OS11E-01
Eight Year Timeseries of Weddell Sea Bottom Water Outflow
Two moorings positioned south of the South Orkney Islands have provided a nearly 8-year record of the currents and thermohaline stratification within the lower 500 m of the outflow of dense Weddell AABW, revealing significant seasonal and interannual variability during the measurement period (April 1999 - February 2007). At mooring M3 (4565 m depth), an annual pulse of the coldest bottom water is evident in the May-July period, though the precise timing and duration varies with year. Intensification of the near bottom stratification is observed as the bottom water attains its coldest values. The coldest bottom events occurred in 1999 and 2002, while in 2000 it was absent. At bottom temperatures <-0.8°C the salinity fluctuations produce a 'fan-like' appearance in T/S space suggesting a varied source of dense shelf water. The coldest bottom water <-1.0°C is relatively salty indicating a source in the southwest Weddell Sea, about 1300 km along isobaths to the mooring site. The typical bottom speed at M3 of 10-15 cm/sec implies shelf water export of deep and bottom water components during the austral summer. A record at a second mooring (M2) at 3059 m depth displays a much reduced annual cycle, but it too records a relatively warm period in 2000. Correlations of the M3 time series with NINO3.4 and SAM suggest that these indices lead M3 on the order of 14-20 months, implying a likely relationship between the water mass and surface forcing. Both M3 and M2 were reinstalled in March 2007.
OS11E-02 INVITED
Southern Hemisphere westerly wind control over the ocean's thermohaline circulation
Twentieth century climate change has forced a poleward contraction of the Southern Hemisphere (SH)
subpolar westerly winds. The implications of this wind shift for the ocean's thermohaline circulation (THC) is
analyzed in models and, where available, observations. Substantial heat content anomalies can be linked to
changes in the latitude and strength of the SH westerly winds. For example, the Southern Annular Mode
projects onto sea surface temperature in a coordinated annular manner - with a conspiring of dynamic and
thermodynamic processes yielding a strong SST signal. Subantarctic Mode Water (SAMW) change can be
linked to fluctuations in the wind-driven Ekman transport of cool, low salinity water across the Subantarctic
Front. Anomalies in air-sea heat fluxes and ice meltwater rates, in contrast, drive variability in Antarctic
Surface Water, which is subducted along Antarctic Intermediate Water (AAIW) density layers. SAMW
variations also spike T-S variability in AAIW, particularly in the southeast Pacific and southeast Indian
Oceans. The location of zero wind stress curl in the SH controls the distribution of overturning in the
Southern Ocean as well as in the North Pacific / North Atlantic. A southward wind shift can force a stronger
Atlantic THC and enhanced stratification in the North Pacific, whereas a northward shift leads to a
significantly reduced Atlantic THC and the development of vigorous sinking in the North Pacific. This is
because the distribution of wind stress over the Southern Ocean influences the surface salinity contrast
between the Pacific and Atlantic basins. Experiments with additional freshwater flux anomalies will also be
presented. The implications of these findings for oceanic climate change are discussed.
http://www.maths.unsw.edu.au/~ matthew
OS11E-03
Antarctic Circumpolar Current Response to Recent Climate Change
Observations indicate a significant intensification of the Southern Hemisphere westerlies during the last decades. A continuation of this trend is projected by climate scenarios for the 21st century. The response of the Antarctic Circumpolar Current (ACC) and Southern Ocean carbon sink to changes in wind stress and surface buoyancy fluxes is under debate. Here we utilize the Argo network of profiling floats and historical data to assess the changes in temperature, salinity and density across the ACC during the last four decades. We find coherent hemispheric-scale warming and freshening trends extending to depths of more than 1000m. The trends are partly related to changes in water mass properties, consistent with the effect of anthropogenic changes in heat and freshwater fluxes suggested by climate models. However, there is no increase in the tilt of density surfaces across the ACC, in contrast to coarse-resolution model studies. The result implies ACC transport and meridional overturning – and therefore the Southern Ocean carbon sink – are insensitive to decadal changes in wind stress, suggesting wind-driven Ekman transport is compensated by eddy fluxes, as simulated by models with explicit eddies.
OS11E-04 INVITED
Quantifying eddy diffusivity in the Southern Ocean
A diagnostic framework is presented, based on the Nakamura effective diffusivity, to investigate the temporal and regional variation in eddy diffusivity. The temporal variability does not appear to be well correlated with variations in the eddy kinetic energy (EKE), or relatedly, with changes in the wind stress associated with the Southern Annular Mode. Similarly the spatial variability of the eddy diffusivity is not everywhere well correlated with the EKE. Instead, in the region of the ACC, variations in the mean flow u are apparently more significant: in the presence of a strong mean flow the eddy diffusivity can be suppressed even in the presence of moderately strong eddy activity, conversely in a region of weak mean flow the eddy diffusivity can be enhanced even in the presence of only weak eddy activity. This casts doubt on the ability of paramterizations based solely on EKE to adequately characterize the eddy diffusivity in regions of strongly varying mean flow such as the Southern Ocean. The results are, however, consistent with the eddy transport and mixing variability predicted by potential vorticity based arguments. It is shown that both in the ACC and equatorward of it there is a reasonable correlation between the eddy diffusivity and √ EKE/u. We suggest, therefore, that parameterizations which take into account the strength of the mean flow may prove more representative.
