State of the art

The Southern Ocean (SO) is the most remote and the least understood of the world’s oceans, although it plays a crucial role in past and present climate state and changes. It is unique in being the only zonally unbounded ocean. For this reason, it is the major link by which water properties are exchanged among the other oceans. It is a region of large exchanges of heat, fresh-water, momentum and carbon between the ocean and the atmosphere. The large rate at which energy is locally imparted to the ocean by the strong westerly winds forces the Antarctic Circumpolar Current (ACC; Wunsh, 1998), which is the longest and strongest oceanic current on Earth (Figure 1).

The strong eastward-flowing ACC tends to isolate the warm subtropical waters from the cold polar waters, acting primarily as a barrier to the inter-basin exchange of heat and other properties between the SO and the remainder of the global ocean.Nonetheless, the intense air-sea ice-ocean exchanges across the SO, Ekman pumping due to the powerful and persistent wind regime together with the highly turbulent nature of the ACC give rise to two major meridional overturning circulation (MOC) cells, namely upper and lower (cf. Speer et al. 2000). The upper cell corresponds to the upwelling of deep waters and the equatorward return flow of lighter modal and intermediate waters. The lower cell combines the densest part of the deep waters with newly formed northward-flowing bottom waters originating along the Antarctic continental shelf. As a consequence, the deep circumpolar water is efficiently upwelled along the Antarctic divergence and it has been suggested that 65% of this water comes for the first time in contact with the atmosphere in the SO (De vries and Primeau 2011).

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).

General concept of SOCLIM and specific objectives

The SO extends over a vast area of the Earth’s surface and it is located far away from the other continents and most of the research facilities. Extreme weather conditions and significant sea-ice coverage prevail there most of the year. At present, the SO remains very under-explored, and the scarcity of oceanographic data limits our ability to understand key climatic-relevant processes and document ongoing changes.

The present proposal (SOCLIM) intends to implement a cutting-edge approach that will qualitatively and quantitatively improve the observation of the SO via pioneering in situ data acquisition. We propose a coordinated and collaborative deployment of a network consisting of different innovative instrumented platforms. Such a network has not been conceived before, or deployed in the SO. The network (Fig. 3) will be based on newly developed instrumented profiling floats, moorings, and drifting platforms, which, in combination with existing systems will allow unprecedented data acquisition.

We will explore a region of the SO encompassing the longitudes of South Africa and of Tasmania, and the latitudes from 40 °South to the Antarctic continent. This zone represents one third of this vast ocean. Our study will embrace the major types of systems that are relevant to climatic studies. From the circulation viewpoint, it will include regions representing fronts, the large Antarctic upwelling, and the formation of intermediate and modal waters. From the biogeochemical viewpoint, the study will investigate the whole range of productivity regimes, from the typically low productivity encountered in High nutrient Low Chlorophyll regions to large-scale naturally fertilized regions, e.g. Kerguelen, where the productivity is the highest measured in the SO.

A thorough knowledge and understanding of the processes and rapid changes within the Southern Ocean is needed, in order to adequately address today’s climate issues.