HR: 14:10h
AN: OS43C-03    [Abstracts]
TI: Mixing Associated with Sills in a Canyon on the Mid-Ocean Ridge Flank
AU: * Thurnherr, A M
EM: ant@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: St. Laurent, L C
EM: lous@ocean.fsu.edu
AF: Florida State University, 0102 OSB, West Call Street, Tallahassee, FL 32306 United States
AU: Speer, K G
EM: kspeer@ocean.fsu.edu
AF: Florida State University, 0102 OSB, West Call Street, Tallahassee, FL 32306 United States
AU: Toole, J M
EM: jtoole@whoi.edu
AF: Woods Hole Oceanographic Institution, Mail Stop #21, Woods Hole, MA 02543
AU: Ledwell, J R
EM: jledwell@whoi.edu
AF: Woods Hole Oceanographic Institution, Mail Stop #21, Woods Hole, MA 02543
AB: In order to close the global overturning circulation, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion. Microstructure observations from the western basin of the South Atlantic indicate that most of the abyssal mixing there takes place over the topographically rough flank of the mid-ocean ridge. In previous studies it has been suggested that the enhanced mixing is primarily caused by breaking internal waves forced by tidal flows. Here, the results from a detailed analysis of hydrographic data from a ridge-flank canyon, augmented by microstructure profiles, current-meter records and high-resolution bathymetry, are presented. Most of the strong dissipation is observed within the canyon, rather than above the ridge-flank topography. The largest dissipation values were recorded in the lee of a narrow sill extending across the full width of the canyon. Along the entire canyon, there is a strong correlation between the presence of sills and along-axial density gradients, while there is no similar correlation between the presence of depressions and the horizontal density gradients. Together, these observations suggest that sill-related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal-wave breaking, which is not expected to be associated preferentially with sills. The current-meter records furthermore indicate that, within the canyon, the kinetic energy of low-frequency flows is larger than that of the tides, consistent with a significant low-frequency energy source for the enhanced mixing. The available data indicate that while only $\approx$15% of the interfacial area between Antarctic Bottom Water and North Atlantic Deep Water in the western subtropical basin of the South Atlantic lies inside canyons, approximately half of the total energy dissipation takes place there. In contrast, only about a third takes place above the ridge-flank topography. The apparent importance of sill-related processes for mixing in the canyons is therefore of global significance, especially considering that a large portion of the global mid-ocean ridge is associated with deep cross-flank canyons.
DE: 4524 Fine structure and microstructure
DE: 4536 Hydrography
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting