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:
Polzin, K. L., J. M. Toole, J. R. Ledwell and R. W. Schmitt, 1997. Spatial
variability of turbulent mixing in the abyssal ocean. Science, Vol.
276, pp. 93-96.
Montgomery, E. T. and R. W. Schmitt, 1997. Acoustic Altimeter control
of a free vehicle for near-bottom turbulence measurements. Deep-Sea
Research, 44, 6, 1077-1084.
Montgomery, E. T., 1998. Use of the High Resolution Profiler in the Brazil
Basin Tracer Release Experiment. Woods Hole Oceanographic Institution
Technical Report, 98-08, 34 pps.
Zhang, J., R. W. Schmitt, and R. X. Huang, 1998. Sensitivity of the
GFDL Modular Ocean Model to the parameterization of double-diffusive processes.
Journal of Physical Oceanography, 28, 4, 589-605.
Zhang, J., R. W. Schmitt, and R. X. Huang, 1999. The relative influence
of diapycnal mixing and hydrologic forcing on the stability of the thermohaline
circulation. Journal of Physical Oceanography, 29,
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