Deep Ocean Ventilation Through Antarctic Intermediate Layers
  1. Statement of Objectives

The foregoing background leads us to four specific objectives which have been used in constructing the DOVETAIL program. These can be stated as follows:

Objective 1: Assess the quantity, physical and chemical characteristics of Weddell Sea source waters for the Weddell-Scotia Confluence.

This will tell us the maximum volume of Weddell Sea water which is available for ventilation as Antarctic Bottom Water, and will give a "baseline" set of chemical parameters and tracers which can then be compared with downstream conditions in the WSC region and used to estimate ages and sources of waters and to determine mixing relationships during passage through the WSC region. Knowledge of sources is essential for understanding the possible influences of climate change on production [Gordon, 1991b]. Information on ages is essential to determine fractions which may recirculate in the Weddell Sea rather than exiting immediately via the Confluence region [Schlosser et al, 1991].

Objective 2: Describe the dominant physical processes associated with spreading and sinking of dense Antarctic waters within the Weddell-Scotia Confluence region.

Available field data and modeling results suggest two pathways by which Weddell Sea water transits the WSC region to contribute to Antarctic Bottom Water. First, cold Weddell Sea Bottom Water (WSBW, with T down to -0.6=F8C) exits through deep channels in the South Scotia Ridge as topographically steered bottom boundary currents. Second, Warm Deep Water (WDW, with T>0=F8C) and Weddell Sea Deep Water between 400 and 2500 m are transferred northward over the South Scotia Ridge along isopycnal surfaces, sinking to depths greater than 3500 m north of the Ridge and thence contributing to the Antarctic Bottom Waters [Orsi et al., 1993; Whitworth et al., 1994]. Hydrographic conditions in the overlying waters, reflected most obviously in the zonally-trending isopleths of variables, and currents predicted using numerical models [Semtner and Chervin, 1992] are consistent with a predominantly zonal circulation which parallels the Scotia Ridge. Weddell Sea water is likely transferred north through the WSC by way of diapycnal and isopycnal mixing and advectively by mesoscale current features associated with the Weddell and Scotia fronts. Energy for mixing processes and for instabilities leading to the mesoscale features can be derived from the mean flow by baroclinic instabilities [e.g. Klinck, 1985] and by interactions among the mean currents and a locally steep and complex bottom topography. In addition to mixing processes, the regional northward Ekman transport might force subduction of dense surface water of the Weddell-Scotia Confluence below the circumpolar waters of the Scotia Sea, leading to a process akin to the formation of Antarctic Intermediate Water. The proposed program will focus on quantifying both the deep boundary currents and the overlying mixing processes which combine to transfer Weddell Sea waters northward through the WSC.

Objective 3: Estimate the ventilation rate of the World Ocean from the Weddell Sea.

The field program will be coordinated with modelling efforts to estimate the quantity of Antarctic Bottom Water which passes northward through the WSC region and is available for deep ventilation. Existing models, such as the Princeton Ocean Model [Blumberg and Mellor, 1987] will be tuned using improved estimates of lateral mixing parameters derived from the field data. Water chemistry and tracer data will contribute, along with physical process studies, to quantifying the mixing history of water available for deep ocean ventilation north of the Confluence.

Objective 4: Estimate seasonal fluctuations in regional ocean transport and hydrographic structure, and assess the likely influence of interannual variability on rates of ventilation by Weddell Sea waters.

Recent field data from the Weddell Gyre indicate significant seasonal and interannual variability in the gyre-wide circulation [Fahrbach 1994]. Past field observations and modelling results indicate, similarly, interannual variability in the Circumpolar Current [Whitworth et al. 1983]. The Weddell Scotia Confluence lies between, and must be influenced by, these two major ocean regimes. Recent current data timeseries and historical current, hydrographic and sea ice datasets will be integrated with new current and water mass data and numerical model results for the Weddell Gyre and Antarctic Circumpolar Current in order to better understand the physical interactions which link the seasonal and interannual changes and which might link climate change with ventilation rates.