Impact of interocean exchange on the Atlantic overturning circulation

Abstract

The awareness that human activity could change climate has greatly raised public and scientific interest in climate. One issue of present-day climate research is the stability of the thermohaline circulation. This overturning circulation, popularly known as the `conveyor belt', redistributes water, heat and salt over the entire globe, and is, for instance, responsible for the relatively warm climate in western Europe. Despite the fact that this thermohaline circulation is presently very stable, it probably played an important role in climate fluctuations and switches during the Pleistocene era of ice ages. The possibility that the thermohaline circulation may weaken or even come to a halt as response to global warming gives reason for concern in light of climate change scenarios. In this dissertation it is studied how the strength and stability of the overturning circulation are influenced by the exchange of water between the Atlantic Ocean and the rest of the World Ocean. As part of the global thermohaline circulation, the Atlantic exports so-called North Atlantic Deep Water (NADW). NADW is formed by strong atmospheric cooling of surface water in the Nordic and Labrador Seas in the northern North Atlantic. The Atlantic imports water on shallower levels to compensate for this export of NADW. Part of this water returns via the Drake Passage (between South America and the Antarctic continent); the remaining part enters south of Africa, via a process called Agulhas Leakage. This Indian-Atlantic exchange is accomplished by Agulhas rings, which are about 300 km large and filled with warm and salty Indian Ocean water. It has been suggested that this exchange could stimulate the Atlantic overturning circulation. Agulhas Leakage is sensitive to climate change, and palaeoclimatological data suggest that Agulhas Leakage has been strongly reduced during the last ice-age. This could indicate that the restart of Agulhas Leakage at the end of the last ice-age stimulated the restart of the overturning circulation. The studies of the dissertation show that the exchange of heat and salt between the Atlantic Ocean and the rest of the World Ocean exerts a considerable influence on the strength of the Atlantic overturning circulation. In particular, the heat and salt exchange by Agulhas Leakage turns out to strengthen and stabilize the overturning with respect to, for instance, enhanced inflow of freshwater from the Arctic (mainly derived from the North Pacific via the Bering Strait). These results show that the absence of Agulhas Leakage during the latest ice-age may have been partly responsible for weakening of glacial circulation. This may also have decreased its stability, so that it was more vulnerable to disturbances (like periods of enhanced meltwater discharge). The recovery of Agulhas Leakage at the end of the last ice-age may thus well have stimulated the restart of the overturning circulation. At the moment, the variability of Agulhas Leakage is small, so that the effect on European climate is negligible. However, the fact that Agulhas Leakage stabilizes the present-day circulation diminishes the possibility of a catastrophic climate change as result of global warming.