|Robin Muench||Susan Howard||Laurie Padman|
DOVETAIL is an international oceanographic research program whose main goal is to understand the physical processes in the Weddell Scotia Confluence (WSC) region sufficiently to quantify the ventilation if the World Ocean achieved by the Weddell Sea water masses. There are four related objectives in this program:
In August 1997, Temperature (T), Salinity (S), and Dissolved
Oxygen (DO) data, along with upper ocean vertical current profiles, were
obtained in the area extending northeastward from the Antarctic Peninsula across
the South Orkney Plateau (see figure below). This work comprised the US field
component of the international DOVETAIL program which was funded by the
NSF Office of Polar Programs.
The DOVETAIL program is the third in a sequence of integrated field and modeling programs which were initiated in 1992 and which have been coordinated by the international Antarctic Zone (iAnZone) group and carried out in the Weddell Sea. The first two programs have focussed on processes associated with ocean ventilation in the polar waters. DOVETAIL builds upon the results from these two preceding programs by focusing on the escape of recently ventilated deep water from the Weddell Sea into the Global Ocean. In this way, the scientists involved hope to better define and understand the role of Antarctic waters and processes in the global ocean and climate system.
The primary results from this multi-institutional program as of 2000 were summarized by a series of papers published in a special DOVETAIL volume of Deep-Sea Research Part II (Topical Studies in Oceanography), Volume 49(21) (Muench and Hellmer, 2002) (see publication list below). R. D. Muench of ESR was chief Guest Editor for this volume.
At Earth and Space Research,
our scientific contributions to DOVETAIL have covered two main areas:
Topic (1) required detailed analyses of data (ADCP and CTD) collected during the fieldwork portion of DOVETAIL in 1998, analyses of German current meter records acquired in the DOVETAIL region prior to this fieldwork, and improvements to our barotropic tides model to better match the observed tidal velocity fields. These studies led us to believe that the generation of baroclinic tides ("internal waves") by the flow of the barotropic tide over the South Scotia Ridge could provide a significant source of energy for mixing in the DOVETAIL region, thus spurring Topic (2), numerical modeling of baroclinic tide generation. In addition to these primary efforts, research continues on describing and assessing the dynamics of the coastal and shelf-break frontal systems in the WSC region (Heywood et al., 2004).
Vertical profiles of ocean temperature, salinity, dissolved oxygen concentration and currents were obtained during austral winter 1997 from a region extending from the northeastern Antarctic Peninsula to the South Orkney Plateau in the northern Weddell Sea. From these data we estimate the spatial variation of shear-driven turbulent mixing and the mean diapycnal diffusivity (Kz) in the main pycnocline. We conclude that the mean upper ocean heat flux in the northwestern Weddell Sea is in the range 2-10 W m-2. The ocean tide provides a significant fraction of the oceanic kinetic energy in the region: tidal current speeds at the shelf break can approach 50 cm s-1 compared with mean flows of ~5-10 cm s-1. We show that the conversion rate of barotropic to baroclinic semidiurnal tidal energy along the South Scotia Ridge, as a pathway for energy flux to the turbulent microscale, is consistent with the data-based estimates of Kz. Baroclinic tides from the ridge can penetrate into the northern Weddell Sea and may explain observed significant velocity shears in the central Powell Basin, hundreds of kilometers from significant topographic features.
This work has been reported in peer-reviewed publications by
Muench et al. (2002) and by Muench's contribution to Matano
et al. (2002). Our results are also summarized in the online version of
the poster we presented at the XXV
General Assembly of the European Geophysical Society meeting in held in
Nice, April 24-28, 2000:
The field data collected under DOVETAIL auspices have also allowed us to describe, in considerably greater detail than previously, the shelf currents and shelf break fronts in the Weddell-Scotia Confluence (WSC) region (Muench contribution to Heywood et al., 2004). The Antarctic circumpolar shelf-break front is a major oceanographic feature that likely plays a significant role in the seaward transport of dense shelf waters and in the redistribution of low-salinity near-surface layers. This frontal system is the topic of ongoing research that is planned to be developed into a major international project.
