The Global Influence of the Southern Ocean Steve Rintoul Centre for Australian Weather and Climate Research Wealth from Oceans National Research Flagship Antarctic Climate and Ecosystems CRC Hobart, Tasmania, Australia

The ocean is cold Potential temperature in the Atlantic Ocean. ? ? ?

The Southern Ocean overturning circulation: a window to the deep sea Rintoul, 2001

Global warming is ocean warming Levitus et al., 2004 OCEAN LAND ICE SHEETS ATMOSPHERE SEA ICE GLACIERS 84% of warming since 1955 is found in the ocean.

Overturning determines how much carbon dioxide gets stored by the oceans Sabine et al., 2004

Antarctic Circumpolar Current: connecting the ocean basins Rintoul et al., 2001

Eddies play a key role in the dynamics of the Southern Ocean Hallberg and Gnanadesikan, 2006

Antarctic sea ice

Is the Southern Ocean changing? Scambos et al., NSIDC

Loss of Antarctic ice: melt by a warmer ocean? Antarctic mass loss increased by 70% in last decade, as a result of warmer air and sea temperatures. Rignot et al., Nature Geoscience, 2008 New gravity measurements suggest larger than expected ice loss in East Antarctica. Chen et al., Nature Geoscience, 2009

Dense water formed in the Southern Ocean has become fresher and less dense since Rintoul, GRL, 2007

Argo profiling floats

Tracking warming of the Southern Ocean with Argo floats Böning et al., Nature Geoscience, 2008

Elephant seal oceanography: mapping the ocean beneath the sea ice Charassin et al., PNAS, 2008

Acidification of the Southern Ocean: 450 ppm is the “tipping point” Orr et al (2005) McNeil & Matear, PNAS, 2008 If take seasonality into account, SO will be undersaturated with respect to aragonite by Corresponds to atmosphere concentration of 450 ppm.

Large regional changes in Antarctic sea ice Stammerjohn et al. (2008) Changes in sea ice duration: 1979 –  23 days 57  13 days

Future of the Southern Ocean Warmer, more precipitation More stratified (less nutrient input?) Weaker overturning circulation? “Saturation” of carbon sink? More acidic (ecosystem impact?) Increased melt of floating glacial ice, faster sea-level rise Less sea ice (fewer krill + krill predators?) Southward shift of ocean currents and habitat

Southern Ocean science and policy To educate, to inspire and to raise awareness of the global influence of the Southern Ocean. To detect change and assess the likelihood of abrupt change. Source of better climate projections, from models that include a better representation of Southern Ocean processes. Wise stewardship of marine resources. Protecting lives and the environment in the face of a growing human presence in Antarctica.

A Southern Ocean Observing System: Legacy of the International Polar Year

Summary The Southern Ocean has a profound influence on the earth system. Changes in the Southern Ocean will affect climate, sea level and marine resources. Change observed to date likely linked to the Southern Annular Mode. To respond to the challenges of a changing climate, knowledge of how the Southern Ocean will respond to and drive climate change is critical. Sustained observations are key. The IPY demonstrated that a Southern Ocean Observing System is feasible, cost-effective and essential.

Changes in SO winds: Southern Annular Mode Thompson and Solomon (2002) TREND CONGRUENT WITH SAM

Trends in sea surface height: Sokolov and Rintoul, submitted

Rintoul, Hughes and Olbers 2001; Sokolov and Rintoul 2007a,b Bottom pressure torque (color); streamfunction (black) Barotropic vorticity (~ angular momentum) balance ß  x =  p b  H +  +  F North-south = bottom pressure + curl of wind + curl of nonlinear flow torque stress terms Hot spots of biological productivity

A dynamical recipe for the world’s largest ocean current winds and buoyancy forcing drive a current around Antarctica density surfaces tilt up to the south, in balance with current eventually the current goes unstable, spawning eddies eddies transfer momentum downward, and heat and mass poleward (so east-west and north-south flows are coupled) vertical transfer of momentum sets up deep currents interaction of deep currents with bathymetry establishes stresses on the sea floor to balance momentum and vorticity budgets. We now understand the circumpolar current and overturning circulation are intimately linked:

“Saturation” of the Southern Ocean carbon sink?

Summary The Southern Ocean and Antarctica are changing. SO changes will have global impact. Models still fail to capture key SO processes, so nature of future change remains uncertain. Observations are critical for model testing and improvement and to determine climate trajectory. A Southern Ocean Observing System is feasible, cost-effective, and urgently needed. Need to add something re policy.

Summary The Southern Ocean links the upper and lower limbs of the global overturning circulation, and therefore has a strong influence on the earth’s climate. The three-dimensional circulation of the Southern Ocean reflects a delicate balance between wind and buoyancy forcing, water mass transformation, eddy fluxes and topographic interactions. The present revolution in ocean observations will enable a major leap forward in our understanding of ocean climate processes and in our ability to predict future changes in climate and their impacts.

Acknowledgments Students: Bernadette Sloyan, Helen Phillips, Guy Williams, Andrew Meijers, Katy Hill, Laura Herraiz-Borreguero, Steph Downes Collaborators: Serguei Sokolov, Dirk Olbers, Kevin Speer, Mark Rosenberg Mentors: Trevor McDougall, John Church, Carl Wunsch, Arnold Gordon Support: CSIRO, Australian Antarctic Division, Australian Greenhouse Office, CRC program At sea: officers and crew of RSV Aurora Australis, Astrolabe, RV Franklin, FRV Southern Surveyor, family coping with long absences

Sea Mammal Research unit MAMVis-AD CEBC-CNRS Kerguelen Antarctica 1200 m Sea temperature Biuw et al., PNAS, 2007

Global influence of the Southern Ocean