South Atlantic Connections (SACO)

The circulation of the Atlantic Ocean may be idealized as a combination of separate vertical and horizontal cells that are gradually interconnected through relatively slow vertical motions. In the vertical cell, deep waters form at high latitudes of the North Atlantic and follow a long meridional trip to end in the lower layers of the Antarctic Circumpolar Current (ACC, Fig.1). These deep ACC waters leave the Atlantic Basin for long times, up to hundreds of years, before returning to the Atlantic basin as intermediate subantarctic or Indian-Ocean central waters, at shallower thermocline levels that will progress northwards to ultimately reach the high-latitude surface North Atlantic. In the horizontal cells, these thermocline waters flow in partly isolated wind-driven transoceanic tropical, subtropical and subpolar gyres, which are connected precisely through the returning branch of the vertical cell.

This simple view of the vertical cell constitutes the basics of what is known as the Atlantic Meridional Overturning Circulation (Atlantic MOC or AMOC), with the deep-water formation supplying the mean-southward departure limb and the returning limb being formed by the mean-northward flowing intermediate and surface waters (Fig. 1). The vertical structure of the AMOC is, however, much more complex because of the formation of Antarctic Bottom Waters (AABW), wind-induced downwelling and upwelling (subduction and induction along isopycnal or constant-density surfaces) at different latitudes, near-surface boundary retroflections and bottom-flow constrictions, and spatiotemporal intermittent vertical and horizontal mixing. Together with mixing, the returning limb of the AMOC is the major latitudinal connector between the intermediate and upper layers of the entire ocean. Thanks to these returning AMOC waters, the transoceanic gyres exchange mass, heat and all sorts of properties, causing that no single part of the Atlantic gyres remains isolated.

The AMOC has a major role on heat transport to high latitudes of the Atlantic Ocean, helping temperate the weather in northern Europe. Changes in deep-water formation are expected to drive major variations in the returning limb of the AMOC, e.g. a decrease would imply less subtropical water and heat arriving to high latitudes, hence increasing the density of high-latitude surface waters in what would be a regulating feedback mechanism. However, this decrease could be counteracted by an increase in the heat content of the subtropical waters and by the larger gain of radiative heat at high latitudes as a result of greenhouse and albedo changes, as well as by the associated ice melting. Either way, warming of the surface ocean would drive substantial weather changes, such as increased ocean evaporation and precipitation at high latitudes and changes in the mean position of the Intertropical Convergence Zone, with high impact in precipitation patterns of the Mediterranean and tropical regions.