The thermohaline circulation (THC) is a global system of ocean currents which is primarily driven by density (or buoyancy). It is sometimes called the meridional overturning circulation (MOC) when referring to the arm in the Atlantic Ocean.
Of course, in a turbulent ocean each water parcel could take any number of different pathways, but averaged and simplified it might look something like this figure (left). Most importantly, a water parcel will sink near the poles at deep water formation sites in the North Atlantic and Southern Ocean, where freezing winds reduce it’s buoyancy, and only gradually rise again to the surface on its journey through the Pacific and Indian Oceans and back to the Atlantic.
Through the thermohaline circulation, the ocean is able to regulate our climate by redistributing heat from the equator, where the Earth receives most energy from the Sun, towards the poles. Hence, if this circulation slows or accelerates there are global consequences on our climate.
The arm of the THC in the Atlantic Ocean (MOC) is especially important, because most of the world’s deep water is formed in the North Atlantic and Arctic Oceans. As a result, surface waters must flow into the Atlantic basin from the south to feed these deep water formation sites. This is where the Agulhas comes in! Agulhas leakage is an important component of the THC because it provides most of the surface waters which flow northward in the Atlantic basin. Hence, Agulhas leakage is linked to climate through its link to the Atlantic MOC and oceanic heat transport.