nasa-gesdisc/es-publications-researchareas huggingface dataset:
Sea ice circulation around the Beaufort Gyre: The changing role of wind forcing and the sea ice state (2016)

Sea ice drift estimates from feature tracking of satellite passive microwave data are used to investigate seasonal trends and variability in the ice circulation around the Beaufort Gyre, over the multidecadal period 1980-2013. Our results suggest an amplified response of the Beaufort Gyre ice circulation to wind forcing, especially during the late 2000s. We find increasing anticyclonic ice drift across all seasons, with the strongest trend in autumn, associated with increased ice export out of the southern Beaufort Sea (into the Chukchi Sea). A flux gate analysis highlights consistency across a suite of drift products. Despite these seasonal anticyclonic ice drift trends, a significant anticyclonic wind trend occurs in summer only, driven, in-part, by anomalously anticyclonic winds in 2007. Across all seasons, the ice drift curl is more anticyclonic than predicted from a linear relationship to the wind curl in the 2000s, compared to the 1980s/1990s. The strength of this anticyclonic ice drift curl amplification is strongest in autumn and appears to have increased since the 1980s (up to 2010). In spring and summer, the ice drift curl amplification occurs mainly between 2007 and 2010. These results suggest nonlinear ice interaction feedbacks (e.g., a weaker, more mobile sea ice pack), enhanced atmospheric drag, and/or an increased role of the ocean. The results also show a weakening of the anticyclonic wind and ice circulation since 2010.

Research areas: Cryospheric Indicators
Surface velocity of the Northeast Greenland Ice Stream (NEGIS): (2020)

Abstract. The Northeast Greenland Ice Stream (NEGIS) extends around
600 km upstream from the coast to its onset near the ice divide in interior
Greenland. Several maps of surface velocity and topography of interior
Greenland exist, but their accuracy is not well constrained by in situ
observations. Here we present the results from a GPS mapping of surface
velocity in an area located approximately 150 km from the ice divide near
the East Greenland Ice-core Project (EastGRIP) deep-drilling site. A GPS
strain net consisting of 63 poles was established and observed over the
years 20152019. The strain net covers an area of 35 km by 40 km, including
both shear margins. The ice flows with a uniform surface speed of
approximately 55 m a1 within a central flow band with longitudinal and
transverse strain rates on the order of 104 a1 and increasing by
an order of magnitude in the shear margins. We compare the GPS results to
the Arctic Digital Elevation Model and a list of satellite-derived surface
velocity products in order to evaluate these products. For each velocity
product, we determine the bias in and precision of the velocity compared to the
GPS observations, as well as the smoothing of the velocity products needed
to obtain optimal precision. The best products have a bias and a precision
of 0.5 m a1. We combine the GPS results with
satellite-derived products and show that organized patterns in flow and
topography emerge in NEGIS when the surface velocity exceeds
approximately 55 m a1 and are related to bedrock topography.

Research areas: Cryospheric Indicators
Sensitivity of Arctic sea ice to melt pond processes and atmospheric forcing: A model study (2021)

Melt ponds are pools of meltwater forming principally on Arctic sea ice during the melt season. The albedo of melt ponds is a key component of the surface energy balance. For this reason, various melt pond schemes have been developed for climate models. These schemes require assumptions on the physical processes governing melt ponds as well as a knowledge of the atmospheric state, which are not perfectly known. In this study, we investigate the effects of the sources of uncertainty from the prescribed atmospheric surface state, the melt pond scheme definition and the refreezing formulation of melt ponds on the simulated Arctic sea ice and melt pond properties with the NEMO-LIM3 oceansea ice general circulation model. We find that the simulated melt pond state is largely controlled by the freezing point of melt ponds. The representation of melt ponds is in better agreement with observations when using the freezing point of 0.15C compared to the value of 2.00C, in our model set-up. All the simulations feature positive trends in melt pond area fraction over the past decades. However, only 3 out of 8 simulations have significant positive trends in melt pond volume per sea ice area. This suggests an influence of the sea ice state for melt ponds over the last 30 years. Overall, we find that the simulated sea ice state, and in particular sea ice volume, is more affected by changes in the prescribed atmospheric forcing than by changes in the prescribed melt pond scheme or refreezing formulation. Including explicit melt pond schemes in large-scale sea-ice models offer the possibility to improve the representation of the surface energy balance in climate general circulation models. Our results underline that, in parallel to these efforts in model developments, improved estimates of surface atmospheric conditions will be required to achieve more realistic sea ice states.

