A proposal for a Southern Ocean biogeochemical observations and modeling program (SOBOM)
Professor Jorge Sarmiento, Princeton University, USA
Because the Southern Ocean surrounding the Antarctic is the primary window through which the vast volume of the intermediate, deep, and bottom waters of the ocean interact with the surface layer of the ocean and thus the atmosphere, this region has a profound influence on the Earth’s climate and ecosystems. Indeed, prior modeling and observational studies suggest that, despite occupying just over a quarter of the surface ocean area,
the Southern Ocean accounts for up to half of the annual oceanic uptake of anthropogenic carbon dioxide from the atmosphere;
vertical exchange in the Southern Ocean supplies nutrients (nitrate and phosphate) that fertilize three-quarters of the biological production in the global ocean north of 30°S;
the Southern Ocean accounts for about 70% ± 30% of the excess heat that is transferred from the atmosphere into the ocean each year and which is currently slowing the rate of global warming; and
Southern Ocean winds and buoyancy fluxes are the principal sources of energy for driving the large scale meridional overturning circulation of the deep ocean.
Furthermore, model simulations of the future project that climate change will have a profound impact on vertical exchange of deep and surface waters in the Southern Ocean, with corresponding changes in the ocean carbon cycle, heat uptake, and ecosystems; and that, due to acidification, the Southern Ocean will become undersaturated in aragonitic calcium carbonate by 2030, with potentially major impacts on calcifying organisms and Antarctic ecosystems. Despite its disproportionate importance, the Southern Ocean is the least observed and least understood region of the world ocean, and the studies underlying these results are thus highly controversial. The two most critical issues are that models of the Southern Ocean are too coarse to resolve critical features of the ocean circulation; and that we have only limited observations to assess the models due to the great difficulty of obtaining data in this region. We propose taking advantage of recent advances in computational capacity and biogeochemical sensors to develop a research program consisting of a strategic and optimal mix of innovative and sustained observations of the carbon cycle, ultra-high resolution modeling, and focused process studies.
Changes in the ventilation of the southern oceans
Professor Darryn Waugh, John Hopkins University, USA
Surface westerly winds in the Southern Hemisphere have intensified over the past few decades, primarily in response to the formation of the Antarctic ozone hole. I will discuss the impact of this intensification on the transport of surface waters into the interior (“ventilation”) of the southern oceans. Measurements of CFC-12 made in the southern oceans in the early 1990s and mid- to late-2000s will be used to show large-scale coherent changes in the ventilation, with a decrease in the age of subtropical subantarctic mode waters and an increase in the age of circumpolar deep waters. Model simulations will be used to examine the possible mechanisms involved with these changes in ventilation, and the possible impact on the oceanic uptake of heat.
Circulation in the Southern Ocean: a conspiracy between wind, buoyancy, eddies and geometry
Dr Andy Hogg, ANU, Australia
Disentangling the individual contributions of surface wind stress and surface buoyancy forcing to the Southern Ocean circulation is complicated by the dynamical role played by eddies, as well as interactions between flow and topography in this region. Here we show a suite of recent results from idealised (but high resolution) ocean models, which are helping to unravel the governing dynamics of the Southern Ocean. It is now clear that eddies may partially moderate the Southern Ocean response to future changes in wind stress, but that the sensitivity of the overturning circulation and the circumpolar transport differ considerably. Surface buoyancy forcing (both local and remote) plays a strong role in controlling the system response, and is likely to dominate Southern Ocean change on long timescales. Idealised model have the twin advantages of complete equilibration and model efficiency; however, an important caveat on the application of idealised model results is that details of the model topography can dominate the behaviour of the system.
Glacial – interglacial changes in CO2 and the links to Southern Ocean dynamics
Professor Andrew Watson FRS, University of Exeter, UK
The causes of reduced atmospheric CO2 in the Quaternary glaciations remain imperfectly understood, and difficult to model from first principles. It is clear however, from the correlation observed in Antarctic ice cores between temperatures and CO2, that Southern Ocean processes play key roles in driving these changes. Among these, there is good reason to believe that changes in the overturning and bottom water formation processes are very important. We discuss here some aspects of these processes in the modern ocean, and highlight differences in glacial time that we believe would have contributed to decreasing atmospheric CO2. We concentrate on (1) decreased Southern Ocean upwelling due to a weaker residual circulation, (2) a greater remoteness of the upwelling sites from those of bottom water formation, (3) a seasonal rectification effect due to the proximity of winter sea ice formation to the polar front, resulting in greater salinity stratification of the oceans, (4) Decreased air-sea equilibration of newly densified surface water, due to the intense and rapid cooling that occurs in coastal polynas which would have been the main formation region for bottom waters in glacial time.
What's happening at the poles?
Professor John Marshall FRS, MIT, USA
The Arctic is warming and sea ice is disappearing. But the Antarctic is (mainly) cooling and sea ice is growing. Why? We discuss the role of the ocean in the asymmetric response of the poles to Greenhouse Gas and Ozone Hole forcing and suggest that part of the answer might lay in inter-hemispheric asymmetries in the mean ocean circulation.