The Subduction experiment : cruise report RRS Charles Darwin cruise number 73 subduction 3 mooring deployment and recovery cruise, 30 September-26 October 1992

Abstract

Subduction is the mechanism by which water masses formed in the mixed layer and near the surface of the ocean find their way into the upper thermocline. The subduction process and its underlying mechanisms were studied through a combination of Eulerian and Langrangian measurements of velocity, measurements of tracer distrbutions and hydrographic properties and modeling. An array of five surface moorings carrying meteorological and oceanographic instrumentation were deployed for a period of two years beginning in June 1991 as part of an Office of Naval Research (ONR) funded Subduction experiment. Three eight month deployments were planned. The moorings were deployed at 18°N 34°W, 18°N 22°W, 25.5°N 29°W, 33°N 22°W and 33°N 34°W. A Vector Averaging Wind Recorder (VAWR) and an Improved Meteorological Recorder (IMET) collected wind speed and wind direction, sea surface temperature, air temperature, short wave radiation, barometric pressure and relative humidity. The IMET also measured precipitation. The moorings were heavily instrumented below the surface with Vector Measuring Current Meters (VMCM), and single point temperature recorders. Expendable bathythermograph (XBT) data were collected and meteorological observations were made while transitting between mooring locations. In addition a series of 59 cm stations were made and water samples taken to be analyzed for tritium levels, salinity and dissolved oxygen content. This report describes the work that took place during RRS Charles Darwin cruise number 73 which was the third scheduled Subduction mooring cruise. During this cruise the second setting of the moorings were recovered and redeployed for a third eight month period. This report includes a description of the instrumentation that was deployed and recovered, has information about the underway measurements (XBT and meteorological observations) that were made including plots of the data, includes a description of the work conducted in conjunction with the tracer/hydrography program and presents a chronology of the cruise events.