During the last few decades we have advanced our knowledge of the dominant processes of the southern ocean shelf regime and adjacent deep ocean regime, but what has lagged, mostly because of difficult scientific and logistic problems, is the interface across which these two distinct regimes interact: the continental slope regime. The slope regime is often characterized by strong thermohaline gradients (the Slope Front) and irregular topography with incised canyons.
As the shelf/deep regime interaction is related to major ventilation of the global ocean, it is important that it be properly represented within global climate models.
An important climate related question remains unresolved:
What is the role of the Southern Ocean slope front and slope canyons in the exchange of mass, heat, and freshwater between the shelf and oceanic regimes, in particular those leading to deep reaching plumes?
An iAnZone Slope Front workshop was held on 27-29 September 1999 to discuss what we know about shelf/deep regime exchange and to structure an observational and modeling research plan which would help develop a quantitative understanding of the cross slope exchange processes.
The workshop participants, representing US, Italy, UK, Germany, Russia and Japan, are listed in Table I. Written reports from Australia and Norway were also available. The meeting agenda is attached as Appendix A.
The meeting began with science presentations, with a focus on water mass exchange between the shelf and deep water regimes; slope front structure and variability and its role in shelf/deep water exchange; descending dense waters on the slope and within canyons, and the relationships to atmospheric and sea ice conditions. The presentations can discuss observational and modeling information at any site along the Antarctic margins. However, as the Ross Sea remains the most likely site of observational research, emphasis on the Ross Sea or on the relevance of results to the Ross Sea region, was encouraged.
The science presentations and national reports were followed by a discussion and further development of the June 30, 1999 Summary Statement (strawman plan), prepared by the US iAnZone group.
The group agrees with summary statement that from both logistics and scientific considerations that the Ross Sea offers the most promising site to study cross slope exchange. The slope regime in the Ross Sea is generally ice free in the summer months, enabling use of mooring and Lagrangian technology, and a greater chance of sampling of the thermohaline and tracer fields at the proper scales. Dense shelf water abounds, including Ice Shelf Water. Its bathymetry is better surveyed than other segments of the continental margins. It is close to the logical bases of McMurdo and Christchurch. The alternative site within the Weddell Sea, is considered too logistically difficult and remote to allow implementation of a properly designed experiment. While the Ross Sea is clearly the region of choice, where in the Ross Sea is still an open question: western, central or eastern dense water escape routes. Each has positives and negatives.
It is recommended that US and Italian programs be closely coordinated to further develop the experimental approach. Coordination will enable more than one site within the Ross Sea to be studied. Coordination with the Australian program along the Adelie coast to the west (which probably represents the downstream condition) will also be explored.
The workshop participants more sharply focussed the initial Summary Statement towards addressing the nature of the descending plumes of dense shelf water over the continental slope regime, and their relationship to topography and Slope Front structure and variability.
Draft statement of the research program:
Title: Cross-slope fluxes of Antarctic Shelf Water along the Antarctic continental slope.
Goal: Determine the principal physical processes that cause the injection of dense shelf water into the intermediate and deep layers of the deep water surrounding the Antarctic continent.
Hypothesis: The property characteristics and rate of transport of dense shelf water across the shelf break is determined by the following processes:
1. Topographic effects (role of canyons, isobath divergence/convergence, proximity to end of western boundary);
2. Cross-slope advection by tides, other "high frequency" forcing (weather-band wind stress, frontal instabilities), and benthic Ekman dynamics associated with the along-isobath mean barotropic circulation;
3. Nonlinear equation of state effects (thermobaricity, cabbeling);
4. Shelf water mass properties and spatial variability of cross-slope surface and benthic layer fluxes.
Figure 1: Conceptualization of slope front mixing processes
Methodology: Obtain measurements and develop models that describe the structure of cross-slope flow of shelf water at a specific "characteristic" site, use the data to improve the models local predictive skill, then use the upgraded models to extrapolate our estimate of cross-slope benthic flow to the major regions of AABW production in the Antarctic.
The processes described in "Hypothesis" require a coordinated program involving mesoscale and process modeling, and observations capable of resolving the small scales of variability that are expected from our present knowledge of the small space and time scales of the dominant processes near the shelf break. We expect the cross-slope transport of shelf water mass properties to be highly variable, almost certainly actually intermittent, in both time and space.
