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- Author or Editor: David A. Smeed x
- The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) x
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Abstract
Mesoscale eddies are universal features of the ocean circulation, yet the processes by which their energy is dissipated remain poorly understood. One hypothesis argues that the interaction of strong geostrophic flows with rough bottom topography effects an energy transfer between eddies and internal waves, with the breaking of these waves causing locally elevated dissipation focused near the sea floor. This study uses hydrographic and velocity data from a 1-yr mooring cluster deployment in the Southern Ocean to test this hypothesis. The moorings were located over a small (~10 km) topographic obstacle to the east of Drake Passage in a region of high eddy kinetic energy, and one was equipped with an ADCP at 2800-m depth from which internal wave shear variance and dissipation rates were calculated. Examination of the ADCP time series revealed a predominance of upward-propagating internal wave energy and a significant correlation (r = 0.45) between shear variance levels and subinertial near-bottom current speeds. Periods of strong near-bottom flow coincided with increased convergence of eddy-induced interfacial form stress in the bottom 1500 m. Predictions of internal wave energy radiation were made from theory using measured near-bottom current speeds, and the mean value of wave radiation (5.3 mW m−2) was sufficient to support the dissipated power calculated from the ADCP. A significant temporal correlation was also observed between radiated and dissipated power. Given the ubiquity of strong eddy flows and rough topography in the Southern Ocean, the transfer from eddy to internal wave energy is likely to be an important term in closing the ocean energy budget.
Abstract
Mesoscale eddies are universal features of the ocean circulation, yet the processes by which their energy is dissipated remain poorly understood. One hypothesis argues that the interaction of strong geostrophic flows with rough bottom topography effects an energy transfer between eddies and internal waves, with the breaking of these waves causing locally elevated dissipation focused near the sea floor. This study uses hydrographic and velocity data from a 1-yr mooring cluster deployment in the Southern Ocean to test this hypothesis. The moorings were located over a small (~10 km) topographic obstacle to the east of Drake Passage in a region of high eddy kinetic energy, and one was equipped with an ADCP at 2800-m depth from which internal wave shear variance and dissipation rates were calculated. Examination of the ADCP time series revealed a predominance of upward-propagating internal wave energy and a significant correlation (r = 0.45) between shear variance levels and subinertial near-bottom current speeds. Periods of strong near-bottom flow coincided with increased convergence of eddy-induced interfacial form stress in the bottom 1500 m. Predictions of internal wave energy radiation were made from theory using measured near-bottom current speeds, and the mean value of wave radiation (5.3 mW m−2) was sufficient to support the dissipated power calculated from the ADCP. A significant temporal correlation was also observed between radiated and dissipated power. Given the ubiquity of strong eddy flows and rough topography in the Southern Ocean, the transfer from eddy to internal wave energy is likely to be an important term in closing the ocean energy budget.
Abstract
Lee waves are thought to play a prominent role in Southern Ocean dynamics, facilitating a transfer of energy from the jets of the Antarctic Circumpolar Current to microscale, turbulent motions important in water mass transformations. Two EM-APEX profiling floats deployed in the Drake Passage during the Diapycnal and Isopycnal Mixing Experiment (DIMES) independently measured a 120 ± 20-m vertical amplitude lee wave over the Shackleton Fracture Zone. A model for steady EM-APEX motion is developed to calculate absolute vertical water velocity, augmenting the horizontal velocity measurements made by the floats. The wave exhibits fluctuations in all three velocity components of over 15 cm s−1 and an intrinsic frequency close to the local buoyancy frequency. The wave is observed to transport energy and horizontal momentum vertically at respective peak rates of 1.3 ± 0.2 W m−2 and 8 ± 1 N m−2. The rate of turbulent kinetic energy dissipation is estimated using both Thorpe scales and a method that isolates high-frequency vertical kinetic energy and is found to be enhanced within the wave to values of order 10−7 W kg−1. The observed vertical flux of energy is significantly larger than expected from idealized numerical simulations and also larger than observed depth-integrated dissipation rates. These results provide the first unambiguous observation of a lee wave in the Southern Ocean with simultaneous measurements of its energetics and dynamics.
Abstract
Lee waves are thought to play a prominent role in Southern Ocean dynamics, facilitating a transfer of energy from the jets of the Antarctic Circumpolar Current to microscale, turbulent motions important in water mass transformations. Two EM-APEX profiling floats deployed in the Drake Passage during the Diapycnal and Isopycnal Mixing Experiment (DIMES) independently measured a 120 ± 20-m vertical amplitude lee wave over the Shackleton Fracture Zone. A model for steady EM-APEX motion is developed to calculate absolute vertical water velocity, augmenting the horizontal velocity measurements made by the floats. The wave exhibits fluctuations in all three velocity components of over 15 cm s−1 and an intrinsic frequency close to the local buoyancy frequency. The wave is observed to transport energy and horizontal momentum vertically at respective peak rates of 1.3 ± 0.2 W m−2 and 8 ± 1 N m−2. The rate of turbulent kinetic energy dissipation is estimated using both Thorpe scales and a method that isolates high-frequency vertical kinetic energy and is found to be enhanced within the wave to values of order 10−7 W kg−1. The observed vertical flux of energy is significantly larger than expected from idealized numerical simulations and also larger than observed depth-integrated dissipation rates. These results provide the first unambiguous observation of a lee wave in the Southern Ocean with simultaneous measurements of its energetics and dynamics.
