4-18 September, 1996 
Jim Ledwell

Our dye/microstructure cruise on Oceanus, 4 - 18 September, is finished and was a success in spite of threats from the weather. It turns out that we had mostly fog with calm seas, Hurricane Hortense passing to the east of us. The worst weather came on the last day, but we were able to finish the planned final adcp/ctd line.

There were 17 in the science party, including:
Neil Oakey, BIO
Blair Greenan, BIO
Tim Duda, WHOI
Harvey Seim, Skidaway

The cruise started with the recovery of a wayward "Wave Rider" buoy from the CMO central mooring group and an adcp/ctd line from the 400 meter isobath to the 40 meter isobath, with ctd stations every 5 km. This was a reoccupation of a line occupied by Steve Lentz earlier in the summer, and passes by the offshore, central, and inshore mooring sites. We found that Hurricane Edouard, which had come through the area on 9/2, had mixed the water on the shelf, creating very low stratification throughout the water column at some sites inshore of the 70-meter isobath, and SST of around 16 C generally on the shelf (see the avhrr image for 9/3/96). Our work was performed between the 65 and 75 meter isobaths where the hydrography was more seasonable, except for relatively weak stratification in the upper pycnocline and cool surface temperatures of 16 to 17 C. The salinity gradients were weak in this region, less than about 0.5 ppt(psu) over the water column, and stable with respect to double diffusion.

Two dye release experiments were performed, the first with Rhodamine-WT released on sigma-theta = 24.063 at around 32 meters depth, N = 5 cph, and the other with fluorescein released on sigma-theta = 24.300 at around 48 meters depth, N = 10 cph. Both releases were performed near the along-shore mooring at around 70-19 W, 40-28 N, and both patches drifted roughly along isobaths past the central mooring array at a mean speed of 10 to 15 cm/s. 940 EPSONDE profiles were obtained during the cruise, many of them along the path of the dye patch.

The first experiment lasted for about 84 hours, with a complete survey performed at the beginning and the end, and a partial survey in the middle. Tracking by integration of the 300 khz broad band adcp record of flow under the ship was remarkably accurate, and at least as reliable as the 6 drogues that had been released with the patch. There was a clear growth in the diapycnal spread of the tracer during this experiment. Quantitative estimates of the diapycnal diffusivity must await final analysis, but preliminary estimates indicate something between 0.1 and 1 cm^2/s. The buoyancy frequency varied a great deal during the experiment, but averaged around 5 cph.

The second experiment lasted for approximately 100 hours, with 3 complete surveys performed. Fluorescein proved to be a superior tracer to rhodamine because of far less background signal. The fluorescein was released deep enough that, even if the sun had been out, photolysis would not have been significant. Again, the diapycnal spreading of the patch will be readily quantifiable, although the mixing was exceedingly weak. Preliminary estimates put K_z somewhere between 0.01 and 0.2 cm^2/s, depending on the period examined and whether the upper half or lower half of the distribution is examined. Mixing downward was greater than mixing upward. The buoyancy frequency for this experiment was around 10 cph.

The results of the EPSONDE surveys and the dye results seem to be consistent with one another, although we are at present only at the stage of comparing orders of magnitude.

The broadband adcp performed well, except for bottom tracking. The shear data and unreferenced velocity were quite good, but spikes appeared in processed absolute velocity. The signal was degraded whenever anything was lowered from the starboard boom on Oceanus. We look forward to evaluating the shear statistics at scales of 2 meters. The ctd data obtained during dye sampling as tow-yo transects with 30 to 50-meter depth aperture should enable us to also evaluate statistics for the gradient Richardson number at this scale.

© Woods Hole Oceanographic Institution