Turbulent Mixing and Lee-Wave Radiation in Drake Passage: Sensitivity to Topography

Abstract

Radiation and breaking of internal lee waves are thought to play a significant role in the energy and heat budget of the Southern Ocean. Open questions remain, however, regarding the amount of energy converted from the deep flows of the Antarctic Circumpolar Current (ACC) into lee waves and how much of this energy dissipates locally. This study estimated the linear lee-wave energy radiation using a unique 4-year time series of stratification and near-bottom currents from an array of Current and Pressure measuring Inverted Echo Sounders (CPIES) spanning Drake Passage. Lee-wave energy was calculated from two 2D anisotropic and one 1D isotropic abyssal hill topographies. Lee-wave energy radiation from all three topographies was largest in the Polar Front Zone associated with strong deep meandering of the ACC fronts. Both baroclinic and barotropic instabilities appeared to modulate the conversion to lee waves in the Polar Front Zone. Fine structure temperature, salinity, and velocity profiles at the CPIES locations were used to estimate turbulent dissipation due to breaking internal waves by employing a finescale parameterization. High dissipation near the bottom was consistent with upward-propagating, high-frequency lee waves as found by earlier studies. In contrast to idealized numerical predictions of 50% local dissipation of lee-wave energy, this study found less than 10% dissipated locally similar to some other studies. Improving the representation of the abyssal hills by accounting for anisotropy did not reduce the discrepancy between radiated lee-wave energy and local dissipation. Instead, alternative fates must be considered for the excess radiated lee-wave energy.