New Multi-Season Measurements of Currents and Hydrography from Profiling Floats and Altimetry in the Amundsen Sea: Implications for Antarctic Shelf-Slope Exchange and Sea-Ice Thermodynamics
Abstract:
(CDW) to the Antarctic continent is a well-known feature of the
Meridional Overturning Circulation, but the specific mechanisms
responsible for this transport and its variability have not been fully
established. We report new results from an ongoing field effort
focused on CDW pathways to the West Antarctic shelf, where ice sheet
mass losses have been particularly rapid in recent years.
Satellite altimetry measurements, together with a shipboard section
across the Amundsen Sea with CTD, ADCP, and expendable instruments
(XBT, XCTD) during a January 2015 transit by the R/V Nathaniel
B. Palmer, reveal interannual variability in the summertime current
structure and the apparent influence of a line of seamounts
north of the Amundsen shelf. In combination with 3-month trajectories
of the CDW layer from EM-APEX profiling floats launched on the same
transit, these observations suggest that the dominant southward flow
has shifted east of the seamounts and is directed at the central,
rather than the western Amundsen Sea. These variations are discussed
in the context of forcing variability and their connection with
previous hydrographic measurements and shelf CDW observations.
The under-ice EM-APEX represent a promising new approach to year-round
sampling of subsurface shear and water properties in the seasonal ice
zone, using minimal contact with the underside of the ice to determine
draft and saving subsurface data until surface telemetry is
available. Preliminary data have now been returned successfully
through wintertime leads, including ice draft measurements ranging
from 0.5 to 1.3 m and T, S, and water velocity up to the ice
underside. The contrasting environments of open-water and ice-covered
conditions are described--particularly the varying internal wave shear
and strain levels and their implications for the mixing between the
surface near-freezing layer and the 3-4 C warmer CDW layer below.