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Pictures from the Workshop

A group of participants in the international program for study of Deep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) met for an informal workshop at the Universitat Polytecnica de Catalunya in Barcelona on 2-3 May 2000 directly following the EGS Assembly in Nice. The purpose of the workshop was to present preliminary scientific results from the international DOVETAIL program and to coordinate and integrate ongoing research activities. Participating nations include Argentina, Brazil, Germany, Spain, the UK and the US. DOVETAIL falls under the purview of the SCOR-affiliated iAnZone (international Antarctic Zone) committee, and is seen as a contribution to the international CLIVAR program.

The DOVETAIL program, which commenced in 1996, has the goal of assessing the volume transports and mechanisms associated with northward flow of Weddell Sea Deep and Bottom Waters across the South Scotia Ridge into the Scotia Sea, whence it is available to ventilate the deep global ocean. This goal is approached through acquisition and analyses of new field data, dedicated numerical modeling, and integration of final results.

This report is intended to be a brief summary of the workshop and to provide a general idea of what topics were discussed. More detailed information can be requested from Robin Muench, who chaired the meeting, or from any other participant (see the list included below).

Field data acquired as a result of DOVETAIL have provided new and quantitative results and have greatly increased our confidence in predicting primary transport pathways through the South Scotia Ridge region. These predictions rely on "traditional" hydrographic, tracer and current measurements, the latter both as instantaneous (ADCP and LADCP) and as multiyear time series observations.

Field results suggest two primary deep ventilation pathways that originate from the southern and southwestern Weddell Sea shelf regions. The shallower flow exits the Weddell Sea via the channel west of the South Orkney Islands, whereas the deeper exits northward through Orkney Passage and other passages east of the South Orkney Islands and comprises the major outflow. Northward flow appears primarily along the western sides of passages, with weaker southward counterflows along the eastern sides.

Details of the deep flows depend upon bottom topography associated with sills in the South Orkney Ridge, and this topography is not well known. Sill depths influence throughflows and circulation cells interior to Bransfield Strait and Powell Basin, and it is possible that they "modulate" seasonal or even interannual flow variability between source and sink regions by trapping water for a period.

Shelf source regions in the northwestern Weddell Sea appeared to impact primarily Bransfield Strait and the waters between the Antarctic Peninsula and the South Orkneys. Weddell-Scotia Confluence waters were not always present east of the South Orkneys, and questions were raised concerning seasonality of this feature and of related upstream shelf processes. It does not appear that Bransfield Strait waters contribute to the Confluence.

Time series observations have revealed a system rich in temporal fluctuations varying from tidal through "meteorological" to interannual. Tidal currents were pronounced over shelf and bank regions and contributed to water column mixing. An 11-13 day signal in the NW Weddell was of uncertain origin, and sudden shifts were observed in deep water mass characteristics at some sites. Conversely, atop the bank west of the South Orkneys, bottom flow was steady toward the northwest. Seasonal signals were not evident in the deep waters. Long-term warming of the Weddell Deep Water continued from 1998-2000.

The Antarctic Circumpolar Current (ACC) influences the region by injecting high energy events, or perhaps by meandering, southward. The Powell Basin, in particular, might be influenced by the ACC. These interactions are not well understood. The ACC also influences He tracer results by introducing He that has been injected along mid-ocean ridge systems.

Three separate modeling efforts were discussed, including derivatives of the SPEM and MOM models. Mean circulation results derived from numerical models were generally consistent with information derived from the field data, and the models were perceived as being especially useful as potential trackers for bottom water flow using trajectories. General modeled distributions of variables agreed with field data, though there was some disagreement in places. Discussion focused on restoring parameters (e.g., winter, summer or Levitus), boundary conditions (open, closed, or specified) and surface forcing with emphasis on ice cover influence. Some difficulty exists reproducing flow in passages due to grid size constraints. Needed work includes improved bottom topographic data for model runs, which will be obtained by integrating existing regional (e.g. ETOPO5 and Smith-Sandwell) and shipboard topographic data. Models underestimate ages because of lack of diffusion along trajectories, so there needs to be some inclusion of diffusion, especially in highly turbulent regions, and there needs to be some inclusion of bottom plume dynamics. There was discussion also of "nesting" of models in order to attain sufficiently small grid spacing in crucial subregions.

Goals of the ongoing model development are seen to be improvement of our mean circulation information, definition and explanation of seasonal cycles, and process studies. An effort is needed to adjust model bottom topography to observed topography, especially near the passages through the South Scotia Ridge. The general developmental plan is to (i) obtain the best possible agreement between model and observational results, (ii) carry out sensitivity studies, and (iii) carry out process studies using the models. For example, lagrangian drift studies might be used in conjunction with injected tracers in order to track bottom flow pathways.

Ongoing work includes broad-band analyses of the field data, model development, and integration of the model and field data results. Additionally, new field data are being acquired by way of long-term US current measurement and multiyear Brazilian shipboard programs. These two programs are being coordinated at the scientific level. These observations are being obtained along the same grid as used for earlier DOVETAIL activities and will be of great value in assessing the ongoing interannual changes. We are particularly indebted to the Brazilian Navy for making possible the ongoing multiyear series of shipboard observations.

Planned significant activities, in addition to continuing data analyses addressing the central DOVETAIL issues, are:

1. Compilation of DOVETAIL and related historical ocean data and distribution to DOVETAIL investigators on a CD-ROM, target date of winter 2001.
2. Publication of a special journal issue of DOVETAIL results, first choice being Deep-Sea Research, with a target submission date for papers of 1 January 2001.
3. Coordinated efforts to compare model results with the extensive field database, with particular attention to modeled representation of convective and mesoscale processes.
4. Continuation, over at least the next two years, of a coordination effort focussed on integrating new field data into the existing database and using these data to reassess interannual change and to carry out ongoing re-examination of our results.
5. Convening of a one-day DOVETAIL workshop in conjunction with the October 2001 iAnZone meeting and Ross Sea Conference scheduled to take place near Naples. The primary purpose will be to discuss and integrate final results and to assess interannual changes detected by the two ongoing programs.

It was felt by the workshop participants that satisfactory progress is being made toward understanding and quantifying the complex processes that impact northward flow of Weddell Sea Water into the Scotia Sea. Data acquired under the DOVETAIL umbrella have vastly improved the spatial and temporal data coverage of the region, finally allowing a mapping of the mean circulation that maintains consistency among physical and tracer observations and modeling results. We are now approaching a sufficiently long time series of observations that we can reasonably assess interannual variability and perhaps guess at mechanisms.

A special "Thank You" is due to Marc Garcia and Maria Ruiz for hosting the workshop and creating a pleasant environment at UPC!

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