OS11E-05 INVITED
Long-Term Observations in Drake Passage from ISOS to IPY: Highlights from Recent Analyses.
Drake Passage is unique in that it has relatively long and extensive data coverage in the chronically under- sampled Southern Ocean. Because it is the narrowest constriction through which the Antarctic Circumpolar Current must pass on its global journey, it has provided a convenient chokepoint to measure and study Southern Ocean processes. The International Southern Ocean Study (ISOS) intensive field program in Drake Passage from 1974-81 contributed much to our present knowledge of oceanography within the Southern Ocean: the distinct near-zonation of the flow and water masses; the coherence of the horizontal and vertical scales of velocity and properties within the frontal regions; the ubiquitous presence of mesoscale frontal meanders and eddies; and the need for a poleward oceanic eddy-heat flux to balance heat loss to the atmosphere. During ISOS, a net transport for the ACC of 134 Sv was arrived at through a progression of hydrographic surveys, a picket-fence of moorings, and bottom pressure gauge data. In the ensuing years, this transport value has proven remarkably robust, especially given the relatively sparse and primitive instrumentation that was available at the time of ISOS. More recently, the relative accessibility and proximity to numerous international bases along the Antarctic Peninsula, has permitted ship-of-opportunity measurements en route across Drake Passage. Underway in situ measurements from XBT, XCTD and ADCP instrumentation, along with concurrent shipboard meteorological and pCO2 sampling, have been relatively routinely acquired over the past decade. These high-resolution measurements on year-round, repeating transects across Drake Passage have been used to examine seasonal and spatial variability in upper ocean diapycnal eddy diffusivities, eddy heat and momentum fluxes, and reveal long-term trends in upper ocean temperature related to large-scale climate variability. The talk will review the various measurement programs that have been undertaken in Drake Passage, from ISOS through to the International Polar Year (IPY), in particular highlighting the results from some recent analyses of the in situ shipboard observations from the repeat transects.
OS11E-06
Spin-up, adjustment and equilibrium state of the Antarctic Circumpolar Current.
Results will be presented for the spin-up, adjustment and equilibrium state of the Antarctic Circumpolar Current (ACC), forced by wind stress, buoyancy forcing and diapycnal mixing. First, an analytical model will be presented for the spin-up of an ACC in a reduced-gravity ocean, starting from a shallow pycnocline (consistent with Sandstrom's theorem). The initial spin up is controlled by diapycnal mixing, but the eventual equilibration/adjustment time-scale is controlled by the Gent and McWilliams eddy transfer coefficient. This result has important implications for climate model studies: eddy closures greatly underestimate the sensitivity of the eddy fluxes to subtle changes in the mean strength of the ACC, and the adjustment time- scale is therefore likely to be overestimated. We also find that diapycnal mixing plays an important in setting the mean strength of the ACC, in particular in the limit of strong diapycnal mixing (10-4 m2 s- 1), for precisely the same reasons that diapycnal mixing may play a role in setting the strength of the Atlantic Meridional Overturning Circulation (AMOC). These theoretical predictions are compared with the results of numerical calculations using the MITgcm, in a global configuration with parameterized eddies, and in an idealized basin/reentrant channel configuration with partially resolved eddies. The relation of the adjustment of the ACC to the global pycnocline and the AMOC, and the possibility of exciting seiching modes between the different basins, will also be discussed.
OS11E-07
Recent Decadal Warming and Freshening of Antarctic-Derived Abyssal Waters
Waters of Antarctic origin ventilate much of the abyssal global ocean and a significant fraction of the entire global ocean. WOCE hydrographic sections occupied mostly in the 1990's provide a high-accuracy survey of global water properties. A subset of these sections has been reoccupied in the current decade. Comparison of these repeat sections reveals statistically significant warming in relatively vertically and laterally homogenous abyssal layers in the Pacific, western Atlantic, and southeast Indian oceans. Abyssal freshening is also evident in some of the basins adjacent to Antarctica. Transient tracer data analysis shows that a substantial fraction of the abyssal waters in these basins have been ventilated within the past few decades. While the sparse data makes quantification of these changes difficult, they do appear large enough to be significant contributors to global heat and sea level budgets.
OS11E-08
Observations of atmosphere/ocean interaction in the Southern Ocean region
In this contribution I will review observational evidence of coupling between large-scale atmospheric variability and sea-surface temperatures (SSTs) in the Southern Ocean region. I will first overview the most important patterns of large-scale atmospheric variability in the high latitude Southern Hemisphere, and will then demonstrate how these structures impact the SST field. I will examine the coupling as a function of season, will examine the physical mechanisms that drive the observed coupling, and will explore lead/lag relationships between the SST field and the atmospheric flow on weekly time scales. I will close with a brief discussion of the processes which give rise to the observed persistence in Southern Ocean SSTs.