Jim Ledwell

Senior Scientist
Applied Ocean Physics & Engineering
Woods Hole Oceanographic Institution


Attached is my favorite graph from my work and one that has found it's way into text books and papers by others. It is a picture of a tracer patch in the eastern North Atlantic, in the Canary Basin, at various stages. The lines to the right marked "INJ" are tracks along which Oceanus injected tracer (139 kg of sulfur hexafluoride) on a surface of constant neutral density at around 300 m depth in May 1992. The contour map just to the west of the injection streaks shows the tracer patch, as found during the weeks after the release by RRS Charles Darwin. The little square marked 'CM' is a current meter mooring deployed by Bob Weller as part of the ONR Subduction Experiment. The long sinuous streaks marked Oct and Nov were sampled by Oceanus on painstaking 30-day and 25-day back to back cruises in October and November of 1992. The dashed lines in the figure show where we towed a cumbersome array of samplers through the water at over 2 knots for 10 hours at a time to try to find the tracer in October. The CTD wire was led astern and through the A-Frame, and the CTD winch was automated to keep the center of the array (and also the injection sled in May) on the target density surface. Oceanus captains (Mike Palmieri and Paul Howland), chief engineer (Buzz McLauchlin) and chief mate (Larry Bearse) were totally cooperative in facilitating this complex operation, as usual. During the cruise with all the towing the brake on the CTD winch gave trouble. After several ingenious fixes, it finally failed completely, and Buzz made a hand brake from scratch operated by one of the science party on the main deck with a line to the new creation - it worked fine. In November we could not tow the sampling array because of other work with microstructure profiling by Chief Scientist Neil Oakey, and deployments of Tim Duda's Cartesian Diver that were scheduled. So we poked along to find more of the tracer streak with CTD/Rosette casts as well as we could ('+' is where there was no tracer, 'o' was were there was a little, and filled 'o's are where there was a lot.) The solid lines are an estimate of the boundaries of the tracer streaks. The streaks are of interest because they confirm the space and time scales of mesoscale eddies stirring the ocean and because their width gives a unique estimate of mixing at scales smaller than those of the eddies.

If we add in a site survey with the High Resolution Profiler of Ray Schmitt, John Toole and Kurt Polzin prior to the tracer release, plus transit and port times, Oceanus spent a third of 1992 supporting the North Atlantic Tracer Release Experiment, which launched both my career and the tracer release technique in open ocean studies.