AnSlope
Cross-Slope Exchanges at the Antarctic Slope Front
This page summarizes the AnSlope program. Further information on AnSlope
can be found at the
LDEO AnSlope Home Page.
ESR's specific contributions to AnSlope are described at the
ESR Contributions page.
Goals and Objectives
AnSlope is a multi-institution experiment seeking to answer 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. AnSlope
addresses 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:
- 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;
- Determine the influence of slope topography (canyons, proximity to a
continental boundary, isobath divergence/convergence) on frontal location
and outflow of dense Shelf Water;
- Establish the role of frontal instabilities, benthic boundary layer
transports, tides and other oscillatory processes on cross-slope advection
and fluxes; and
- 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.
Approach
AnSlope addresses these objectives with an integrated observational and
modeling program. Acquisition of a set of measurements focused over the outer
continental shelf and upper slope of the northwestern Ross Sea began with
a cruise on the icebreaker N. B. Palmer in February-April 2003. Field work
will continue with a second cruise in early 2004, and a final cruise in late 2004.
The core elements of the field work are:
- Moorings, obtaining 1-2 years of time series of ocean currents,
temperature, and salinity;
- CTD/LADCP and CTD-based microstructure; and
- tracer chemistry.
In addition, basic modeling studies will be undertaken to study tides,
plume dynamics, and frontal dynamics.
The various components of AnSlope studies overlap in many ways. However,
a broad summary of the main foci of the principal investigators is as follows:
- 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 was deployed prior to the
CTD/tracer surveys.
- 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.
- Microstructure on CTD; L. Padman, R. Muench (ESR): A microstructure
package is 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.
- Tide modeling; L. Padman (ESR): Tides are expected to be the
primary contributor to ocean currents in the study region. The cross-slope
advection due to these periodic currents must be taken into account when assessing
data from the CTD/ADCP/LADCP survey. ESR, in collaboration with OSU, has
created a ~10 km tides model (the
Circum-Antarctic Tidal Simulation: "CATS")
which does quite a good job of estimating current strengths, but presently
does poorly in estimating the tidal current phase along the Ross Sea
continental shelf. ESR and OSU have now created a higher-resolution tidal model
for the Ross Sea, and have improved the model quality for the AnSlope region
by assimilating the vessel-mounted ADCP (VM-ADCP) data collected during
AnSlope Cruise 1 and 2 earlier Ross Sea cruises. We will continue to improve
this model by assimilating more VM-ADCP data from AnSlope cruises 2 and 3, as
well as from the TAMU/OSU moorings when they are recovered.
Acknowledgements
AnSlope is funded by the National Science Foundation, Office of Polar
Programs. We gratefully acknowledge Dr. Bernhard Lettau's faith in our ability
to carry out this complex experiment in a difficult environment.
AnSlope is the 4th in a series of projects under the SCOR-affiliated
International Antarctic Zone (iAnZone) program.
