Abstract

Slow subinertial variations in Antarctic Bottom Water (AABW) are investigated interacting with internal waves and associated turbulent mixing in the Puerto Rico Trench (PRT), northwest Atlantic. Just below the PRT's top at 5,500 m, a deep-sea mooring was deployed for 14 months. Around 6,100 m, the line held a 200 m long string of 101 high-resolution temperature sensors and a current meter. Around 8,250 m, a similar string held 102 sensors, of which the lowest was 9 m above the bottom. As was measured with shipborne conductivity-temperature-depth profiling down to 7,150 m, PRT waters are very weakly stratified, with local mean buoyancy frequency equaling 1–1.7 times the semidiurnal tidal frequency. The observations show alternating quiescent and relatively turbulent periods that correspond with relatively cooler and warmer AABW, respectively. Over the 2,100 m distance between the two temperature strings, only semidiurnal tidal variations significantly correlate with an average phase difference of 90° (3 h) for the entire recording period. This suggests a dominant baroclinic rather than a barotropic coupling. During quiescent periods, the vertical internal wave scale is rather small with out-of-phase tidal motions between the two data sets and vertical excursions of 40 m. During turbulent periods, internal wave motions at all frequencies are close to in phase between the two data sets, suggesting fast vertical propagation with excursions exceeding 200 m. Increased subinertial energy levels, probably reflecting trapped internal waves, are found near the bottom during such periods. It is suggested that internal wave turbulence dominates the deep-sea transport of heat and suspended materials over the entire PRT height and possibly beyond.

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