earth map with SAFDE location

The Sub-Antarctic Flux and Dynamics Experiment (SAFDE)

Douglas S. Luther and the SAFDE PIs*

*SAFDE PIs (alphabetically)


[This article appeared in WOCE Notes, October, 1997, 9(2), 8-12;
and in the International WOCE Newsletter, December, 1997, #29, 32-35.]


List of Figures

Figure 1. (112 kB) Locations of most of the recovered SAFDE instruments, overlaid on Smith and Sandwell (1994) topography. Triangles are inverted echo sounders; stars are horizontal electrometers; large (small) circles are full (partial) moorings; the diamond indicates coincident electrometer and partial mooring; and, the square is a pressure recorder. All locations shown yielded full two-year records, except the pressure gauge which had a one-year record.

Figure 2. (25 kB) Section of temperature along the WOCE SR3 line in March, 1993, truncated in latitude and depth to emphasize the SAF (isotherms in Deg. C). The locations of the horizontal electrometers (the middle row of instruments in Fig. 1 ) are shown to indicate the positioning of the SAFDE array relative to the SAF.

Figure 3. (34 kB) Zonal currents from the eastern mooring (easternmost circle in Fig. 1 ) at depths of 300 m, 600 m, 1000 m, and 2000 m. The 300 m current is the most energetic, then 600 m and so forth.

Figure 4. (13 kB) Coherence amplitude between zonal current measured at 2000 m by a moored current meter and conductivity-weighted, vertically-averaged zonal current measured by a horizontal electrometer near the mooring (both instruments are indicated by the diamond in Fig. 1 ). The 95% level of no significance is provided.

Figure 5. (29 kB) Conductivity-weighted, vertically-averaged horizontal water speed measured by the five HEMs nearest the center of the SAFDE array (indicated by the 4 stars surrounding the diamond in Fig. 1 ).


Introduction

WOCE initiatives have resulted in a large increase in the number of direct measurements of the Antarctic Circumpolar Current (ACC) and its associated weaker circulation systems. There are now many more hydrographic sections, XBT sections, drifter and float tracks, and moored current and temperature measurements than were available before WOCE began (WOCE, 1997). Despite these in situ observations and the accumulation of satellite sensor measurements of sea height and sea surface temperature, the ~25,000 km long ACC remains one of the most poorly observed ocean currents.

While another call for more in situ observations of the ACC is easy to make, the reality is that logistical and cost barriers are so high that observations in the Southern Ocean will always be sparse in time and space. Advances in our knowledge of ACC dynamics must come through a synergy of models and observations. What this means for observations is that they must focus on measuring quantities, such as non-geostrophic terms of the momentum equations and terms of the vorticity equation, that can provide better discrimination of differing model assumptions and parameterizations.

The Sub-Antarctic Flux and Dynamics Experiment (SAFDE) was designed with this principle in mind, to achieve observations of the ACC south of Tasmania that would permit direct evaluation of the momentum, energy and vorticity budgets. That these budgets differ in different sectors of the Southern Ocean is as obvious as the differing bottom topographies that exert such a great influence on the ACC. The assumption of zonal invariance of the dynamics of the ACC, occasionally necessary for the development of basic dynamical concepts, must certainly be rejected in detail (see, for instance, the analysis of FRAM output by Wells and de Cuevas, 1995). One way that limited area, high- resolution observation programs such as SAFDE can be employed in the study of the ACC's dynamics over larger regions is through comparison with, and validation of, conclusions based on satellite-derived sea surface heights; and this certainly is one of our goals. But perhaps the greatest value will be achieved when the high resolution observations are used to provide the kind of information that is most useful for discriminating the validity of differing model dynamics. Models validated with such observations would then be expected to more accurately simulate the flow and variability in other locations of the Southern Ocean.

SAFDE observations alone are insufficient for satisfactory model validation, but prudent experiments like SAFDE, as well as thoughtful analyses of surface drifter velocities and altimeter-derived velocity components at cross-over points, will take us a long way toward that goal. A good example of a comparison of higher moment statistics derived both from a numerical model of the Southern Ocean and from various altimeter, drifter and current meter datasets can be found in Wilkin and Morrow (1994).

SAFDE has achieved multi-year observations of currents and temperatures in both a small current meter mooring array with a diameter of ~70 km, and along a SSW-NNE section perpendicular to the expected mean axis of the ACC at the Sub-Antarctic Front (SAF). The experiment lasted two years, from April, 1995, through March, 1997. The measured variables are coherent horizontally and vertically in broad, sub-inertial frequency bands. The rarity of such horizontally coherent observations, required for estimating momentum budgets, etc., is revealed by noting that when SAFDE was proposed six years ago only three locations in the ACC had been sampled with moored current meters for as long as one year, and only one location for as long as two years, with the horizontally coherent records all being no more than about a year long.

