DOI: 10.5061/dryad.3ffbg79rm
title: Widespread seawater intrusions beneath the grounded ice of Thwaites Glacier, West Antarctica

description: Warm water from the Southern Ocean has a dominant impact on the evolution
 of Antarctic glaciers and in turn on their contribution to sea level rise.
 Using a continuous time series of daily-repeat satellite
 synthetic-aperture radar interferometry data from the ICEYE constellation
 collected in March-June 2023, we document an ice grounding zone, or region
 of tidally-controlled migration of the transition boundary between
 grounded ice and ice afloat in the ocean, at the main trunk of Thwaites
 Glacier, West Antarctica, a strong contributor to sea level rise with an
 ice volume equivalent to a 0.6-m global sea level rise. The ice grounding
 zone is 6 km wide in the central part of Thwaites with shallow bed slopes,
 and 2 km wide along its flanks with steep basal slopes. We additionally
 detect irregular seawater intrusions, 5-10 cm in thickness, extending
 another 6 km upstream, at high tide, in a bed depression located beyond a
 bedrock ridge that impedes the glacier retreat. Seawater intrusions align
 well with regions predicted by the GlaDS subglacial water model to host a
 high-pressure distributed subglacial hydrology system in between
 lower-pressure subglacial channels. Pressurized seawater intrusions will
 induce vigorous melt of grounded ice over kilometers, making the glacier
 more vulnerable to ocean warming, and increasing the projections of ice
 mass loss. Kilometer-wide, widespread seawater intrusion beneath grounded
 ice may be the missing link between the rapid, past, and present changes
 in ice sheet mass and the slower changes replicated by ice sheet
 models. The dataset includes grounding line positions, all ICEYE
 radar interferograms and parameter files, files of tidal predictions and
 corrections for change in atmospheric pressure, and output products from
 the GlADS subglacial hydrology model.
description: <strong>Differential interferometry </strong>measures
 a differential change in ice displacement between 3 (or 4) epochs after
 eliminating the horizontal motion of ice through differencing of two
 pairs: we subtract two consecutive one-day pairs to eliminate the (steady)
 horizontal motion of the ice and leave the short term (vertical) motion of
 the ice, i.e. the vertical motion caused by changes in oceanic tide and
 atmospheric pressure, plus noise. If there is a data gap in data
 acquisition, we combine 2 x 1-day pairs acquired more than one day apart,
 hence 4 scenes.  <strong>Predictions of
 SSH.</strong> We calculate changes in SHH using the CATS2008 tidal
 model corrected for IBE using ERA-5 atmospheric pressure
 fields.   <strong>Subglacial
 hydrology</strong> is reproduced using the 2D finite-element Glacier
 Drainage System (GlaDS) model, which predicts the presence of long,
 high-pressured subglacial channels and an adjacent distributed drainage
 system. GlaDS calculates water discharge, flux, and pressure beneath
 Thwaites Glacier. The model allows coincident development of distributed
 and channelized systems. We test a range of conductivity of the
 distributed and channelised systems to converge to a stable solution. We
 use outputs that produce a) an upper limit for water pressure, beyond
 which the model does not converge, b) a lower limit when solutions have
 water with pressures far below the overburden pressure, and c)
 intermediate levels of pressure. We present an output (c) with a
 distributed system conductivity of 1 x 10<sup>-4
 </sup>m<sup>3/2</sup> kg<sup>-1/2 </sup>and
 a channel conductivity of 5 x 10<sup>-2</sup>
 m<sup>3/2</sup> kg<sup>-1/2</sup>. The model is
 applied with basal sliding velocity and water production (geothermal and
 frictional heat) from the ISSM IMSIP-6 Antarctic control run along with
 surface and bed topographies from BedMachine Antarctica version 1. The
 model is run on a mesh of 19,340 nodes with refining near the grounding
 line giving a minimum edge length of 280 m. The model is run for 20,000
 days until near steady state.
description: # Widespread seawater intrusions beneath the grounded ice of Thwaites
 Glacier, West Antarctica ## Description of the data and file structure All
 the data are geocoded in Polar Stereographic format, with a coordinate
 reference system (CRS) EPSG:3031, at 25 m posting. 1. Three
 differential_interferograms folders: 1) AscRight, 2) AscLeft, and 3)
 DscRight. * The differential interferograms are 27 in descending
 right-looking, 26 in ascending right-looking, and 20 in ascending
 left-looking tracks. The naming convention is
 "gl_ICEYE-X6_9999_9999_ASC_YYYYMMDD_HHMMSS-YYYYMMDD_HHMMSS_YYYYMMDD_HHMMSS-YYYYMMDD_HHMMS_XXXXX_XXXX_XXXXX_XXXXX_2s.tiff". The first "YYYYMMDD_HHMMSS_HHMMSS" stands for the date and time of the primary interferogram. The second "YYYYMMDD_HHMMSS_HHMMSS" after hyphen "-" stands for the date and time of the secondary interferogram. XXXX is several seconds in the day for the 4 dates. tiff means geotiff format. The projection is ESPG 3031 WGS 84 Antarctic Polar Stereographic. The posting is 25 meters. * A subfolder "Parameters" lists parameter files for the native interferograms, the native individual scenes, and the interferometric baselines generated by the Gamma Remote Sensing software in ASCII format, an IDL code was written to read the tidal data and produce the results in Figure 3 of the PNAS paper, and example outputs in postscript format. 2. All grounding lines hand-drawn by the authors using differential interferograms are stored in the gl_iceye_TWG_2023_new.zip file using an ESRI shapefile format. The file gl_iceye_TWG_2023_new.count.tif is a geotiff file that counts the number of times the grounding line positions fall into an image element at 25 m posting. All data are in ESPG 3031 format, WGS 84 Antarctic Polar Stereographic. 3. Ancillary data folder: There are two products: 1) 1 x geotiff file of bed topography of the Thwaites region extracted from BedMachinev3.7 at 500 m spacing and 2) 1 x geotiff shaded relief map of the surface elevation generated from the Copernicus Digital Elevation Model (DEM) at 450 m spacing. These files are ancillary files used for display background. 4. Tide folder: There is 1 CSV file that lists the tide and IBE correction at the time of passage of the satellites from March 30, 2023, at midnight to June 15, 2023, at midnight. Time is with date (YYYY/MM/DD) and time (XX:XX) in hours is given on row 1 every 15 minutes. Pressure in kPa is listed in row 2. IBE correction is listed in row 3 in centimeters. The tidal height differential from the CATS2008 tidal model is listed in row 4 in centimeters. The tidal height differential corrected for IBE is listed in row 5 in centimeters. 5. GlADS folder: we have 3 x geotiff files of the Thwaites region in ESPG 3031 format, WGS 84 Antarctic Polar Stereographic at 250-meter posting: 1) Thwaites_high_cond_wh_refined.tif: geotiff file of subglacial water height, in centimeters; 2) Thwaites_high_cond_water_pressure2.tif: geotiff file of water pressure as a percentage of the ice overburden pressure, in percent; and 3) Thwaites_high_cond_channels.tif: geotif file of the subglacial water discharge in cubic meter per second. * We also include a fourth file: Thwaites_high_cond_water_pressure2_contour.gpkg, which is a geopackage file of the isocontours of water pressure, with 1% contour levels, generated from Thwaites_high_cond_water_pressure2.tif.