Research areas: Cryospheric Indicators
The cooling signature of basal crevasses in a hard-bedded region of the Greenland Ice Sheet (2021)

Abstract. Temperature sensors installed in a grid of nine full-depth boreholes drilled in the southwestern ablation zone of the Greenland Ice Sheet recorded cooling in discrete sections of ice over time within the lowest third of the ice column in most boreholes. Rates of temperature change outpace cooling expected from vertical conduction alone. Additionally, observed temperature profiles deviate significantly from the site-average thermal profile that is shaped by all thermomechanical processes upstream. These deviations imply recent, localized changes to the basal thermal state in the boreholes. Although numerous heat sources exist to add energy and warm ice as it moves from the central divide towards the margin such as strain heat from internal deformation, latent heat from refreezing meltwater, and the conduction of geothermal heat across the icebedrock interface, identifying heat sinks proves more difficult. After eliminating possible mechanisms that could cause cooling, we find that the observed cooling is a manifestation of previous warming in near-basal ice. Thermal decay after latent heat is released from freezing water in basal crevasses is the most likely mechanism resulting in the transient evolution of temperature and the vertical thermal structure observed at our site. We argue basal crevasses are a viable englacial heat source in the basal ice of Greenland's ablation zone and may have a controlling influence on the temperature structure of the near-basal ice.

Research areas: Cryospheric Indicators
Unravelling the high-altitude Nansen blue ice field meteorite trap (East Antarctica) and implications for regional palaeo-conditions (2019)

Antarctic blue ice zones, the most productive locations for meteorite recovery on Earth, contain old ice that is easily accessible and available in large quantities. However, the mechanisms behind these meteorite traps remain a topic of ongoing debate. Here, we propose an interdisciplinary approach to improve our understanding of a meteorite trap in Dronning Maud Land (East Antarctica) on the Nansen blue ice field meteorite trap (26003100 m above sea level), where more than half of the Asuka meteorites have been collected. Based on 185 surface blue ice samples, one of the largest observed spatial patterns in oxygen isotopic variation to date is found. Relying on meteorites for which the terrestrial ages are determined using 14C and 36Cl, this surface ice is interpreted to date from the Last Interglacial up to the present-day. By combining state-of-the-art satellite derived surface velocities, surface mass balance modelling and ice flow modelling, we estimate that about 7585% of the meteorites found on the ice field were supplied by ice flow after entering the ice sheet in an accumulation area of a few hundred square kilometres located south (upstream) of the ice field. Less than 0.4 new meteorites per year are supplied to the ice field through ice flow, suggesting that the hundreds of meteorites found 25 years after the first visit to this ice field mostly represent meteorites that were previously not found, rather than newly supplied meteorites. By combining these findings, the infall rate of meteorites from space is estimated, which is in line with values from the literature, but situated at the higher end of the range. A comparison of the oxygen isotopic variation of the surface blue ice to that of the European Project for Ice Coring in Antarctica (EPICA), Dronning Maud Land (EDML) ice core (located 750 km to the west, at the same elevation), suggests that the regional changes in topography have been relatively limited since the Last Interglacial, supporting theories of an overall stable East Antarctic Ice Sheet (EAIS) over this time period.

