Use of the High Resolution Profiler (HRP) in the Salt Finger Tracer Release Experiment (SFTRE)

Abstract

The Salt Finger Tracer Release Experiment (SFTRE) was conducted in the tropical North Atlantic in 2001. The experimental area was east of Barbados and is characterized by thermohaline staircase features prevalent in the depth range of 200−600 meters. The goal of this experiment was to quantify the distribution and intensity of vertical mixing in a region of thermohaline staircases. Two cruises were required to accomplish this goal: one to survey with the High Resolution Profiler (HRP) and inject sulfur hexafluoride (SF6) tracer, and another ten months later to map the spatial distribution of tracer and obtain additional estimates of diffusive and turbulent mixing rates using the HRP. The first cruise of the SFTRE experiment took place between January 15 and February 12, 2001 on the R/V Oceanus, leg 365-2 (OC365). An XBT survey identified an area of robust staircases that became the injection site. Then 175 kg of SF6 tracer was injected in nine streaks in a layer with temperature of about 10°C. When the injection mechanism was being replenished, HRP profiles were made in the area of the tracer patch. The profiles yielded estimates of the mixing rates at the start of the experiment. Near the end of the cruise, water samples from the patch were used to map the actual tracer distribution immediately after deployment. The second cruise occurred between October 29 and December 4 on the R/V Seward Johnson, leg 01−12 (SJ0112). Its objective was to sample and map the vertical and horizontal distribution of tracer after ten months. The work completed included 172 CTD casts with chemical analysis performed on the water samples, and 165 HRP profiles. Despite covering an area of 500,000 nautical miles², only 50−60% of the tracer was found, suggesting higher than expected lateral mixing. The SFTRE included the deployment of a Moored Profiler. The profiles acquired by the MP provide background on the temporal variation of the temperature, salinity, and velocity fields where it was deployed. To share costs of personnel, the MP was deployed and recovered on cruises that followed ours, in conjunction with other mooring activities. The MP was deployed in February 2001 from R/V Oceanus and recovered by the R/V Knorr in April 2002. The program was a success, despite not fully delimiting the tracer distribution, because the observations allow more complete quantification of the mixing processes occurring in this region. The inferred mixing intensity was stronger and the influence of the thermohaline staircases more widespread than initially expected.