We use the Princeton Ocean Model (POM), a 3-D primitive equation model that has been successfully applied to other tide modeling, notably the Hawaiian Mid-Ocean Ridge (M. Merrifield, P. Holloway, and collaborators), to study the baroclinic tidal response to barotropic tidal flow over the South Scotia Ridge. So far, we have run the model for the semidiurnal tides, M2 and S2, which we expect will provide most of the baroclinic tidal signal in the DOVETAIL region. We currently run the model at 3-4 km resolution using Smith and Sandwell bathymetry. The model has only been run for idealized stratification (horizontally homogeneous throughout the domain): we are presently constructing laterally varying stratification based on the World Ocean Atlas. The seasonal variation in surface layer properties, associated with radiation changes and sea-ice melt/freeze is also worth exploring, since these seasonal changes in upper-ocean density can be as large as the total full-depth winter stratification.
The conclusions from this modeling exercise are as follows:
Figure 1: Major axis of (top) M2 depth-averaged current, UBT(M2) and (bottom) total M2 surface current, USURF(M2). Maximum values are much higher when baroclinicity is included, and the area affected by tides expands far out from the ridge. The region encompassed by the white dashed line is roughly the DOVETAIL study area, and is the focus for testing model sensitivity to grid resolution and other model parameters.
Figure 2: (a) Vertical profile of at-rest potential density (σ) used to initialize POM runs for DOVETAIL-area baroclinic tides. (b) "Snapshot" transect of the northward component of baroclinic velocity for the M2 tide. Note the complexity of the upper-ocean currents, and their dependence on the location of the sharp pycnocline centered near 200 m. (c) "Snapshot" of the northward component of barotropic velocity (blue) and surface baroclinic velocity (red) along a north-south line running across the South Scotia Ridge (SSR) through Powell Basin. (d) "Snapshot" of the divergence of the barotropic velocity field (blue) and surface baroclinic velocity field (red) along a north-south line running across the SSR through Powell Basin. The RMS divergence for the baroclinic tide is about a factor of 10 greater than for the barotropic tide, indicating significant potential to modify the sea-ice concentration and properties such as mean thickness, thickness distribution, and roughness.
More details are available in a downloadable PowerPoint poster.
While this latter study was driven by our interest in mixing in the DOVETAIL region, predictions of mixing rates based on this modeling depends on our faith in the parameterization of mixing in POM. At this time, the principal results relating to mixing are that: the patchiness of mixing observed in the DOVETAIL cruise data is consistent with the radiation of internal tides from the South Scotia Ridge; and the mixing rates estimated from cruise CTD/ADCP data are consistent with a tidal source for the energy driving most observed upper-ocean mixing. The patchiness itself is of interest, also, since it impacts the conversion of turbulent parameters to a measure of effective mixing.
One of the less anticipated outcomes from this study is the apparently critical role of baroclinicity on upper-ocean divergence fields. Figure 2 illustrates how the generation of internal tides over Scotia Ridge creates surface divergence. This divergence is important because the upper ocean velocity provides a major component of total stress divergence on sea-ice: Padman and Kottmeier ("PK2000"; JGR, 2000) showed that significant additional mean open-water ("lead") fraction could be created by tidal divergence. Even though tidal stress periodically expands and compresses the ice pack, the net effect of the "ice accordion" is an increase in mean lead fraction, and a consequent increase in area-mean ocean-to-atmosphere heat loss in winter (plus additional ice growth). PK2000 showed, for the southern and western Weddell Sea, that ice divergence based on satellite-tracking of ice-mounted drifters was consistent with our barotropic tides model for the Weddell Sea. In the DOVETAIL region, baroclinic tides are much stronger than most regions further south, and so the RMS divergence of the ocean-applied tidal stress on the ice is expected to be much larger (see Figure 2). The impact of surface convergence/divergence due to baroclinic tidal currents on the ice cover in the Dovetail region cannot be overstated. The periodic creation of leads through such convergence allows, during the austral winter, dense water formation through surface freezing. Results shown by Muench et al. (2002) show that densification near the Antarctic Peninsula had led to convection to the seafloor in the near-coastal region strongly impacted by tidal currents. This dense water is then available as a contribution to Weddell Deep and Bottom Waters, and this process likely occurs at many sites surrounding Antarctica where the barotropic to baroclinic tidal current conversion process is active.
The exact impact of baroclinic tides on sea-ice concentration and properties can only be determined once sea-ice dynamical-thermodynamical models are improved to the point where they accurately reflect short time- and space-scale ice dynamics processes (including floe-floe interactions for different thickness categories). This is not yet possible, but points to a role of tides that may in many regions be more important than the tide's contribution to ocean mixing.
Gordon, A.L., M. Visbeck, and B. Huber, 2001: Export of Weddell Sea deep and bottom water, J. Geophys. Res., 106(C5), 9005-9017.
Gordon, A.L., M. Mensch, Z.Q Dong, W.M. Smethie Jr., and J. de Bettencourt., 2000: Deep and Bottom water of the Bransfield Strait eastern and central basins. J. Geophys. Res., 105(C5), 11337-11346.
von Gyldenfeldt, A.B., E. Fahrbach, M.A. García, and M. Schröder, 2002: Flow variability at the tip of the Antarctic Peninsula, Deep Sea Res. II, 49, 4743-4766.
Heywood, K.J., A.C. Naveira-Garabato, D.P. Stevens, and R.D. Muench, 2004: On the fate of the Antarctic Slope Front and the origin of the Weddell Front, J. Geophys. Res. , 109, C06021, doi:10.1029/2003JC002053.
Matano, R.P., A.L. Gordon, R.D. Muench, and E.D. Palma, 2002: A numerical study of the circulation in the northwestern Weddell Sea, Deep Sea Res. II, 49, 4827-4841.
Muench, R.D., and H.H. Hellmer, 2002: The international DOVETAIL program, Deep Sea Res. II, 49, 4711-4714.
Muench, R.D., L. Padman, S.L. Howard, and E. Fahrbach, 2002: Upper ocean diapycnal mixing in the northwestern Weddell Sea, Deep Sea Res. II, 49, 4843-4861.
Naveira Garabato, A.C., E.L. McDonagh, D.P. Stevens, K.J. Heywood, and R.J. Sanders, 2002: On the export of Antarctic Bottom Water from the Weddell Sea, Deep Sea Res. II, 49, 4715-4742.
Palma, E. D., and R. P. Matano, 2000: On the Implementation of open boundary conditions for a general circulation model: The three-dimensional case. J. Geophys. Res., 105, 8605-8627.
Piola, A. R., and R. P. Matano, 2001: The South Atlantic Western Boundary Currents Brazil/Falkland (Malvinas) Currents.in Encyclopedia of Ocean Sciences. J. M. Steele, S. A. Thorpe, and K. K. Turekian Eds. Academic Press. 1, 340-349.
Robertson, R., M. Visbeck, A.L. Gordon, and E. Fahrbach, 2002: Long-term temperature trends in the deep waters of the Weddell Sea, Deep Sea Res. II, 49, 4791-4806.
Schodlok, M.P., H.H. Hellmer, and A. Beckmann, 2002: On the transport, variability and origin of dense water masses crossing the South Scotia Ridge, Deep Sea Res. II, 49, 4807-4825.
Schröder, M., H.H. Hellmer, and J.M. Absy, 2002: On the near-bottom variability in the northwestern Weddell Sea, Deep Sea Res. II, 49, 4767-4790.
Smith, D.A., and J.M. Klinck, 2002: Water properties on the west Antarctic Peninsula continental shelf: a model study of effects of surface fluxes and sea ice, Deep Sea Res. II, 49, 4863-4886.
Padman, L., Tidal Effects on Sea Ice and Ocean-Air Interaction, presented at the Seventh Conference on Polar Meteorology and Oceanography and Joint Symposium on High-Latitude Climate Variations, May 2003, Hyannis, MA.
Howard, S.L., L. Padman, R.D. Muench, Internal Tides in the Weddell-Scotia Confluence Region, Antarctica, presented at AGU, Dec 2002.
Muench, R., S.L. Howard, L. Padman, Upper Ocean Mixing in the NW Weddell Sea During Winter, presented at EGS, April 2000.
Muench, R., S.L. Howard, and L. Padman, Mixing in a weakly stratified upper ocean: the Weddell-Scotia confluence region, presented at SOLAS, Feb 2000.
Howard, S.L., R.D. Muench, and L. Padman, Upper Ocean Stability and water mass formation in the northwestern Weddell Sea during winter, presented at Ocean Sciences, Jan 2000.