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New models and observations of the Southern Ocean, its role in global climate and the carbon cycle

Event

Starts:

July
162013

09:00

Ends:

July
172013

17:00

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Image courtesy of Professor Mike Meredith, British Antarctic Survey, Cambridge, UK

Theo Murphy international scientific meeting organised by Professor Andrew Watson FRS, Professor John Marshall FRS and Professor Mike Meredith.

Event details

The Southern Ocean is the most remote and the least understood of the world’s oceans, but plays a crucial role in past and present climate change. Currently it is the focus of intense physical and biogeochemical research. This meeting will bring together observationalists and modellers to exchange their latest insights, and will reach across the disciplines to bring together physical oceanographers, climatologists and carbon cycle scientists.

Biographies of the key contributors and recorded audio files of the presentations are available below.

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Event organisers

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Schedule of talks

Session 1: The Southern Ocean: large-scale circulation and climate

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Dense water export in the Atlantic sector of the Southern Ocean: mechanisms, changes and consequences

Professor Mike Meredith, British Antarctic Survey, UK

Abstract

The waters that form around the periphery of the Antarctic continent include the densest components of the global overturning circulation. These flow northward to flood the abyss of the global ocean by navigating various topographical obstacles, including complex ridge systems, narrow passages and deep trenches. These dense waters are undergoing significant changes in climate, with a circumpolar freshening now recognized, and a marked warming along the major outflow routes. Determining the causes of the latter is important, since the rate of warming is believed to be significant for assessments of sea level rise, the global heat budget, and benthic biodiversity. The rate of warming is strongest in the Atlantic, into which deep waters are fed from the Weddell Sea. This presentation will present a synopsis of recent advances in our understanding of the processes and controls that lead to variability in the properties and flux of the dense waters that exit the Weddell Sea northward, including wind-forced controls, topographic interactions, and abyssal mixing. Priorities for future research effort will be highlighted.

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Glacial versus Holocene Antarctic Circumpolar Current flow speeds through the Scotia Sea

Professor I Nick McCave, University of Cambridge, UK

Abstract

There are differences of opinion as to whether during the last glacial maximum (LGM) ACC flow speed was faster than present or unchanged, under either stronger or weaker winds, and whether the whole current shifted to the north or not.  We have examined whether the ACC flow speed and transport was greater or less at a past climate extreme with sediment data on the LGM to Holocene difference of ACC flow through the Drake Passage/Scotia Sea flow constriction. We find essentially no change in the average flow of the ACC through the region.  However, ACC flow at the LGM was significantly slower in the southern ice-covered portion of the area (south of 56° S), and only slightly (insignificantly) faster in the north, which implicates shielding from wind stress by fast-ice in the flow’s spatial variability. Slower flow over rough topography in the south implies reduced diapycnal mixing in this key region, consistent with a reduced overturning circulation. Added to the most probable scenarios that LGM winter sea-ice extent was ~5° further north and the frontal system was likely 5°-7° further north, some models can now be further constrained.

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Poleward heat transport in the Southern Ocean: identifying roles of winds and fronts

Dr Sarah Gille, University of California San Diego, USA

Abstract

Observed long-term warming trends in the Southern Ocean have been interpreted as a sign of increased poleward eddy heat transport or of a poleward displacement of the entire Antarctic Circumpolar Current (ACC), possibly in response to a decadal-scale intensification of the Southern Annular Mode (SAM).  The two-decade-long record from satellite altimetry is an important source of information for evaluating the mechanisms governing these trends.  Satellite altimeter data indicate that short-term meandering of the ACC tracks the SAM on monthly to seasonal scales and is closely linked to El Nino variability on seasonal to interannual timescales, but the signal is not yet long enough to intrepret decadal-scale trends in the ACC. The long-term warming trend suggests the possibility that more (or warmer) Upper Circumpolar Deep Water may be coming into contact with the Antarctic Ice Shelves, particularly in regions where the southern edge of the ACC is closest to the Antarctic continent (such as in the Amundsen Sea Embayment, near Pine Island Glacier.)  The Southern Ocean State Estimate (an assimilating model that is constrained both by altimetry and Argo observations) allows us to evaluate the relative contributions of air-sea fluxes, horizontal advection, and diffusion for the Amundsen Sea region.

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Processes at the Antarctic continental slope important for climate and the carbon cycle

Professor Karen J Heywood, University of East Anglia, UK

Abstract

Acceleration of Antarctic ice sheet loss is mainly driven by basal ice shelf melt, in turn determined by ocean-ice interaction and related to the heat transport onto the Antarctic continental shelf. Processes of water mass transformation through sea-ice formation/melting and ocean-atmosphere interaction on the Antarctic continental shelf are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models however cannot include such small-scale processes and struggle to reproduce the water mass properties of the region.

