Mean jets, mesoscale variability and eddy momentum fluxes in the surface layer of the Antarctic Circumpolar Current in Drake Passage

Abstract

High-resolution Acoustic Doppler Current Profiler (ADCP) observations of surface-layer velocities in Drake Passage, comprising 128 sections over a period of 5 years, are used to study the surface-layer circulation of the Antarctic Circumpolar Current (ACC). These observations resolve details of the mean flow including the topographic control of the mean Subantarctic Front (SAF) and the multiple filaments of the Polar Front (PF) and Southern ACC Front (SACCF) that converge into single mean jets as the ACC flows through Drake Passage. Subsurface definitions of the SAF and PF applied to expendable bathythermograph temperatures generally coincide with mean jets, while the SACCF is better defined in velocity than temperature. The mean transport in the top 250-m-deep surface layer, estimated from the cross-track transport along three repeat tracks, is 27.8 +/- 1 Sv.Eddy momentum fluxes were estimated by ensemble averaging Reynolds stresses relative to gridded Eulerian mean currents. Eddy kinetic energy (EKE) is surface intensified in the mixed layer because of inertial currents and decreases poleward in Drake Passage, ranging from similar to 800 cm(2) s(-2) to similar to 200 cm(2) s(-2). ADCP EKE estimates are everywhere significantly higher than altimetric EKE estimates, although the pattern of poleward decrease is the same. Horizontal-wavenumber spectra of velocity fluctuations peak at wavelengths in the 250-330 km range and are significantly anisotropic. Along-passage fluctuations dominate at wavelengths less than 250 km; cross-passage fluctuations dominate at wavelengths greater than 250 km. Mesoscale eddies dominate the variance in northern Drake Passage. Inertial variability is constant with latitude and together with baroclinic tides accounts for some but not all of the discrepancy between the ADCP surface-layer EKE and altimetry-inferred EKE.