Eddy-Induced Modulation of Turbulent Dissipation over Rough Topography in the Southern Ocean

Alberto C Naveira Garabato,Katy L Sheen,J Alexander Brearley,Stephanie Waterman,David A Smeed

doi:10.1175/jpo-d-12-0222.1

Abstract

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.

Highlights

In situ and satellite altimetric observations have shown that the ocean kinetic energy (KE) field in mid-to-high latitudes is dominated by mesoscale eddies on scales of 50–100 km
Low-frequency temperature and salinity changes are largely governed by the north–south movement of the Subantarctic Front (SAF), clearly visible in altimetry (Figs. 2e,f show examples of times when the mooring is located to the south and north of the front, respectively)
The mooring is located to the north of the SAF until 25 January, when a large northward meander moves cold water across the array in concert with a strong jet (Figs. 2a,b,d)

Summary

Introduction

In situ and satellite altimetric observations have shown that the ocean kinetic energy (KE) field in mid-to-high latitudes is dominated by mesoscale eddies on scales of 50–100 km. Circumpolar Current (ACC) as a result of baroclinic instability (Smith 2007), with a particular enhancement of EKE in frontal zones and immediately downstream of Drake Passage (Gille 1994; Morrow et al 1994). The strength of this Southern Ocean eddy field is believed to be sensitive to climatic changes in forcing on a range of time scales (Thompson and Solomon 2002; Meredith and Hogg 2006; Meredith et al 2012). Molemaker et al (2010) suggest that loss of balance is unlikely to be a large term in the deep ocean

Results

Discussion

Conclusion