Changes in temperature and salinity of Southern Ocean water masses have been identified regionally. Here we discuss recent changes in water mass properties on the Antarctic continental shelf. Some of the mechanisms through which the warm waters offshore in the Southern Ocean may penetrate onshore are discussed, including eddies and along-slope waves.

In early 2012 the GENTOO project deployed three Seagliders for up to two months to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. We discuss evidence in the Seaglider data of exchanges across the shelf-break front (the Antarctic Slope Front), including observations of dense water spilling off the continental shelf, and of a subsurface lens of Warm Deep Water on the shelf emanating from offshore. GENTOO demonstrated the capability of ocean gliders to play a key role in a future Southern Ocean Observing System.

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Southern Ocean circulation, variability and links to climate

Dr Stephen Rintoul, CSIRO and Antarctic Climate and Ecosystems Cooperative Research Centre, Australia

Abstract

The Southern Ocean circulation connects the ocean basins, providing the dominant oceanic teleconnection and allowing a global-scale overturning to exist.  But the region is much more than a pipe: water mass transformations driven by air-sea fluxes and diapycnal mixing in the Southern Ocean link the deep and upper ocean and close the global overturning.  By returning deep waters to the upper ocean, and thereby countering the relentless export of organic material from the upper ocean, the Southern Ocean overturning is central to global budgets of carbon, nutrients and other properties.  The downward limbs of the various Southern Ocean overturning cells ventilate the ocean interior and set the capacity of the ocean to store heat and anthropogenic carbon dioxide and thereby regulate climate.  Given the imprint of the Southern Ocean on global ocean circulation, climate, and biogeochemical cycles, change in the Southern Ocean would have widespread consequences.  New measurements reveal aspects of the Southern Ocean circulation that are changing rapidly (eg the volume of Antarctic Bottom Water), while others are more resilient (eg the transport of the Antarctic Circumpolar Current).  These observations provide insight into the sensitivity of the Southern Ocean circulation to changes in forcing, whether natural or anthropogenic.

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Session 2: Southern Ocean mixing and controls on circulation

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Dr Andrew Meijers, British Antarctic Survey, UK

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Diapycnal mixing from coordinated tracer and turbulence measurements

Dr James R Ledwell, Woods Hole Oceanographic Institution, USA

Abstract

Measurements of the dissipation rate of turbulent kinetic energy in the southeast Pacific sector of the Antarctic Circumpolar Current (ACC) imply a diapycnal diffusivity of order 10-5 m2/s at nearly all depths. A tracer released in this same region agreed with this result in the lower part of the Upper Circumpolar Deep Water. Both the tracer and dissipation profiles give diffusivities more than an order of magnitude greater in the topographically rough Scotia Sea than in the relatively smooth southeast Pacific, although it presently appears that integration of the dissipation-based diffusivity falls short of the diffusivity measured by the tracer. The leading hypothesis for enhanced mixing in the Scotia Sea is that lee waves, generated by geostrophic flows over the topography, propagate upward and become unstable and break, to generate turbulent mixing well above the bottom. Extrapolation of the measurements to all of the area of the ACC suggests an average diffusivity of nearly 10-4 m2/s. An effort will be made to estimate from the diffusivity measurements the convergence of buoyancy in various water masses of the ACC, and therefore of the role played by diapycnal mixing in the ACC in the Meridional Overturning Circulation of the ocean.

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Dr Jan Zika, University of Southampton, UK

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Recent observations of Southern Ocean mixing and their implications

Professor Raffaele Ferrari, MIT, USA

Abstract

The Meridional Overturning Circulation (MOC) of the ocean is a critical regulator of the Earth's climate processes. Climate models have been shown to be highly sensitive to the representation of lateral eddy mixing in the southern limb of the MOC, within the Antarctic Circumpolar Current latitudes, although the lack of extensive in situ observations of Southern Ocean mixing processes has made evaluation of mixing somewhat difficult. We present the first direct estimate of the rate of lateral eddy mixing across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is approximately 700 m^2/s at 1500 m depth. The estimate is based on an extrapolation of the tracer based diffusivity using output from numerical tracers released in a 1/20th of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m^2/s and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. The implications of these results for the ventilation of deep water masses and for the representation of oceanic turbulence in ocean models used for climate studies will be discussed.

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Session 3: Southern Ocean overturning and ventilation

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A proposal for a Southern Ocean biogeochemical observations and modeling program (SOBOM)

Professor Jorge Sarmiento, Princeton University, USA

Abstract

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.

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Changes in the ventilation of the southern oceans

Professor Darryn Waugh, John Hopkins University, USA

Abstract

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.