The elements of the required program must include:
1. Bathymetric surveys of the chosen principal region of study, with resolution of better than 2 km (preferably Seabeam);
2. Hydrographic (CTD/LADCP) surveys at spacing finer than the Rossby radius (<5 km spacing), concentrating on the benthic plume and Slope Front, and including tracer chemistry for water age (CFCs, O2) and ice shelf water concentration (stable isotopes). Several sufficiently rapid transects across the plume to limit aliasing of the transect, using XCTD and hull ADCP;
3. Moorings to augment the hydrographic surveys, provide the spatial and temporal scales of variability of the ambient flow, and allow extrapolation of hydrographic data to one annual cycle;
4. Lagrangian methods (floats, profilers and injection of tracers) may be used to conduct measurements in and near the plume that would allow us to develop at least a qualitative understanding of the dominant processes responsible for mixing of the sinking plume with ambient waters.
5. Analytical and numerical process modeling, plume flow over rough/variable topography, role of nonlinear equation of state);
6. Mesoscale modeling to help map the small-domain measurement program to the circum-Antarctic level via improved parameterizations tied to the process models.
NSF Grant Number: OPP99-10317
Prepared by Arnold L. Gordon (chairman), Glen Gawarkiewicz, Alex Orsi and Laurie Padman 21 October 1999
Slope Front Workshop
27-29 September 1999
Monday, 27 September [9 AM to 5 PM]
Gordon, Arnold: workshop
Review of Summary Statement
II Science Talks: Presentations up to 20 minutes long, followed by discussion of up to 10 minutes. Presentations should focus on: water mass exchange between the shelf and deep water regimes; slope front structure and variability and its role in shelf/deep water exchange; and descending dense waters on the slope and within canyons. The presentations can discuss observational and modeling information at any site along the Antarctic margins. However, emphasis on the Ross Sea or on the relevance of results to the Ross Sea region, would be most helpful.
Orsi, Alex: Observations
Whitworth, Tom: Observations
Jacobs, Stan: Observations
Bergamasco, Andrea: "Results of the CLIMA-PNRA Cruises in the Ross Sea.
Padman, Laurie: "Hydrography over the continental shelf and slope of the southwestern Weddell Sea during the 1997/98 austral summer"
Comiso, Joey: "Variability of the Sea Ice Cover"
Gawarkiewicz, Glen: Modeling of exchange across the continental slope
Akitomo, Kazunori: "Model study of descending current and dense water transport along continental slope"
Nof, Doran: "The export of upper water from the Southern Ocean and the associated upwelling"
Hellmer, Hartmut: Weddell Margins
Padman, Laurie: "High-frequency (tides and shelf waves) variability at the Weddell Sea shelf/slope front".
Robertson, Robin: "Effects of the critical latitude & fronts on internal tides in the southern Weddell Sea"
Muench, Robin: "Turbulence and mixing related issues"
Monday, evening: Lamont's 50th anniversary party will be held in Casa Italiana (affiliated with Columbia University; Amsterdam St and 118th st NYC) from 6 to 9 PM. Workshop participant's can attend, if they wish.
Tuesday, 28 September [9 AM to 5 PM]
III Review of related projects already underway or planned:
Zambianchi, Enrico: Italian Ross Sea Program Plans
Jacobs Stan: NBP cruise Feb-Mar 2000
Jenkins, Adrian: BAS
IV Revisit the Summary Statement (June 30, 1999 version was distributed earlier):
A group discussion, chaired by the Steering Committee (Gordon, Orsi, Padman, Gawarkiewicz); objective: to consider and revise as necessary the document.
Wednesday, 29 September [9 AM to 5 PM]
V What's Needed? Discussion of how we might address the objectives expressed in the (revised) Summary Statement to advance understanding of the southern ocean shelf/deep water exchange; slope front, and descending dense waters through observational & modeling research. All are invited to participate but those named are encouraged to prepare and lead the discussion.
Models: what are outstanding questions, what obs are needed? Gawarkiewicz (lead discussion), Hellmer, Akitomo, Nof, Robertson
Sea Ice: Ackley, Comiso, Martinson
Moorings (current meters, ADCP, other): Whitworth, Orsi ,
Drifters: Hellmer, Martinson
Floats: Hebert, Prater, Visbeck
CTD/LADCP: Gordon, Jacobs, Visbeck
Tracers: Smethie, Schlosser
Purposeful Tracers: Ledwell, Schlosser
VI Conclusion & what next? Gordon