This presentation summarizes data collected with vessel-mounted acoustic Doppler current profilers (ADCPs) on 5 cruises during the first year (2001) of Southern Ocean GLOBEC. This program took place on the western Antarctic Peninsula (WAP) continental shelf, focused on Marguerite Bay.

In this talk we will summarize the ADCP measurements, and review some preliminary results.

The configuration parameters were chosen based on optimal settings determined from previous cruises for the GOULD and PALMER ADCP systems. The ADCP on the GOULD is generally setup to use a 16 meter pulse length.  The pulse length is (approximately) the vertical distance over which the velocity is averaged when producing velocity vectors for each depth bin.  A 16 m pulse length and 8 m bin size produces a smooth velocity field but reduces the resolution.  We switched to an 8 meter pulse because we wanted the better resolution.

Here is a map of the locations of good ADCP measurements at 200 m, for all S. O. GLOBEC cruises in Year 1. Each of the five cruises is shown in different colors and the cruise key is on the map. The NBP-0103 cruise (blue) provided the most complete coverage.  Missing data arises on this cruise during strong storms. Ice also significantly interferes with data collection.  When ice is present, we only get good data when vessel is on station.  You can see this in the NBP-0104 data (shown in red).

For the remainder of this talk we will review major circulation features seen in the full ADCP data set, then investigate whether there is any indication of turbulent mixing. The latter study combines ADCP data with CTD data to see whether shear instabilities might be a common occurrence in the pycnocline in the WAP region.

Here we consider the ADCP record of currents at the continental shelf break. Most data (blue) come from a multi-day occupation of LMG-0104 Process Site #1. Vectors from 3 different cruises are shown in the above plot.  They are at a depth of ~ 205 m.  The summary of the data is: (2) Peak speeds are over the central slope. The current is at the edge of the Antarctic Circumpolar Current (ACC). This can be seen from comparisons of the ADCP records with hydrographic data from CTD.

Along with the large scale mean features, there is a lot of high-frequency variability in the data. This is a plot showing depth and time dependence of the east-component of velocity and shear magnitude from the Gould winter cruise, LMG-0106 (Process Site #3). Interpretation of the variability is really only possible when the vessel is stationary or only slowly moving for a significant time interval. In general, this only occurs during the Gould process sites. In this example, we see a strong semidiurnal oscillation. From checking its period, we can see that this oscillation is clearly inertial (period ~13 h), not tidal (periods of ~12.42 h and 12.00 h). The currents are up to 30 cm/s in the surface mixed layer, and weaker but still significant in the lower layer (stratification is shown in the left-hand panels).   From a moving platform like the Palmer survey cruises, oscillations this strong will mask all but the strongest mean flows. The high shear across the pycnocline near 110 m suggests to us that there is a potential for mixing to occur through the onset of dynamical instabilities.  We look at this in the next slides.

There is evidence of strong shear in much of the data, and we are interested in whether this shear is strong enough to initiate dynamical instabilities leading to vertical mixing to lead to higher vertical heat (and salt and nutrient) fluxes.  We use the Richardson number (Ri) calculated with ADCP and CTD data,  as a proxy for mixing.  It is basically a ratio of the Buoyancy Frequency to Shear^2. Formally, Ri<1/4 is the criterion for dynamic shear instability starting from an infinitesimal disturbance.  Ri values of 1 or less indicate the potential for instability (basically, wave-breaking) in a finite-amplitude disturbance such as one might get from high-frequency internal gravity waves. Because of ADCP resolution issues, any values less than 1 suggests that turbulence might also be possible through dynamic shear instability occurring on spatial scales smaller than the ADCP can resolve (~8 m).

This slide shows some of the steps we use to get our estimates of Ri. For a given CTD station, we average U and V during that time period to get an average U and V as functions of depth.  From this we can find the shear (top right).  Here,  you can see that there is a large amount of shear between 160 - 200 m. We then use the CTD data to calculate the buoyancy frequency (lower middle panel). We use the ration of N^2 to Shear^2 to estimate Ri (see previous slide).  Here you see that Ri is less than 1 in the area of large shear around 200 m.

In this plot, we zoomed in on the area of low Richardson  number from the previous slide. The Ri plot is shown on the right and the corresponding potential temperature (PT) profile is on the left.  One interesting thing to us is near 200 m, where the PT profile shows inversions. We think this might indicate that there is turbulence associated with the large shear and suggested by the low Ri.

This is the plot we showed earlier, but with Ri next to the time-depth transects. Ri at the pycnocline near 120 m is less than 1, and so we think mixing might be possible. Once we have a Ri, we can estimate diffusivities and then heat fluxes, using parameterizations such as Pacanowski and Philander.  We find that the region of low Ri produces an upward heat flux of 25 W m-2.

Finally, we look at tides as another source of high-frequency variability in the ADCP data.  For the WAP region, the strongest predicted tidal currents are diurnal topographic vorticity waves along the shelf break. Part of our task is to improve the tidal models of this region, but we need the revised bathymetry and data from the moorings before we can do this. This plot shows the RMS diurnal-band velocity for WHOI surface drifter #4.  Dick Limeburner provided these data.  The left hand panel codes this value by size of the dot, and gives a value once per day.  It only shows points where there are enough ARGOS position data to do a good fit. The upper right panel shows U and V components for the buoy motion.  You can see that peak currents are about 15 cm/s.  The lower right panel shows water depth under the buoy track, interpolated from our tide model grid (CATS01.02). The main point of this figure is to show that there are strong currents associated with tides in some regions.  So, if we want to use the ADCP data to look at mean circulation over the WAP region, we need to think about possibly large signals from near-inertial oscillations and tides.

The next steps in ADCP analysis and interpretation are shown here.  Additional data sets may come from international GLOBEC partners working in the region (Germany and UK), and from US “LTER” cruises.  All present ADCP data sets require further editing.  Once moorings are recovered and analyzed, we will be in a position to improve our tidal model, with the goal of creating a detided ADCP data set.  We note, however, that we presently know of no way to remove the inertial variability, and so need to keep this in mind when interpreting sparse ADCP data from the vessels. Finally, we will be extending our studies of mixing, using Ri as a proxy, to the entire survey area, in an attempt to identify regions that are particularly prone to turbulent upward fluxes of heat, salt and nutrients into the surface mixed layer and up to the sea ice and/or atmosphere.

Eric Firing, Jules Hummon, and Teri Chereskin are funded by NSF-OPP to routinely acquire and edit ADCP data from the Gould and Palmer.  Their work, and additional efforts made on our behalf to edit the data and to improve the ship ADCP systems, has played a major role in the quality of the data sets acquired during S. O. GLOBEC.