Research areas: Validation, Cryospheric Indicators, Environmental Impacts
Towards understanding the pattern of glacier mass balances in High (2020)

Abstract. Glaciers in High Mountain Asia (HMA) provide an important water
resource for communities downstream, and they are markedly impacted by global
warming, yet there is a lack of understanding of the observed glacier mass
balances and their spatial variability. In particular, the glaciers in the
western Kunlun Shan and Karakoram (WKSK) ranges show neutral to positive
mass balances despite global warming. Using models of the regional climate
and glacier mass balance, we reproduce the observed patterns of glacier mass
balance in High Mountain Asia of the last decades within uncertainties. We
show that low temperature sensitivities of glaciers and an increase in
snowfall, for a large part caused by increases in evapotranspiration from
irrigated agriculture, result in positive mass balances in the WKSK. The pattern
of mass balances in High Mountain Asia can thus be understood from the
combination of changes in climatic forcing and glacier properties, with an
important role for irrigated agriculture.

Research areas: Cryospheric Indicators
Quantifying vulnerability of Antarctic ice shelves to hydrofracture using microwave scattering properties (2018)

Recent ice shelf disintegrations on the Antarctic Peninsula and subsequent increases in ice sheet mass loss have highlighted the importance of tracking ice shelf stability with respect to surface melt ponding and hydrofracture. In this study, we use active microwave scatterometry in time-series to estimate melt season duration, and winter backscatter levels as a proxy for relative concentration of refrozen ice lenses in Antarctic ice shelf firn. We demonstrate a physical relationship between melt days and firn/ice backscatter using scatterometry and field data from Greenland, and apply the observed relationship to derive and map a vulnerability index for Antarctica's ice shelves. The index reveals that some remaining Antarctic Peninsula ice shelves have already reached a firn state that is vulnerable to hydrofracture. We also show that the progression of an ice shelf towards vulnerability is affected by many factors, such as surface mass balance, internal stresses, and ice shelf geometry.

Research areas: Cryospheric Indicators
Potential microbial enzyme activity in seasonal snowpack is high and (2022)

Microbes in snow and ice ecosystems in polar regions contribute substantially to C, N, and P cycling, but few studies have explored microbial activity in seasonal snow. The purpose of this study was to explore the relative importance of snow microbial processing of C, N, and P compounds in atmospheric deposition and litter and detect elemental limitations of snow microbes in Rocky Mountain conifer forests. Enzyme activity in snow was orders of magnitude greater than activity reported for lentic and lotic waters in similar environments. Proportions of C/P‑ and C/N‑acquiring enzymes suggest that snow samples were P limited, or C and P co‑limited, while lentic and lotic waters were more N limited. As such, microbes in seasonal snow may change the composition of nutrients and carbon, but these processes are vulnerable to changes in atmospheric deposition and snow extent and duration, which could affect nutrient processing across large areas.

Research areas: Ecosystems
Trends in Winter Light Environment Over the Arctic Ocean: A Perspective (2020)

The last few decades have seen a decrease in Arctic ice cover, leading to changes in the structure and function of marine ecosystems. Yet sustained long-term observations of the marine environment are difficult to acquire. Harsh environments limit in situ measurements, while low light and high solar angles hinder ocean color observations from satellite. Here we use masks of valid-invalid ocean color pixels to diagnose ocean conditions and find strong positive trends in the Arctic open-water season close to the ice sheet, generally consistent with sea ice products from satellites. The North East Atlantic with no seasonal ice cover shows weaker, but significant trends indicative of decreasing winter cloud cover. Decreases in both sea ice and cloud cover will increase light availability at the sea surface and potentially enhance phytoplankton growth. Our method allows the winter light conditions to be studied at temporal and spatial scales relevant for phytoplankton dynamics.

Research areas: Cryospheric Indicators
Greenland Ice Sheet solid ice discharge from 1986 through March 2020 (2020)