Array Description

Figure 1 displays the SAFDE array overlaid on Smith and Sandwell (1994) topography. The current associated with the SAF, containing the large majority of the ACC transport at this longitude, flows from the west at about 49.5S on the north side of the Southeast Indian Ridge. After the current crosses the 140E fracture zone into deeper water its mean position tends toward the east-southeast (e.g., Gille, 1994) though it executes large meanders as predicted by McCartney (1976). The SAFDE array was oriented along a SSW-NNE line overlapping the WOCE SR3 repeat hydrography track that was designed to be perpendicular to the approximate mean axis of the SAF. Figure 2 shows the locations of the horizontal electrometers (the middle row of symbols in Fig. 1 ) relative to the position of the SAF, as observed in March, 1993, by S. Rintoul along the WOCE SR3 line.

At the center of the array, 9 sub-surface, nearly full depth moorings were deployed as a local dynamics array (LDA). Of these, 4 complete moorings and 3 partial moorings were recovered. Three of the complete moorings (indicated by circles that are overlaid by IES triangles in Fig. 1 ) were situated along the SSW-NNE SR3 track, and the last complete mooring was to the east. Figure 3 displays the zonal current from the four current meter depths (300 m, 600 m, 1000 m and 2000 m) on the easternmost mooring (easternmost circle in Fig. 1 ). The high vertical coherence characteristic of currents at other locations along the ACC (e.g., Sciremammano et al., 1980; Bryden and Heath, 1985) is clearly evident here as well.

The three full moorings along SR3 are at the same locations as three moorings deployed by S. Rintoul from March, 1993, through January, 1995. This earlier experiment had a fourth mooring off to the west. Therefore, the combined experiments have yielded compact arrays of four moorings at nearly identical locations for a duration of almost four years, with measurements of currents and temperatures at 4 depths (at least).

The three partial moorings recovered in March, 1997, yielded 2000 m current and temperature records at the locations of the diamond and two small circles in Fig. 1 , two to the west of the line of 3 full moorings and one to the east. Therefore, at the 2000 m level, there are 7 two-year records of currents and temperatures.

The most novel aspect of SAFDE was the deployment of a suite of horizontal electrometers (HEMs) and inverted echo sounders (IESs) to obtain time series of the vertically-averaged horizontal water velocity, of the temperature structure, and of the dynamic height structure. Seventeen electrometers were deployed with fifteen recovered, yielding 13 complete records at the locations shown in Fig. 1 . The HEMs measure the horizontal electric fields (HEFs) which are theoretically related to the conductivity-weighted, vertically-averaged horizontal water velocity (Sanford, 1971; Chave and Luther, 1990). For those readers unfamiliar with the published empirical demonstrations of the relationship between HEFs and water velocity (e.g., Luther et al., 1991), Figure 4 displays the coherence between zonal current from a 2000 m current meter and the HEF-derived vertically-averaged zonal current from a nearby HEM (the instruments are indicated by the diamond in Fig. 1 ). The coherence is high at periods longer than 4 days, and would be higher if the single current meter record could be replaced with vertically-averaged current measured by a full mooring. The high coherence in Fig. 4 is actually another demonstration of the high vertical coherence of fluctuations in the ACC. The low coherence at periods shorter than 4 days is due to the existence of strong electric fields generated in the ionosphere; see Luther et al., 1991, for a simple demonstration of the spectral structure of the ionospheric fields.

The conductivity weighting noted above means that the HEF is not a pure measure of vertically-averaged water velocity. Where both the vertical shear of the horizontal currents is large and the conductivity varies strongly with depth, as in the Gulf Stream for instance, the conductivity correction can be up to 25%. At mid- to high-latitudes, conductivity varies weakly with depth, resulting in a small correction (Chave and Luther, 1990). In the SAF we expect the conductivity correction to be only a few percent. Even so, where the IES data are available the conductivity correction can be directly estimated and removed from the HEF measurement to yield vertically-averaged water velocity.

