Rapid submarine ice melting in the grounding zones of ice shelves in West Antarctica.

Enhanced submarine ice-shelf melting strongly controls ice loss in the Amundsen Sea embayment (ASE) of West Antarctica, but its magnitude is not well known in the critical grounding zones of the ASE's major glaciers. Here we directly quantify bottom ice losses along tens of kilometres with airborne radar sounding of the Dotson and Crosson ice shelves, which buttress the rapidly changing Smith, Pope and Kohler glaciers. Melting in the grounding zones is found to be much higher than steady-state levels, removing 300-490 m of solid ice between 2002 and 2009 beneath the retreating Smith Glacier. The vigorous, unbalanced melting supports the hypothesis that a significant increase in ocean heat influx into ASE sub-ice-shelf cavities took place in the mid-2000s. The synchronous but diverse evolutions of these glaciers illustrate how combinations of oceanography and topography modulate rapid submarine melting to hasten mass loss and glacier retreat from West Antarctica

Evaluating Greenland surface-mass-balance and firn-densification data using ICESat-2 altimetry

peer reviewedAbstract. Surface-mass-balance (SMB) and firn-densification (FD) models are widely used in altimetry studies as a tool to separate atmospheric-driven from ice-dynamics-driven ice-sheet mass changes and to partition observed volume changes into ice-mass changes and firn-air-content changes. Until now, SMB models have been principally validated based on comparison with ice core and weather station data or comparison with widely separated flight radar-survey flight lines. Firn-densification models have been primarily validated based on their ability to match net densification over decades, as recorded in firn cores, and the short-term time-dependent component of densification has rarely been evaluated at all. The advent of systematic ice-sheet-wide repeated ice-surface-height measurements from ICESat-2 (the Ice Cloud, and land Elevation Satellite, 2) allows us to measure the net surface-height change of the Greenland ice sheet at quarterly resolution and compare the measured surface-height differences directly with those predicted by three FD–SMB models: MARv3.5.11 (Modèle Atmosphérique Régional version 3.5.11) and GSFCv1.1 and GSFCv1.2 (the Goddard Space Flight Center FD–SMB models version 1.1 and 1.2). By segregating the data by season and elevation, and based on the timing and magnitude of modelled processes in areas where we expect minimal ice-dynamics-driven height changes, we investigate the models' accuracy in predicting atmospherically driven height changes. We find that while all three models do well in predicting the large seasonal changes in the low-elevation parts of the ice sheet where melt rates are highest, two of the models (MARv3.5.11 and GSFCv1.1) systematically overpredict, by around a factor of 2, the magnitude of height changes in the high-elevation parts of the ice sheet, particularly those associated with melt events. This overprediction seems to be associated with the melt sensitivity of the models in the high-elevation part of the ice sheet. The third model, GSFCv1.2, which has an updated high-elevation melt parameterization, avoids this overprediction

Vertical and latitudinal variation of the intertropical convergence zone derived using GPS radio occultation measurements

Using GPS radio occultation refractivity data collected over the period of 2002-2013, we present a new method for identification of the intertropical convergence zone (ITCZ). The ITCZ is identified by estimating the maximum in the monthly meridional refractivity and specific humidity field by applying a Gaussian fit at each longitude. The interannual variability and climatology of the ITCZ is presented from 12. years of refractivity data. This new method captures all the general features of ITCZ extent and its variability. We also examine the effects of the ITCZ vertically during different seasons. The ITCZ is observed mostly at eastern Pacific in May month, and it is zonally distributed in the September and October months of each year. The zonal variability is large between lower and higher levels, particularly over the Indian monsoon and western Pacific. The latitudinal difference in the vertical extent between 850. hPa and higher levels is larger during the northern hemisphere (NH) summer than NH winter

Mass balance of the Greenland Ice Sheet from 1992 to 2018

In recent decades, the Greenland Ice Sheet has been a major contributor to global sea-level rise1,2, and it is expected to be so in the future3. Although increases in glacier flow4–6 and surface melting7–9 have been driven by oceanic10–12 and atmospheric13,14 warming, the degree and trajectory of today’s imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet’s volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. Although the ice sheet was close to a state of balance in the 1990s, annual losses have risen since then, peaking at 335 ± 62 billion tonnes per year in 2011. In all, Greenland lost 3,800 ± 339 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.6 ± 0.9 millimetres. Using three regional climate models, we show that reduced surface mass balance has driven 1,971 ± 555 billion tonnes (52%) of the ice loss owing to increased meltwater runoff. The remaining 1,827 ± 538 billion tonnes (48%) of ice loss was due to increased glacier discharge, which rose from 41 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. Between 2013 and 2017, the total rate of ice loss slowed to 217 ± 32 billion tonnes per year, on average, as atmospheric circulation favoured cooler conditions15 and as ocean temperatures fell at the terminus of Jakobshavn Isbræ16. Cumulative ice losses from Greenland as a whole have been close to the IPCC’s predicted rates for their high-end climate warming scenario17, which forecast an additional 50 to 120 millimetres of global sea-level rise by 2100 when compared to their central estimate

