Berger and Wefer (1996) surmise that this narrow portal is directly related to the position of the sup-tropical convergence zone. Therefore, a northward displacement of this front could reduce (Gersonde et al., 2003; Paul and Schäfer-Neth, 2003) or even pinch off access, leading to speculation that the reopening of the Agulhas gap at the end of the last ice age may have played a role in restarting the Atlantic THC (Berger and Wefer, 1996). Since the South Atlantic is characterized by a number of unique dynamical features, such as the large Agulhas Rings (Schouten et al., 2002) that form a key link in the THC, it is of interest to investigate the fl ywheel and its respective energy source, using high-resolution models of the South Atlantic, to obtain a more detailed view of this region.