The Agulhas Leakage: the missing link in the interhemispheric climate seesaw?

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) modulates climate on a range of temporal and spatial scales. The northward heat transport associated with its upper limb ameliorates the North Atlantic climate, while its southward flowing lower limb transfers carbon from the atmosphere into the ocean interior (Visbeck 2007; Lozier 2012). Processes taking place in the Northern Hemisphere are historically regarded as the main drivers of the AMOC through their direct influence on the North Atlantic Deep Water (NADW) formation (Lozier 2012). Mounting evidence, however, emphasizes that the inter-ocean exchange of water south of Africa (Beal et al. 2011) and the upwelling of deep water offshore Antarctica (Visbeck 2007) are also potentially important control factors for the AMOC. We are focusing here on the transport of warm and saline waters from the subtropical Indian Ocean by the Agulhas Current, which flows southward along the shelf edge of southern Africa. While most of the Agulhas Current water recirculates into the Indian Ocean, a variable fraction, Agulhas Leakage (AL), escapes into the South Atlantic Ocean (Beal et al. 2011). recent studies contend that the AL sets the southern control for the Atlantic upper ocean buoyancy budget and thus ultimately for the AMOC variability. Potential mechanisms for buoyancy control include planetary-wave adjustments in the Atlantic thermocline and/or advection of salt to the NADW formation sites (Beal et al. 2011 and references therein).