R.D. MUENCH, Earth and Space Research, Seattle, Washington 98102-3620
A major portion of the Southern Hemisphere water conditioning that leads to formation of bottom water in the world ocean occurs in the Weddell Sea. The goal of the international program for study of Deep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) is to understand physical processes in the Weddell-Scotia Confluence (WSC) region sufficiently to quantify the ventilation of the world ocean achieved by Weddell Sea water masses. The WSC is a site of energetic interactions among water masses flowing from the Weddell Sea, the Pacific Ocean and Bellingshausen Sea, the Bransfield Strait, and shelf-conditioned waters from the regions surrounding the Antarctic Peninsula. It overlies a region of steep, complex bathymetry dominated by the South Scotia Ridge, which limits north-south exchange to a few deep, narrow channels, and is thought to represent a gateway for the most direct and largest of these flows of antarctic water (figure 1). It is imperative that we understand the associated physical processes so we can assess their sensitivity to changes in regional forcing, hence, the impact of such changes on world ocean ventilation.
Four objectives contribute to the overarching DOVETAIL goal. The first is to assess the quantity as well as the physical and chemical characteristics of Weddell Sea source waters for the WSC region. This assessment will tell us the maximum volume of Weddell Sea water that is available for deep-ocean ventilation as Antarctic Bottom Water. Additionally, it will provide initial values for chemical parameters and tracers that 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. Information about the mechanisms of source-water formation is particularly important to understanding the possible influences of climate change on bottom-water production.
The second objective is to describe the dominant physical processes associated with spreading and sinking of dense antarctic waters within the WSC region. Available information based on both field and modeling efforts suggests that these waters exit the Weddell Sea either as boundary currents in deep channels or as flow along isopycnal surfaces (surfaces of equal water density). Water as shallow as about 400 meters in the Weddell Sea, following isopycnal surfaces northward, attains depths exceeding 3,000 meters during its northward passage through the WSC. The latter mechanism likely includes water-mass modification by turbulent mixing and transport by mesoscale processes such as eddies.
The third objective is to estimate the role of the Weddell Sea in ventilation of the world ocean. The northward volume transport of water, along with its physical and chemical properties, will help to quantify the mixing history of water available for deep-ocean ventilation north of the WSC.
The final objective is to estimate seasonal fluctuations in regional ocean transport and hydrographic structure and to assess the likely influence of interannual variability on rates of ventilation by Weddell Sea waters. Past field and modeling results indicate significant seasonal and interannual variability in both the Weddell Gyre and in the Antarctic Circumpolar Current (ACC). The WSC forms part of the northwestern limb of the Weddell Gyre and is bounded on the north by the ACC, and hence, must be influenced by variability in both regimes. New field measurements coupled with modeling efforts and synthesis with older results will help us to understand the physical interactions that link the seasonal and interannual changes and that might link climate change with ventilation rates.
DOVETAIL pursues its goal and objectives using a closely integrated field and modeling program. The field component measures critical hydrographic, tracer, and chemical parameters and currents in the northwestern Weddell Sea, a source region for the WSC, through the WSC itself, and in the southern Scotia Sea, which is the sink for water that has passed through the WSC. The hydrographic, tracer, and chemical observations take place during cruises in the austral summer, autumn, and winter seasons. Currents are being measured in the same region using moored current meters deployed as long as 26 months. These observations will be used to describe the mean hydrographic and current fields, to estimate seasonal differences, and to assess the roles of mesoscale processes. Numerical models will use these field data, in conjunction with historical data, to specify boundary conditions, for para-meterizations, and for verification. The models will serve to interpolate and extrapolate the data and will aid in identification and quantification of regional processes.
Two major field programs have been carried out, to date, under the DOVETAIL umbrella. The first of these took place during March 1996 when the German icebreaking research vessel Polarstern carried out a sampling program for water column physical, chemical, and tracer parameters and deployed six long-term current-meter moorings as part of a joint German/Spanish study (figure 2). The moorings are planned to be recovered by Polarstern in late autumn 1998. The second took place during August 1997 when the U.S. icebreaking research vessel Nathaniel B. Palmer occupied a series of transects in the WSC along which physical, chemical, and tracer parameters were measured (figure 1). On this latter cruise, 11 long-term current moorings were deployed and will commence returning data as early as April 1998 (figure 2).
The DOVETAIL program is the third in a sequence of integrated field and modeling programs that were initiated in 1992 and carried out in the Weddell Sea. All three programs have been coordinated by the international Antarctic Zone (iAnZone) group, now a Commission of the Scientific Council on Ocean Research (SCOR). The first two programs have focused on processes associated with ocean ventilation in the polar waters. DOVETAIL proposes to build upon the results from these two preceding programs by focusing on the escape of recently ventilated deep water from the Weddell Sea into the world ocean. In this way, it hopes to better define and understand the role of antarctic waters and processes in the ocean and climate systems.
DOVETAIL priorities parallel, and the results will contribute to, ongoing global change research. The processes responsible for vertical and horizontal fluxes within the ocean and associated interaction with the sea ice and atmosphere in polar regions must be properly represented. The DOVETAIL study region, off the tip of the Antarctic Peninsula, serves as the primary gateway between the southern polar waters and the world ocean. Results from the DOVETAIL experiment will aid in establishing a basis for long-range monitoring of this critical region, inasmuch as both the Global Ocean Observing System (GOOS) and the ocean component of the Global Climate Observation System (GCOS) have been established by a number of international bodies to provide such monitoring data.
This research was funded by National Science Foundation grant OPP 95-27667.