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Circulation in the Southern Ocean: a conspiracy between wind, buoyancy, eddies and geometry

Dr Andy Hogg, ANU, Australia

Abstract

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.

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Glacial – interglacial changes in CO2 and the links to Southern Ocean dynamics

Professor Andrew Watson FRS, University of Exeter, UK

Abstract

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.

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What's happening at the poles?

Professor John Marshall FRS, MIT, USA

Abstract

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.

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Session 4: Carbon cycle and biogeochemical processes

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Biological response to variable dust supply in the South Atlantic sediment record

Professor Robert Anderson, Lamont-Doherty Earth Observatory of Columbia University, USA

Abstract

Iron, which can be supplied by mineral aerosols (dust), is thought to limit phytoplankton growth in most of the Southern Ocean. However, paleo records of Southern Ocean biological productivity generally show little to no response to enhanced flux of dust. The Subantarctic South Atlantic (SASA, roughly 40°-50°S) is anomalous in this regard. The greater biological response seen in the SASA than elsewhere in the Southern Ocean may reflect the upwind proximity of the principal dust source for high southern latitudes, which is located in Patagonia. Previous studies have shown a tight coupling between dust and biological productivity in the SASA at the glacial-interglacial time scale.  Here, by analyzing cores at higher temporal resolution, we demonstrate coupling at millennial time scales as well.  Specifically, we show that 230Th-normalized fluxes in SASA sediments of lithogenic minerals (primarily dust) and of C37-alkenones (produced by coccolithophorids) and of other biogenic tracers are clearly correlated with the dust flux records from two ice cores in Antarctica over the past 100,000 years.  As the correlation is now evident in three high-resolution sediment cores spanning a large portion of the SASA, we conclude that a widespread biological response to variable dust supply. The lack of response in other regions of the Southern Ocean demonstrates that other processes may be at play.

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Penetration of anthropogenic carbon into the deep Southern Ocean with special emphasis on the Weddell Sea

Dr Mario Hoppema, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Abstract

Using 10 cruises spanning 1984 to 2011, we investigate the time rate of change of TCO2 in the Weddell Gyre (i) along the Prime Meridian, (ii) on the continental slope near the tip of the Antarctic Peninsula, and (iii) at the bottom of the Weddell Sea interior. In the Weddell Sea Bottom Water at the Prime Meridian, the spatial distribution of the increase in DIC bears a high resemblance to that of CFCs, suggesting that the changes in Cant are propagated from the surface. However, other variables like dissolved oxygen and silicate also show trends through time, pointing to non-steady state conditions which might also affect the derived CO2 increase. Near the tip of the Peninsula, the coldest and most recently ventilated waters, hugging the continental slope, exhibit increasing DIC over time in clear dependence of temperature. In the bottom layer of the Weddell Sea interior, no relationship is found between DIC and potential temperature. The mean values of DIC in these waters are observed to have remained essentially constant, suggesting that no significant ventilation of these waters has taken place over the time scale of observations. This finding is in line with the low levels of CFCs at this location. Co-authors: Steven van Heuven, Centre for Isotope Research, University of Groningen, The Netherlands Elizabeth Jones, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany Hein J W de Baar, Royal Netherlands Institute for Sea Research, The Netherlands

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Recent trends in the Southern Ocean CO2 sink

Professor Corinne Le Quéré, Tyndall Centre, University of East Anglia, UK

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Southern Ocean carbon: accumulation, fluxes and transport from the Weddell Gyre to the global ocean

Dr Peter Brown, University of East Anglia, UK

Abstract

In the Southern Ocean, the Weddell Gyre (WG) is regarded as the primary location for the formation of deep and bottom waters and is potentially a significant area for the sequestration of carbon, nutrients and atmospheric gases. Here, measurements of the inorganic carbon system and transient tracers from four cruises that cross and enclose the Weddell Gyre combined with velocity fields from a box inverse model are used to investigate the accumulation, fluxes and transports of contemporary and anthropogenic carbon into/out of the gyre. The gyre is found to be a sink for both Canth and contemporary carbon dioxide for the summer/fall period under investigation; substantial undersaturation of pCO2 (up to 80 μatm) of the surface layer down to the depth of the Winter Water temperature minimum is found, possibly related to the recent retreat of sea-ice in the area. Little accumulation of carbon is found to occur in the WG, although elevated concentrations associated with sea-ice production/anthropogenic uptake are observed to be exported through the gaps in the South Scotia Ridge, primarily as transports of Antarctic Bottom Water. These results highlight the role of the region in injecting human-derived carbon into the global abyss.

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New models and observations of the Southern Ocean, its role in global climate and the carbon cycle Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