Brazil Basin Tracer Release Experiment (BBTRE) - Year 1

The goal of the experiment was to use two independent methods to quantify mixing in the eastern Brazil Basin. The methods employed were: 1) injecting SF6 tracer, and then tracking it's dispersion over time and 2) making discrete, instantaneous measurements of the turbulence using the HRP.   The HRP was also used in this experiment to choose the best site for the tracer injection. A cross basin survey was made en route to the Mid-Atlantic Ridge. The left side of this figure shows the HRP dive locations and basin topography. On the right, the solid lines show the paths of tracer injection runs, the dashed lines show sampling lines used to verify that tracer was actually deployed at the target depth. A total of 114 KG of tracer was deployed on eight injection lines.

To obtain mixing estimates associated with various topographic features, temporal and spatial series of HRP dives were made near the site of the injection and along the western flank of the Mid-Atlantic Ridge.  73 of the 75 HRP profiles made during this experiment were full ocean depth, which in this area entailed going to 4500 to 5500 meters. Click here to see how close we were able to get to the bottom, using the altimeter in addition to pressure as a dive termination condition. By being able to make deep, near-bottom measurements, we were able to obtain the first full depth section of turbulent dissipation across an ocean basin.  A longitudinally averaged section of diffusivity shows little mixing in the western part of the basin, over relatively smooth topography. The eastern area, however, is over rougher topography, and shows enhanced mixing well into the water column. This result was somewhat unexpected, and suggests that internal wave generation by tidal flow over the rough bathymetry is an important deep ocean mixing mechanism, and that classical bottom-generated turbulence is relatively unimportant.

Publications resulting from this project are:

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