2.3 Southern Ocean

Recent scientific advances have led to growing recognition that Southern Ocean processes influence climate and biogeochemical cycles on global scales. The Southern Ocean connects the ocean basins and links the shallow and deep limbs of the overturning circulation, a global-scale system of ocean currents that influences how much heat and carbon the ocean can store (Rintoul et al., 2001). The upwelling of deep waters returns carbon (e.g. le Queré et al., 2007) and nutrients (e.g. Sarmiento et al., 2004) to the surface ocean; the compensating sinking of surface waters into the ocean interior sequesters carbon and heat and renews oxygen levels. The capacity of the ocean to moderate the pace of climate change is controlled strongly by the circulation of the Southern Ocean. The future of the Antarctic ice sheet, and therefore sealevel rise, is increasingly understood to be determined by the rate at which the relatively warm ocean can melt floating glacial ice around the margin of Antarctica (Rignot and Jacobs, 2002). The expansion and contraction of Antarctic sea ice influences surface albedo, air-sea exchange of heat and of gases, such as carbon dioxide and oxygen, and the habitat for a variety of marine organisms (Thomas and Dieckmann, 2002). The Southern Ocean is also home to unique and productive ecosystems and rich biodiversity. Given the significance of the Southern Ocean to the Earth system, any change in the region would have impacts that extend well beyond the high southern latitudes. Recent studies suggest change is underway: the Southern Ocean is warming and freshening throughout most of the ocean depth (Gille, 2008; Böning et al., 2008); major currents are shifting to the south, causing regional changes in sea-level (Sokolov and Rintoul, 2009a,b) and the distribution of organisms (Cubillos et al., 2007), and supplying additional heat to melt ice around the rim of Antarctica (Jacobs, 2006); and the future of the Southern Ocean carbon sink is a topic of vigorous debate (le Queré et al., 2007; Böning et al., 2008). Climate feedbacks involving ocean circulation, changes in sea ice (hence albedo) and the carbon cycle have the potential to alter rates of climate change in the future, but the magnitude and likelihood of such feedbacks remains poorly understood. Progress in understanding Southern Ocean processes has been slowed by the historical lack of observations in this remote part of the globe. Growing recognition of the importance of the Southern Ocean has resulted in an increasing focus on the region; at the same time, new technologies have led to great improvements in our ability to observe the Southern Ocean. International Polar Year 2007–2008 effectively harnessed the human and logistic resources of the international community and exploited technology developments to deliver an unprecedented view of the status of the Southern Ocean, provided a baseline for assessing change and demonstrated the feasibility, value and timeliness of a Southern Ocean Observing System (Chapter 3.3). During IPY, a circumpolar, multidisciplinary snapshot of the status of the Southern Ocean was obtained for the first time; many properties, processes or regions had not been measured before. Scientists from more than 25 nations participated in Southern Ocean IPY. Here, we summarize the rationale, field programs and early scientific highlights from IPY programs in the Southern Ocean to show that the IPY has provided significant advances in our understanding of the Southern Ocean.