OS12A-01
Diagnosing Meridional Meandering of the Antarctic Circumpolar Current
Long-term warming trends in the Southern Ocean appear consistent with a multi-decadal scale poleward migration of the Antarctic Circumpolar Current (ACC), possibly linked to a long-term shift in the Southern Annular Mode. Since historic in situ hydrographic data are sparse, definitive assessments of the actual trends and mechanisms governing them have proved difficult. Satellite data offer some promise for evaluating the meridional migrations of the ACC. However, studies carried out using microwave sea surface temperature (SST) data as well as satellite altimetry have sometimes reached differing conclusions. In part these differences may stem from the variety of definitions used to locate the frontal jets that define the ACC, which have been based on absolute sea surface height, gradients in sea surface height, or gradients in SST. In addition alternate statistical measures, such as the variance and skewness of sea surface height, have been proposed as means to identify the position of the ACC. This study represents an effort to identify consistent and robust definitions of the ACC jets from satellite altimeter data and to make use of these to investigate the physical processes governing ACC variability.
OS12A-02
Evidence of an Eddy-Driven, Intrinsic Interannual Variability in the Southern Ocean.
Sea Level Anomalies (SLAs) simulated by the DRAKKAR multi-resolution (2, 1, half- and quarter-degree) hierarchy of global ocean/sea-ice 50-year hindcasts, and derived from altimetry since 1993, are jointly investigated. Simulated SLAs are first collocated onto their observed counterparts in time and space ; all fields are then low-passed filtered to extract observed and simulated large-scale interannual SLA variabilities. The mean state, overall magnitude and structure of the interannual SLA variability are shown to improve with increased model resolution in most areas of the World Ocean, including Southern latitudes; this helps quantify the benefits of model resolution for climate-orented ocean hindcasts/forecasts. Unlike in other regions, the enhanced interannual variability which emerges at eddy-admitting resolution in the Southern Ocean does not project well on the leading EOFs of observed variability, suggesting its possible eddy-driven, non-linear, intrinsic character. Our methods and results, this interpretation and possible implications will be discussed.
OS12A-03
Determining Vertical and Along-Isopycnal Mixing Processes in the Southern Ocean
Here the strength and structure of the Southern Hemisphere Meridional Overturning Circulation (SMOC) is
related to the along-isopycnal and vertical mixing coefficients (K and D respectively) by
analysing tracer and density fields from hydrographic data. The total transports across temperature contours
on isopycnals, inferred from both tracer and density distributions suggest a ratio of K to D
of order 106 particularly on deeper layers of Upper Circumpolar Deep Water (UCDW). Analysis of the
overturning circulation for such a balance reveals a view of the SMOC where deep water is transferred to
lighter and denser layers in the Southern Ocean not only at the surface but also in the ocean interior with
vertical mixing playing an important role.
Relating the cross contour transport to a geostrophic streamfunction on ispoycnal surfaces allows for a great
advancement on existing inverse techniques. In this method points sharing the same temperature and
salinity, such as along the Antarctic Circumpolar Current, are connected by Tracer Tubes. Here vertical and
along isopycnal mixing processes as well as the mean and residual circulations may be inferred directly from
the observed hydrography.
http://www.marine.csiro.au/~zik003/SOoverturning_Zika_etal_080509.pdf
OS12A-04
The Force Balance of the Southern Ocean Meridional Overturning Circulation
An eddy-permitting state estimate was used to diagnose the Southern Ocean's momentum budget. The budget was zonally averaged to enable identification of the forces driving the meridional overturning circulation (MOC). The analysis was carried out in isopycnal coordinates, a natural coordinate system for the ocean as eddy fluxes are directed along these surfaces. Many recent theoretical studies surmised that the Southern Ocean MOC results from winds driving a clockwise MOC, and eddy transport of potential vorticity (PV) forcing a counterclockwise MOC. Analysis of the state estimate suggests a different force balance. A clockwise MOC is driven by mean and transient winds. This circulation is largely compensated by a counterclockwise geostrophic MOC. The geostrophic circulation is supported by pressure gradients at the continental margins and at isopycnal outcrops where such margins do not exists, i.e. in the Drake Passage latitude band. The geostrophic circulation dominates in the most buoyant density classes, making it responsible for the Southern Ocean's observed poleward heat transport.
OS12A-05
A Hydrographic and CFC Survey on the Adelie Land Shelf
During 16 Dec 07 - 27 Jan 08, a hydrographic survey of the Antarctic shelf adjacent to Adelie Land was carried out as part of the joint Australian programs - Climate of Antarctica and the Southern Ocean (CASO) and Collaborative East Antarctic Marine Census (CEAMARC) - from aboard the RSV Aurora Australis. Over 80 CTD stations were occupied on the shelf or adjacent slope in the region between 139° 13' E and 145° E. In addition to hydrographic parameters, dissolved oxygen and nutrients, CFCs, dissolved inorganic carbon, and total alkalinity were measured at nearly all of these stations. Several features of the CFC distributions stand out in this formation region of Adelie Land Bottom Water (ALBW) and appear to be related to the bathymetry of the shelf. There are two depressions in this region, both deeper than 800 m - one on the western edge of the study region and the other adjacent to the Mertz Glacial Tongue on the eastern side of the study region. Throughout most of the study area, the presence of Highly-Modified Circumpolar Deep Water (HMCDW) is reflected in mid-depth CFC concentration minima. However, HMCDW is not present in the shallower region between the depressions. Beneath the HMCDW, CFC concentrations generally increase towards the seafloor. The bottom water CFC concentrations below 600 m in the easternmost of these basins are 5-10% higher than those of the westernmost depression. The bottom water dissolved oxygen concentrations are also higher by approximately 15 μmol kg-1 in bottom waters of the eastern depression. The circulation in the eastern depression is cyclonic and bottom waters can flow out of the basin through a trough in the shelf break near 143° E. Waters with high CFC concentrations were detected on the downslope side of the trough - indicating that ALBW was being supplied to the deep Australia-Antarctic Basin even during summer. The data from this expedition will be compared to previous CFC measurements from this region over the past decade.