Mass balance of the Greenland and Antarctic ice sheets from 1992 to 2020

Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9g€¯mm to global mean sea level, with the rate of mass loss rising from 105g€¯Gtg€¯yr-1 between 1992 and 1996 to 372g€¯Gtg€¯yr-1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9g€¯Gtg€¯yr-1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86g€¯Gtg€¯yr-1 in 2017 to 444g€¯Gtg€¯yr-1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9g€¯Gtg€¯yr-1) and, to a lesser extent, from the Antarctic Peninsula (13±5g€¯Gtg€¯yr-1). East Antarctica remains close to a state of balance, with a small gain of 3±15g€¯Gtg€¯yr-1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at 10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021)

tsutterley/geoid-toolkit: v1.1.1

feat: added field_mapping options to netCDF4 and HDF5 reads (https://github.com/tsutterley/geoid-toolkit/pull/18) docs: slimmer build to prevent rtd memory overutilization (https://github.com/tsutterley/geoid-toolkit/pull/19, https://github.com/tsutterley/geoid-toolkit/pull/24) docs: updated structure of documentation (https://github.com/tsutterley/geoid-toolkit/pull/20) feat: added datetime parser for ascii time columns (https://github.com/tsutterley/geoid-toolkit/pull/21) refactor: single implicit import of geoid toolkit (https://github.com/tsutterley/geoid-toolkit/pull/23) feat: output average Earth's density in ref_ellipsoid (https://github.com/tsutterley/geoid-toolkit/pull/25

tsutterley/IS2view: v0.0.5

<p><strong>Summary:</strong> Updates for Release-3 of the ATL14 and ATL15 product. Adds granule merging to allow visualization of the new Antarctic sub-region granules. Adds dask functionality for reading multiple granules in parallel. Thanks @wsauthoff for help and feedback with this release!</p> <p><strong>Itemized Changes:</strong></p> <ul> <li><code>feat</code>: add case for streaming from s3 using <code>h5netcdf</code> #31</li> <li><code>docs</code>: add <code>conda</code> version badge :tada: #31</li> <li><code>feat</code>: add prototype regions and lags for R003 #31</li> <li><code>feat</code>: add merging of <code>xarray</code> datasets for R003 Antarctica #32</li> <li><code>docs</code>: add plot Release-3 Antarctic subregions #32</li> <li><code>feat</code>: can query multiple granules for merging #32</li> <li><code>docs</code>: switch to RTD <code>build.os</code> #33</li> <li><code>ci</code>: use <code>mamba</code> for CI builds #33</li> <li><code>feat</code>: add option for viewing full screen leaflet map #33</li> <li><code>feat</code>: add option to specify the start and end cycle for a local granule #33</li> <li><code>fix</code>: assert cycle is length 2 when querying #33</li> <li><code>feat</code>: add shortcuts for merging Arctic and Antarctic regions #33</li> <li><code>ci</code>: bump GitHub actions to latest versions #33</li> <li><code>refactor</code>: create wrapper function <code>open_dataset</code> #33</li> <li><code>refactor</code>: pass through <code>HBox</code> and <code>VBox</code> to <code>widgets</code> #33</li> <li><code>docs</code>: use micromamba for RTD builds #33</li> <li><code>feat</code>: add delayed reads with dask for multiple granules #33</li> <li><code>fix</code>: filter CMR request type using regular expressions #33</li> </ul> <p><strong>Full Changelog</strong>: https://github.com/tsutterley/IS2view/compare/0.0.4...0.0.5</p&gt

tsutterley/pyTMD: v2.0.9

<p><strong>Summary:</strong> Number of fixes to the code base. Fixes the units for TPXO9-atlas currents (thanks @cywhale!). Replaces the deprecated <code>pkg_resources</code> (thanks @mrsiegfried!).</p> <p><strong>Itemized Changes:</strong></p> <ul> <li><code>fix</code>: scaling factors for TPXO9-atlas currents for #241 (https://github.com/tsutterley/pyTMD/pull/243)</li> <li><code>refactor</code>: renamed tidal ellipse function (https://github.com/tsutterley/pyTMD/pull/243)</li> <li><code>refactor</code>: renamed constituent parameters function (https://github.com/tsutterley/pyTMD/pull/243)</li> <li><code>refactor</code>: renamed check tide model points function (https://github.com/tsutterley/pyTMD/pull/243)</li> <li><code>feat</code>: can read from netCDF4 or HDF5 variable groups (https://github.com/tsutterley/pyTMD/pull/249)</li> <li><code>fix</code>: spelling mistakes (https://github.com/tsutterley/pyTMD/pull/249)</li> <li><code>feat</code>: add generic wrapper function for reading ATLAS constituents (https://github.com/tsutterley/pyTMD/pull/250)</li> <li><code>feat</code>: can write datetime as time column for csv files (https://github.com/tsutterley/pyTMD/pull/252)</li> <li><code>fix</code>: replace deprecated <code>pkg_resources</code> with <code>importlib</code> (https://github.com/tsutterley/pyTMD/pull/256)</li> </ul> <p><strong>Full Changelog</strong>: https://github.com/tsutterley/pyTMD/compare/2.0.8...2.0.9</p&gt

Remote Sensing Observations of Modern-Day Regional Ice Sheet Change

The Earth's great ice sheets are losing mass at accelerating levels, rising global sea levels and posing a significant problem to society. The ice sheets contain enough water to raise sea level by 65 meters, and are the largest reservoirs of freshwater on the planet. Measurements of current ice sheet mass change are important in order to assess their current contribution to sea level rise, and to constrain future projections. There are three general approaches for measuring the current mass balance of ice sheets: the gravimetric method using time-variable gravity measurements, the altimetric method combining surface elevation change measurements with estimates of the density change, and the mass budget method combining rates of mass input from snow and rain with rates of mass output from meltwater runoff, ice discharge and other processes. In this dissertation, we use multiple independent measurements to assess the current uncertainties in mass balance efforts, and to create new estimates of current ice sheet mass change. We investigate key regions of Antarctica, where changes in the ice sheet velocity structure have led to accelerating mass losses. We compile new assessments of the mass change of the Greenland ice sheet, where increased rates of surface runoff and losses from ice sheet dynamics have dramatically shifted the mass balance regime. The work helps constrain estimation errors from GRACE, provides new constraints to ice sheet and glacial isostatic adjustment models, and helps improve our general understanding of the mechanisms driving current ice sheet mass change