Given the above, embedded in the global thermohaline circulation, SO contributes to the central role played by the SO in regulating the Earth’s climate, through the formation, transformation and redistribution of water masses throughout the other three major oceans.
Accurately describing the circulation of the SO is difficult because of the prominent role of very intense fine-scale dynamics that characterizes the flows in this region all-along their paths, including the ACC, (Gille and Kelly 1996). It has been suggested that this dynamics, which is characterized by small eddies, jets and filaments, has a strong impact on the ocean circulation, biogeochemical properties and ecosystems in setting the density structure and transport properties of the current and in ruling vertical transfers as well as the structure of the upper branch of the MOC (Rintoul et al. 2001).
For example, the SO is a major source of natural CO2 due to the upwelling of CO2-rich deep waters, but a major sink of anthropogenic CO2 due to the formation of intermediate and bottom waters (Takahashi et al. 2012). Moreover, the SO largely contributes to supply nutrients from the deep ocean to upper water layer everywhere in the world ocean (Sarmiento et al., 2004).
For all these reasons, the SO plays a critical role in the control of the Earth’s climate. (Sigman et al. 2010, Kohfeld et al. 2005). In turn it is very sensitive to climate variability (Figure 2). Given the critical role of the SO in the Earth’s climate system, changes in that ocean have global ramifications. In fact, such changes are already under way.
The SO is warming more rapidly and to greater depth than the global ocean average (Böning et al., 2008; Gille, 2008), and freshening has been detected at all depths (Durack and Wijffels, 2010), The SO CO2 sink has decreased (Lequéré et al. 2007). Anthropogenic CO2 concentrations have increased, changing the ocean chemistry (Orr et al. 2005) with impacts on marine organisms (Moy et al, 2009); ocean warming is contributing to enhanced melt of floating ice shelves, with implications for ice sheet mass balance and sea-level rise (Shepherd et al., 2004; Rignot, 2008); and changes in oxygen concentrations have been detected (Helm et al., 2010).
The SO is the largest High Nutrient Low Chlorophyll region in the World Ocean, where the potential for carbon storage by the biological pump is not fully realized. This is mainly due to iron limitation but other factors (light-mixing regime, silicate) also play important roles (Boyd 2002, Blain 2007, 2013). Future changes in atmospheric and oceanic dynamics may alter these factors, and therefore modify the efficiency of the biological carbon pump in a way that is presently very difficult to predict.
For a long time, the overturning circulation, the ventilation of the SO and other properties (e.g. nutrient distributions) have been described in terms of zonal means. However, recent studies have shown that within-zone spatial and temporal differences require detailed regional studies (Sallé et al. 2010).