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At several locations around Antarctica, dense water is formed as a result of intense sea ice formation. When this dense water becomes sufficiently denser than the surrounding water, it descends the continental slope and forms Antarctic Bottom Water (AABW). This study presents the AABW formation off the coast of Cape Darnley [Cape Darnley Bottom Water (CDBW)] in East Antarctica, using a nonhydrostatic model. The model is forced for 8 months by a temporally uniform surface salt flux (because of sea ice formation) estimated from Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) data and a heat budget calculation. The authors reproduce AABW formation and associated periodic downslope flows of dense water. Descending pathways of dense water are largely determined by the topography; most dense water flows into depressions on the continental shelf, advects onto the continental slope, and is steered downslope to greater depths by the canyons. Intense sea ice formation is the most important factor in the formation of AABW off Cape Darnley, and the existence of depressions is of only minor importance for the flux of CDBW. The mechanism responsible for the periodic downslope flow of dense water is further analyzed using an idealized model setup. The period of dense water outflow is regulated primarily by the topographic beta effect. [ABSTRACT FROM AUTHOR]

In the major ocean basins, diapycnal mixing upwells dense Antarctic Bottom Water, which returns southward and closes the deepest cell of the meridional overturning circulation (MOC). This cell ventilates the deep ocean and regulates the partitioning of CO2 between the atmosphere and the ocean. The oceanographic community's conceptual understanding of the deep stratification and MOC has evolved from classic 'abyssal recipes' arguments to a more recent appreciation of along-isopycnal upwelling in the Southern Ocean, consistent with a weakly mixed ocean interior. Both the deep stratification and the deep MOC are shown here to be sensitive to the form of the surface buoyancy forcing in a two-dimensional model that includes a circumpolar channel and northern basin. For a fixed surface buoyancy condition, the deep stratification is essentially prescribed, whereas for a fixed surface buoyancy flux, the deep stratification varies by orders of magnitude over the range of diapycnal diffusivity κ observed in the ocean. These cases also produce different scalings for the deep MOC with κ, in both weak and strong κ regimes. In addition, these scalings are shown to be sensitive not only to the type of surface boundary condition, but also to the latitudinal structure of the surface fluxes. This latter point is crucial as buoyancy budgets and dynamical features of the circulation are poorly constrained along the Antarctic margins. This study emphasizes the need for caution in the interpretation of simple conceptual models that, while useful, may not include all mechanisms that contribute to the MOC's strength and structure. [ABSTRACT FROM AUTHOR]

The major mechanisms of the oceanic poleward heat flux in the Southern Ocean are still in debate. The long-standing belief stipulates that the poleward heat flux across the Antarctic Circumpolar Current (ACC) is mainly due to mesoscale transient eddies and the cross-stream heat flux by time-mean flow is insignificant. This belief has recently been challenged by several numerical modeling studies, which stress the importance of mean flow for the meridional heat flux in the Southern Ocean. Here, this study analyzes moored current meter data obtained recently in the Fawn Trough, Kerguelen Plateau, to estimate the cross-stream heat flux caused by the time-mean flow and transient eddies. It is shown that the poleward eddy heat flux in this southern part of the ACC is negligible, while that from the mean flow is overwhelming by two orders of magnitude. This is due to the unusual anticlockwise turning of currents with decreasing depth, which is associated with significant bottom upwelling engendered by strong bottom currents flowing over the sloping topography of the trough. The circumpolar implications of these local observations are discussed in terms of the depth-integrated linear vorticity budget, which suggests that the six topographic features along the southern flank of the ACC equivalent to the Fawn Trough case would yield sufficient poleward heat flux to balance the oceanic heat loss in the subpolar region. As eddy activity on the southern flank of the ACC is too weak to transport sufficient heat poleward, the nonequivalent barotropic structure of the mean flow in several topographically constricted passages should accomplish the required task. [ABSTRACT FROM AUTHOR]