The IESs measure acoustic travel times from the seafloor to the surface and back. Watts and Rossby (1977) showed that variations in the travel times are dominated by the largest vertical scales (lowest baroclinic mode) of temperature perturbations. Where there is high vertical coherence of the temperature perturbations, irrespective of modal content, each travel time can define almost uniquely a particular profile of temperature. If, in addition, dynamic height variability is dominated by temperature fluctuations, each travel time can also define almost uniquely a particular profile of dynamic height. To make this analysis work there must be high vertical coherence of velocity and temperature fluctuations, a condition known to be met in the ACC as noted above, and there must be sufficient CTD data to fully map the range of measured travel times onto particular profiles of temperature or dynamic height, a condition met south of Tasmania due to the many repeat hydrographic sections conducted by S. Rintoul and collaborators over the past six years.

Of the eighteen IESs deployed in SAFDE, all were recovered and all yielded complete records. Where the IESs surround the HEMs, dynamic heights from the IESs will be used to get top-to-bottom shears that will be combined with vertically-averaged water velocities from the HEFs (after applying the IES-derived conductivity correction) to yield absolute profiles of water velocity as a function of time over a 225 km span of the SAF. These measurements will have many uses, such as providing sections of momentum and heat fluxes, providing information about frontal location and structure for proper analysis of the LDA mooring data, and validating the interpretation of satellite measurements of kinetic energy and momentum flux.

Two of the HEM-IES absolute velocity profile time series exist within the LDA, at locations where moorings were either lost or only partially recovered. Sub-sampling these profiles at the depths of the current meters on the 4 complete moorings of the LDA will produce a full two-dimensional array of six observation sites at 4 depths, thus permitting calculation of higher-order momentum, energy and vorticity equation terms that require estimation of horizontal gradients and divergences of fluxes, etc.

To demonstrate that the horizontal spacing within the array has yielded good horizontal coherence, a requirement for calculation of momentum budget terms, etc., Figure 5 displays vertically-averaged water speeds derived from the five central HEMs (centered on the diamond in Fig. 1 ). Besides the obvious coherence, the data show remarkable variability, with speeds occasionally exceeding 30 cm/s. The IES records are even more coherent.

At low frequencies we expect to be able to use the entire HEM array from 48.7S to 52.8S to calculate the total transport and its variability through this ~450 km section, despite the gaps due to lost data or instruments ( Fig. 1 ). The relationship of this barotropic transport to the baroclinic transport observed along SR3, and the effect of the barotropic variability on property fluxes through this section will be examined.


References

Bryden, H.L. and R.A. Heath. 1985. Energetic eddies at the northern edge of the Antarctic Circumpolar Current in the Southwest Pacific. Prog. Oceanogr., 14, 65-87.

Chave, A.D. and D.S. Luther. 1990. Low-frequency, motionally induced electromagnetic fields in the ocean, 1, Theory. J. Geophys. Res., 95, 7185-7200

Gille, S.T.. 1994. Mean sea surface height of the Antarctic Circumpolar Current from Geosat data: Method and application. J. Geophys. Res., 99, 18,255-18,273.

Luther, D.S., J.H. Filloux and A.D. Chave. 1991. Low-frequency, motionally induced electromagnetic fields in the ocean, 2, Electric field and Eulerian current comparison from BEMPEX. J. Geophys. Res., 96, 12,797-12,814.

McCartney, M.S.. 1976. The interaction of zonal currents with topography with applications to the Southern Ocean. Deep-Sea Res., 23, 413-427.

Sanford, T.B. 1971. Motionally-induced electric and magnetic fields in the sea. J. Geophys. Res., 76, 3476-3492.

Sciremammano, F. Jr., R.D. Pillsbury, W.D. Nowlin, Jr., and T. Whitworth, III. 1980. Spatial scales of temperature and flow in Drake Passage. J. Geophys. Res., 85, 4015-4028.

Smith, W. H. F. and D. T. Sandwell. 1994. Bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry. J. Geophys. Res., 99, 21803-21824.

Watts, D.R. and H.T. Rossby. 1977. Measuring dynamic heights with inverted echo sounders: Results from MODE. J. Phys. Oceanogr., 7, 345-358.

Wells, N.C. and B.A. de Cuevas, 1995: Depth-integrated vorticity budget of the Southern Ocean from a General Circulation Model, J. Phys. Oceanogr., 25, 2569-2582.

Wilkin, J.L. and R.A. Morrow. 1994. Eddy kinetic energy and momentum flux in the Southern Ocean: Comparison of a global eddy-resolving model with altimeter, drifter, and current-meter data. J. Geophys. Res., 99, 7903-7916.

WOCE. 1997. WOCE Data Guide 1997. WOCE Report No. 150/97, March 1997, WOCE International Project Office, Southampton, UK.


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