Observation of a Large Lee Wave in the Drake Passage

Jesse M Cusack,David A Smeed,James B Girton,Alberto C Naveira Garabato

doi:10.1175/jpo-d-16-0153.1

Abstract

AbstractLee 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.

Highlights

Lee waves can be generally defined as internal gravity waves generated by the interaction of a quasi-steady stratified flow with topography
Between 2 and 4 January 2011, two electromagnetic autonomous profiling explorer (EM-APEX) floats were advected eastward over the northern segment of the Shackleton Fracture Zone (SFZ), a chain of seamounts and large bathymetric features that extends between the Antarctic Peninsula and the South American continental shelf
Large overturns of order 10 m in scale are detectable using the Thorpescale method, with dissipation rates in such patches approaching 1026 W kg21, while the majority of overturns ararete,smP a5lleÐ r02Zthra«ndzt,hpise.aTkhs eatd2e0pmthW-inmte2g2r.ated dissipation

Summary

Introduction

Lee waves can be generally defined as internal gravity waves generated by the interaction of a quasi-steady stratified flow with topography. Several methods for estimating vertical water velocity and turbulent kinetic energy dissipation are adapted and applied to the measurements, allowing almost complete characterization of the wave in terms of frequency, wavelength, momentum flux, energy flux, and dissipation rate.

Results

Conclusion