OS12A-06
Impact of Interactive Icebergs in a Global Present Day Climate Simulation
Icebergs are an important component of the global hydrological cycle linking the ice sheets/shelves and the ocean. GCMs used for 21st century climate projections in the IPCC AR4 lacked a proper description of ice sheet disintegration. However, the report states that such processes need to be included to improve estimates, e.g. on sea level rise, and that icebergs represent a major flux of freshwater from ice sheets. The stratification in the Southern Ocean and North Atlantic is sensitive to the spatial and temporal distribution of freshwater input, and both are thus dependent on the iceberg trajectories and melt distribution. In this study we implement a dynamic-thermodynamic iceberg model, based on the works of Bigg et al. (1997) and Gladstone et al. (2001), in a next generation IPCC GCM called CM2G. Icebergs are simulated as Lagrangian objects in the Eulerian framework of the GCM. Lacking an ice sheet model we use the calving field, which is the mass flux of frozen precipitation from ice sheets, generated by CM2G as the source for icebergs. Our simulation produces calving fluxes of about 2100 Gt/yr from Antarctica and 180 Gt/yr from Greenland, which are within the range of observed iceberg calving rates. The annual cycle inherited from the precipitation rate is absorbed by allowing the model to form icebergs of different size. Inclusion of icebergs leads to a more realistic distribution of the freshwater input. In the Southern Ocean iceberg trajectories reach as far north as 45°S and yield an average melt rate of 1.7· 10-6 kg/(m2s). This results in a less stable stratification of the water column on the Antarctic shelf and ventilation is greatly enhanced. CFC concentrations in the AABW below 4000 m depth are 5 to 10 times higher in the iceberg run compared to the control run. However, the sea-ice cover thins by 20 cm on average, and compactness and extent decrease because the calving flux no longer biases the formation of frazil ice.
OS12A-07
Observations of Ekman Currents in the Southern Ocean
Largely zonal winds in the Southern Ocean drive an equatorward Ekman transport that constitutes the shallowest limb of the meridional overturning circulation of the Antarctic Circumpolar Current (ACC). Despite its importance, there have been no direct observations of the open ocean Ekman balance in the Southern Ocean until now. Using high-resolution repeat observations of upper ocean velocity in Drake Passage, we resolve a mean Ekman spiral and compute Ekman transport. The mean Ekman currents decay in amplitude and rotate anticyclonically with depth, penetrating to ~100 m depth, above the base of the annual mean mixed layer at 120 m. The rotation depth scale exceeds the e-folding scale of the speed by about a factor of 3, resulting in a current spiral that is compressed relative to predictions from Ekman theory. Transport estimated from the observed currents is mostly equatorward and in good agreement with the Ekman transport computed from four different gridded wind products. The mean temperature of the Ekman layer is not distinguishable from temperature at the surface. Turbulent eddy viscosities inferred from Ekman theory and a direct estimate of the time-averaged stress were O(102-103) cm2 s-1. The latter calculation results in a profile of eddy viscosity that decreases in magnitude with depth and a time-averaged stress that is not parallel to the time-averaged vertical shear. The compression of the Ekman spiral and the nonparallel shear/stress relation are likely due to time-averaging over the cycling of the stratification in response to diurnal buoyancy fluxes, although the action of surface waves and the oceanic response to high frequency wind variability may also contribute.
OS12A-08
An Intercomparison of IPCC Coupled Climate Models' Simulation Of The Southern Ocean: Climate, Circulation And "Carbon"
Dominated by the Antarctic Circumpolar Current (ACC), the vast Southern Ocean can influence large-scale surface climate features on various time scales. Its climatic relevance stems in part from being the region where most of the transformation of the World Ocean's water masses occurs. In climate change experiments that simulate greenhouse gas-induced warming, Southern Ocean air-sea heat fluxes and three-dimensional circulation patterns make it a region where much of the future oceanic heat uptake occurs, though the magnitude of that heat storage is one of the larger sources of uncertainty associated with the transient climate response in such model projections. These links are explored here in a coupled model context by analyzing a suite of experiments associated with the Intergovernmental Panel on Climate Change's Fourth Assessment Report. The potential impacts of the predicted change in Southern Ocean sea-ice extent, concentration and seasonality, water masses, ocean circulation and frontal positions on heat, carbon and marine ecosystems are examined.