Tides of the northwestern Ross Sea and their impact on dense outflows of Antarctic Bottom Water
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Moored and depth-profile measurements of hydrography and currents during the 2003-2005 Antarctic Slope ("AnSlope") program on the shelf-slope region near Drygalski Trough sill in the northwest Ross Sea have revealed an energetic benthic outflow of cold, dense Antarctic Bottom Water (AABW) produced near the shelf break. This outflow couples dense-water production processes on the continental shelf to ventilation of the global deep ocean. We use time series of bottom pressure and currents on the AnSlope moorings, and a high-resolution, three-dimensional, primitive-equation numerical model, to explore the hypothesis that the tidal currents near the shelf break have a significant impact on outflow volume transport and hydrographic properties. Tidal currents exceed 1 m s-1 at spring tide over the outer shelf and upper slope. Tidal advection at spring tide carries water parcels ∼20 km across-slope during a half tidal cycle, sufficient to advect water from northern Drygalski Trough across the sill to the shelf break, or similarly from the shelf break (∼500 m water depth) down to the ∼1600 m isobath. A tidally rectified Lagrangian mean flow of order 0.05-0.1 m s-1 over the outer shelf and upper slope accompanies the spring tide. Thickness and water mass properties of the benthic layer over the sill and upper slope exhibit pronounced tidal cycles due to advection of hydrographic gradients and to energetic mixing that can extend more than halfway up from the seabed to the ocean surface. Maximum parameterized diapycnal diffusivity in the benthic layer at spring tides approaches 1 m2 s-1. The addition of tides has a profound effect on time-averaged outflows of AABW, primarily through an increase in benthic layer thickness during spring tide. Downslope volume fluxes of AABW over the slope, averaged over a tidal cycle, are ∼2.5 times larger at spring tide than at neap, indicating a need to include the effect of tides in large-scale ocean models attempting to represent shelf water contributions to the global ocean circulation. © 2008 Elsevier Ltd. All rights reserved.
author list (cited authors)
Padman, L., Howard, S. L., Orsi, A. H., & Muench, R. D.