Internal Tides in the Weddell-Scotia Confluence Region, Antarctica Susan L. Howard, Laurence Padman, and Robin D. Muench Introduction Recent observations, backed up by 3-D model simulations of tides, highlight the role of internal tide generation over ridges as a source of velocity variability and ocean mixing. Most research is on low- and mid-latitude ridges. Here, we study this process in a high-latitude environment where stratification is significantly weaker than elsewhere. We focus on the Weddell-Scotia Confluence along the South Scotia Ridge (SSR), which was the focus of the Deep Ocean Ventilation Through Antarctic Intermediate Layers (DOVETAIL) experiment [Muench and Hellmer, 2002]. Water mass mixing and air-sea interaction in the northern Weddell Sea and along the SSR influence the properties of the dense water escaping from the Weddell Sea into the World Ocean. We propose that internal tides generated at the SSR could explain some of the observed velocity shear and mixing during the DOVETAIL field program [Muench et al., 2002]. Our Hypotheses Internal tides are generated through interaction between barotropic tidal currents and the irregular and steep seafloor topography of the SSR. (1)These internal tides are sufficiently energetic to affect winter sea ice properties through shear, strain, and divergence. (2)Locally high mixing in the main pycnocline might result from internal wave shear. Model Setup Currents: M 2 major axes References Merrifield et al., 2001: The generation of internal tides at the Hawaiian Ridge. Geophys. Res. Lett., 28, Morozov, E.G., 1995: Semidiurnal internal wave global field. Deep-Sea Res., 42, Muench, R.D., & H. Hellmer, 2002: The international DOVETAIL program. Deep-Sea Res. II, 48, Muench et al., 2002: Upper ocean diapycnal mixing in the northwestern Weddell Sea. Deep-Sea Res. II, 48, Naveira Garabato, A.C., et al., 2002: On the export of Antarctic Bottom Water from the Weddell Sea. Deep-Sea Res. II, 48, Padman, L., and C. Kottmeier, 2000: High-frequency ice motion and divergence in the Weddell Sea. J. Geophys. Res., 105 (C2), Polzin, K.L., et al., 1997: Spatial variability of turbulent mixing in the abyssal ocean. Science, 276, Acknowledgements The work reported here was carried out with support from National Science Foundation grants OPP and OPP to Earth & Space Research, and is a contribution to the international DOVETAIL program. Conclusions Internal tides are generated at the South Scotia Ridge and propagate south into the Powell Basin and northern Weddell Sea, and north into the Scotia Sea. Energy flux is a factor of ~3 lower than we get using the Morozov [1995] ridge generation model. Vertical displacements due to baroclinic waves can exceed 100 m peak-to-peak. The region in which surface tidal currents are significant is much more extensive around the ridge than is shown by purely barotropic models. Tidal shear, strain and divergence acting on ice are much larger when baroclinicity is included, relative to barotropic- only models. The model supports the patchiness of mixing observed in the Powell Basin during DOVETAIL [Muench et al., 2002]. The model is set up as follows: Resolution: 1/16 o Longitude x 1/25 o Latitude (mean spacing ~3-4 km); 41 sigma levels. Bathymetry: Modified Etopo5. Forcing: M 2 tidal forcing only, normal flow at open boundaries. OBC: On all open boundaries, a flow relaxation scheme in a 10-point sponge layer is applied to the baroclinic u and v velocities as well as T and S. Run time: 15 days. The edge of the sponge layer is shown on maps by the dashed line. Future Work The results reported here are preliminary. To fully understand baroclinic tides in this region, we need to make new runs with: Updated (Smith and Sandwell) bathymetry; Realistic, varying stratification, including seasonal variation in the upper ocean; Forcing from multiple tidal constituents simultaneously; Increased resolution, from our ~4 km to 1-2 km. Model runs were made using POM, which is a 3-D, primitive equation model. The simulations are carried out in a similar way to Merrifield et al. [2001]. We consider only the M 2 tidal constituent, although other runs indicate a similar response for S 2. We have not yet simulated diurnal tides in this region. Our model domain is shown below. For more information, contact: Susan Howard Earth & Space Research (206) x15 South-to-North slice of the baroclinic component of northward velocity (v) for the transect shown in the figure to the left. Well away from the ridge system, wavelengths agree with ray tracing [Muench et al., 2002]. Near the ridge system, currents are complicated as multiple generation sites contribute to the modeled velocities. Little energy escapes into the northern Weddell Sea. Profile of σ θ used to initialize model runs. Most stratification is near the surface, between 100 and 300 m. Stratification at the depth of the ridges (>1000 m) is weak. Vertical displacement of isopycnals due to M 2 tides. Maximum values exceed 50 m, including in the strong stratification over the crest of the SSR. Major axis of (a) M 2 depth-averaged current, U BT (M 2 ) and (b) total M 2 surface current, U S (M 2 ). Maximum values are much higher when baroclinicity is included, and the area affected by tides expands far out from the ridge. Transect of velocity Divergence Instantaneous divergence of (a) M 2 depth-averaged current, U BT (M 2 ) and (b) M 2 surface current, U S (M 2 ). Area shown is indicated by boxes on plot to the left. Maximum values are much higher when baroclinicity is included, because the spatial scales of the internal waves are much less than the barotropic variability. Surface Current Fields Model density profile Energy Flux from the barotropic to the baroclinic tide (M 2 only) Figures show energy flux magnitude (color scale) and direction (arrows). Most generation occurs across the SSR north of Powell Basin. Little generation occurs at the continental slopes surrounding the South Orkney Plateau and Antarctic Peninsula, supporting the prevailing view that ridges rather than continental slopes contribute most internal tide energy. Maximum fluxes are ~200 W m -2, and the ridge average is a factor of ~3 smaller than Morozov [1995] predictions. 68 o S 58 o S Transect of vertical displacement Weddell Powell Scotia Sea Basin Sea SSR Weddell Powell Scotia Sea Basin Sea SSR (a) (b) (a) (b) Energy Conversion and Depth-Dependence of Baroclinic Velocity and Displacements Mixing Upward heat flux through the pycnocline In Muench et al. [2002] we estimated mean upward heat fluxes of ~4 W m -2 based on the ridge generation of Morozov [1995] (500 W m -1 for M 2 only), and an arbitrary 1000 km decay scale. The present model suggests that much less energy is generated along the ridge than this (~100 W m -1 ), but most of this energy remains trapped within ~200 km of the ridge, hence the predicted mean heat flux is about the same, ~4 W m -2 near the ridge, with peak fluxes of >20 W m -2. The patchiness predicted in Powell Basin by the POM model is consistent with DOVETAIL observations. Additional mixing sources? The SSR topography is extremely rough. Our present model bathymetry is relatively smooth, interpolated from our CATS grid (1/4 o x 1/12 o : ~10 km) which is based on ETOPO-5. Higher resolution bathymetry (e.g., Smith and Sandwell) may provide much more internal tide generation, perhaps of high modes [see Polzin et al., 1997]. Increased mixing associated with such baroclinic waves will dilute the dense water outflows through deep passages through the SSR [Naveira Garabato et al., 2002], one of the paths for the Weddell Sea contribution to the Global Ocean.