AnSlope: Cross-slope exchanges at the Antarctic Slope Front
AnSlope seeks an answer to the question: What is the role of the Antarctic Slope Front and continental slope morphology in the exchanges of mass, heat, and freshwater between the shelf and oceanic regimes, in particular those leading to outflows of dense water into intermediate and deep layers of the adjacent deep basins and world ocean circulation?
The importance to the global ocean circulation and climate of cold water masses originating in the Antarctic is now understood, but the processes by which these water masses enter the deep ocean circulation are not. We have developed a program called AnSlope* to address this problem. Our primary goal is to identify the principal physical processes that govern the transfer of shelf-modified dense water into intermediate and deep layers of the adjacent deep ocean. At the same time, we seek to understand the compensatory poleward flow of waters from the oceanic regime. We identify the upper continental slope as the critical gateway for the exchange of shelf and deep ocean waters. Here the topography, velocity and density fields associated with the nearly ubiquitous Antarctic Slope Front (ASF) must strongly influence the advective and turbulent transfer of water properties between the shelf and oceanic regimes.
AnSlope has four specific objectives: [A] Determine the ASF mean structure and the principal scales of variability (spatial from ~1 km to ~100 km, and temporal from tidal to seasonal), and estimate the role of the Front on cross-slope exchanges and mixing of adjacent water masses; [B] Determine the influence of slope topography (canyons, proximity to a continental boundary, isobath divergence/convergence) on frontal location and outflow of dense Shelf Water; [C] Establish the role of frontal instabilities, benthic boundary layer transports, tides and other oscillatory processes on cross-slope advection and fluxes; and [D] Assess the effect of diapycnal mixing (shear-driven and double-diffusive), lateral mixing identified through intrusions, and nonlinearities in the equation of state (thermobaricity and cabbeling) on the rate of descent and fate of outflowing, near-freezing Shelf Water.
AnSlope addresses these objectives with an integrated observational and modeling program structured as follows. A collaborative core program begins in 2002, containing the components considered central to meeting AnSlope objectives, primarily through acquisition of a set of measurements focused over the outer continental shelf and upper slope of the northwestern Ross Sea. This will allow us to assess the regional AABW production rate, and to identify the cross-front exchange processes that must be taken into account when assessing provision of dense water to the deep basins elsewhere around Antarctica. The core elements are: moorings; CTD/LADCP and CTD-based microstructure; tracers; and basic tidal modeling. "Enhancement" proposals, to be submitted separately, request support for the modeling studies that are necessary to fully exploit the measurements and develop the techniques for parameterizing cross-front exchanges in regional and global models. Three cruises are proposed, beginning in Austral summer 2003, over a period of 12 to 14 months. Moorings would be in place throughout this period. The Italian CLIMA program in the Ross Sea provides a valuable international enhancement for the AnSlope observational component. The German BRIOS-2 coupled ice-ocean GCM program is complementary to the US process-oriented modeling studies, and provides a test-bed for AnSlope-generated parameterizations of cross-front exchange.
AnSlope Field Programs and Collaborators
Moored current meters: A. Orsi, T. Whitworth (TAMU), D. Pillsbury (OSU): One year of direct current and temperature measurements will characterize the horizontal and vertical flow structure of the Antarctic Slope Current, its most important time-varying components from tidal to seasonal scales, and allow an assessment of the eddy components of cross-slope fluxes of heat, freshwater and momentum. The current meter array is designed to track the front's location over time. To maximize chances of achieving complete, contemporaneous coverage during the most critical period of intensive measurements (CTD, ADCP, tracers), the array will be deployed prior to the CTD/tracer surveys, and recovered and redeployed at the end of the summer cruise of 2002-03. This combination of data will provide the context for data returned from the year-long deployment.
CTD/ADCP; A. Gordon, S. Jacobs, M. Visbeck (LDEO): This component, providing high resolution measurements of the stratification and velocity shear fields of the outer shelf, slope and intervening fronts, enables study of vertical coherence of these fields under varied bathymetric conditions; detects exchanges of water masses between the shelf and slope regimes, including plumes; and investigates the role of cabbeling and thermobaric effects. Special attention will be paid to the benthic spatial scales, particularly within the confines of submarine canyons. The combined use of hull-mounted and lowered ADCP allows accurate estimates of the width of frontal jets; provides first order estimates of tidal velocities; obtains full ocean depth velocity profiles and accurate estimates of near-bottom flow. Removal of tides will be done with a regional model and assimilated bottom pressure and currents from the moorings.
Microstructure on CTD; L. Padman, R. Muench (ESR): A microstructure package will be mounted on the CTD rosette to obtain direct estimates of the diapycnal diffusivity, Kv, and to identify the primary instability processes driving vertical turbulent fluxes. This instrument has a depth range of ~2000 m and so can sample into the benthic dense plume out to the central slope.
CFC and stable isotope tracers; W. Smethie, P. Schlosser (LDEO): CFCs, isotopes of oxygen and helium, and tritium will be measured on the Coarse and Fine Scale Resolution surveys. Shelf Water is well ventilated and contains high CFC concentrations and low He-3 concentrations. Opposite characteristics are found in the CDW upwelling across the slope. Sharp contrasts in tracer concentrations and ratios (CFC-113:CFC-11, CFC:tritium) are observed between Shelf Water types that form underneath the Ice Shelf or over the continental shelf. CFC, He-3 and hydrographic data are used to map spreading pathways and to determine exchange and mixing history across the ASF; dO-18 and total helium data will determine the location of the sources of outflowing Shelf Water types; ratios will determine residence times. AnSlope CFC measurements will provide critical information on the controlling processes for surface and Shelf Water chemistry, such as the extent to which they are in equilibrium with the atmosphere.
AnSlope is funded by the National Science Foundation/Office of Polar Programs
AnSlope* is the 4th in a series of projects under the SCOR-affiliated program: International Antarctic Zone (iAnZone).