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 HypothesesInternal 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: M2 major axes

ReferencesMerrifield et al., 2001: The generation of internal tides at the Hawaiian Ridge. Geophys. Res. Lett., 28, 559-562.

Morozov, E.G., 1995: Semidiurnal internal wave global field. Deep-Sea Res., 42, 784-791.

Muench, R.D., & H. Hellmer, 2002: The international DOVETAIL program. Deep-Sea Res. II, 48, 4711-4714.

Muench et al., 2002: Upper ocean diapycnal mixing in the northwestern Weddell Sea. Deep-Sea Res. II, 48, 4843-4861.

Naveira Garabato, A.C., et al., 2002: On the export of Antarctic Bottom Water from the Weddell Sea. Deep-Sea Res. II, 48, 4715-4742.

Padman, L., and C. Kottmeier, 2000: High-frequency ice motion and divergence in the Weddell Sea. J. Geophys. Res., 105 (C2), 3379-3400.

Polzin, K.L., et al., 1997: Spatial variability of turbulent mixing in the abyssal ocean. Science, 276, 93-96.

AcknowledgementsThe work reported here was carried out with support from National Science Foundation grants OPP-9527667 and OPP-9615525 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 1/25o Latitude (mean spacing ~3-4 km); 41 sigma levels.

Bathymetry: Modified Etopo5.

Forcing: M2 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 WorkThe 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 M2

tidal constituent, although other runs indicate a similar

response for S2. 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

http://www.esr.org

howard@esr.org

(206) 726-0501 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 σθ 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 M2 tides. Maximum values exceed 50 m, including in the strong stratification over the crest of the SSR.

Major axis of (a) M2 depth-averaged current, UBT(M2) and (b) total M2 surface current, US(M2). 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) M2 depth-averaged current, UBT(M2) and (b) M2 surface current, US(M2). 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 (M2 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.

68oS 58oS

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

MixingUpward 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 M2 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

(1/4o x 1/12o: ~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.