This study explores the role of sea ice freshwater and salt fluxes in modulating twenty-first-century surface warming in the Southern Ocean via analysis of sensitivity experiments in the Community Climate System Model, version 3 (CCSM3). In particular, the role of a change in these fluxes in causing surface cooling, expanding sea ice, and increasing deep oceanic storage of heat in the Southern Ocean is investigated. The results indicate that in response to the doubling of CO2 concentrations in the atmosphere in CCSM3, net freshwater input from sea ice to the ocean increases south of 58°°S (owing to less growth) and decreases from 48°° to 58°°S (owing to less melt). The freshwater source from changing precipitation in the model is considerably less than from sea ice south of 58°°S, but it serves to compensate for the reduction in sea ice melt near the ice edge, leaving almost no net freshwater flux change between about 48°° and 58°°S. As a result, freshwater input principally from sea ice reduces ocean convection, which in turns reduces the entrainment of heat into the mixed layer and reduces the upward heat transport along isopycnals below about 1000 m. The reduced upward heat transport (from all sources) causes deep-ocean heating south of 60°°S and below 500-m depth, with a corresponding surface cooling in large parts of the Southern Ocean in the model. These results indicate that changing sea ice freshwater and salt fluxes are a major component of the twenty-first-century delay in surface warming of the Southern Ocean and weak reduction in Antarctic sea ice in model projections. [ABSTRACT FROM AUTHOR]

Consistency and discrepancy of air--sea latent and sensible heat fluxes (LHF and SHF, respectively) in the Southern Ocean for current-day flux products are analyzed from climatology and interannual-to-decadal variability perspectives. Five flux products are examined, including the National Oceanography Centre, Southampton flux dataset version 2 (NOCS2), the National Centers for Environmental Prediction/Department of Energy Global Reanalysis 2 (NCEP-2), the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data version 3 (HOAPS-3), and the objectively analyzed air--sea fluxes (OAFlux). Comparisons suggest that most datasets show encouraging agreement in the spatial distribution of the annual-mean LHF, the meridional profile of the zonal-averaged LHF, the leading empirical orthogonal function (EOF) mode of the LHF and SHF, and the large-scale response of the LHF and SHF to the Antarctic Oscillation (AAO) and El Niño--Southern Oscillation (ENSO). However, substantial spatiotemporal discrepancies are noteworthy. The largest across-data scatter is found in the central Indian sector of the Antarctic Circumpolar Current (ACC) for the annual-mean LHF, and in the Atlantic and Indian sectors of the ACC for the annual-mean SHF, which is comparable to and even larger than their respective interannual variability. The zonal mean of the SHF varies widely across the datasets in the ACC. There is a large spread in the seasonal cycle for the LHF and SHF among the datasets, particularly in the cold season. The datasets show interannual variability of various amplitudes and decadal trends of different signs. The flux variability of the NOCS2 is substantially different from the other datasets. Possible attributions of the identified discrepancies for these flux products are discussed based on the availability of the input meteorological state variables. [ABSTRACT FROM AUTHOR]

The modulation of air--sea heat fluxes by geostrophic eddies due to the stirring of temperature at the sea surface is discussed and quantified. It is argued that the damping of eddy temperature variance by such air--sea fluxes enhances the dissipation of surface temperature fields. Depending on the time scale of damping relative to that of the eddying motions, surface eddy diffusivities can be significantly enhanced over interior values. The issues are explored and quantified in a controlled setting by driving a tracer field, a proxy for sea surface temperature, with surface altimetric observations in the Antarctic Circumpolar Current (ACC) of the Southern Ocean. A new, tracer-based diagnostic of eddy diffusivity is introduced, which is related to the Nakamura effective diffusivity. Using this, the mixed layer lateral eddy diffusivities associated with (i) eddy stirring and small-scale mixing and (ii) surface damping by air--sea interaction is quantified. In the ACC, a diffusivity associated with surface damping of a comparable magnitude to that associated with eddy stirring (∼∼500 m2 s−−1) is found. In frontal regions prevalent in the ACC, an augmentation of surface lateral eddy diffusivities of this magnitude is equivalent to an air--sea flux of 100 W m−−2 acting over a mixed layer depth of 100 m, a very significant effect. Finally, the implications for other tracer fields such as salinity, dissolved gases, and chlorophyll are discussed. Different tracers are found to have surface eddy diffusivities that differ significantly in magnitude. [ABSTRACT FROM AUTHOR]