Abstract. We present a 1986 through March 2020 estimate of Greenland Ice Sheet ice discharge. Our data include all discharging ice that flows faster than 100 m yr1 and are generated through an automatic and adaptable method, as opposed to conventional handpicked gates. We position gates near the present-year termini and estimate problematic bed topography (ice thickness) values where necessary. In addition to using annual time-varying ice thickness, our time series uses velocity maps that begin with sparse spatial and temporal coverage and end with near-complete spatial coverage and 12 d updates to velocity. The 2010 through 2019 average ice discharge through the flux gates is 48749 Gt yr1. The 10 % uncertainty stems primarily from uncertain ice bed location (ice thickness). We attribute the 50 Gt yr1 differences among our results and previous studies to our use of updated bed topography from BedMachine v3. Discharge is approximately steady from 1986 to 2000, increases sharply from 2000 to 2005, and then is approximately steady again. However, regional and glacier variability is more pronounced, with recent decreases at most major glaciers and in all but one region offset by increases in the northwest region through 2017 and in the southeast from 2017 through March 2020. As part of the journal's living archive option and our goal to make an operational product, all input data, code, and results from this study will be updated as needed (when new input data are available, as new features are added, or to fix bugs) and made available at https://doi.org/10.22008/promice/data/ice_discharge (Mankoff, 2020a) and at https://github.com/mankoff/ice_discharge (last access: 6 June 2020, Mankoff, 2020e).

Research areas: Cryospheric Indicators
Estimating Regional Snow Line Elevation Using Public Webcam Images (2022)

Snow cover is of high relevance for the Earths climate system, and its variability plays a key role in alpine hydrology, ecology, and socioeconomic systems. Measurements obtained by optical satellite remote sensing are an essential source for quantifying snow cover variability from a local to global scale. However, the temporal resolution of such measurements is often affected by persistent cloud coverage, limiting the application of high resolution snow cover mapping. In this study, we derive the regional snow line elevation in an alpine catchment area using public webcams. We compare our results to the snow line information derived from the Moderate-Resolution Imaging Spectroradiometer (MODIS) and Sentinel-2 snow cover products and find our results to be in good agreement therewith. Between October 2017 and the end of June 2018, snow lines derived from webcams lie on average 55.8 m below and 33.7 m above MODIS snow lines using a normalized-difference snow index (NDSI) of 0.4 and 0.1, respectively, and are on average 53.1 m below snow lines derived from Sentinel-2. We further analyze the superior temporal resolution of webcam-based snow cover information and demonstrate its effectiveness in filling temporal gaps in satellite-based measurements caused by cloud cover. Our findings show the ability of webcam-based snow line elevation retrieval to complement and improve satellite-based measurements.

Research areas: Cryospheric Indicators
Weekly to monthly terminus variability of Greenland's marine-terminating outlet glaciers (2023)

Abstract. Seasonal terminus-position variability of Greenland's marine-terminating outlet glaciers is superimposed on multidecadal trends of glacier retreat. To characterize this seasonal variability, we manually digitized terminus positions for 219 marine-terminating glaciers in Greenland from January 2015 through December 2021 using Sentinel-1 synthetic aperture radar (SAR) mosaics. We digitized at a monthly frequency for 199 glaciers and at a 6 d frequency for 20 glaciers. We found that nearly 80 % of glacier termini in Greenland vary significantly on a seasonal basis. For these seasonally varying glaciers, on average, seasonal retreat typically begins in mid-May, and seasonal advance generally commences in early October. The timing of the initiation of the retreat period may be related to the timing of the onset of ice-sheet surface melt. The rate of retreat events peaks in late summer and reaches a minimum in late winter and early spring. The median magnitude of terminus-position seasonality, the difference between glacier length at the dates of peak advance and retreat, is about 220 m. We find a stronger correlation between this magnitude and glacier velocity than between magnitude and glacier width. Terminus-position seasonality can influence longer-term glacier dynamics and, consequently, ice-sheet mass balance. This study contributes to our understanding of terminus-position seasonality for individual glaciers and collectively for glaciers around the entire Greenland Ice Sheet.

Research areas: Cryospheric Indicators
Retrieval and parameterisation of sea-ice bulk density from airborne multi-sensor measurements (2022)

Abstract. Knowledge of sea-ice thickness and volume depends on freeboard observations from satellite altimeters and in turn on information of snow mass and sea-ice density required for the freeboard-to-thickness conversion. These parameters, especially sea-ice density, are usually based on climatologies constructed from in situ observations made in the 1980s and earlier while contemporary and representative measurements are lacking. Our aim with this paper is to derive updated sea-ice bulk density estimates suitable for the present Arctic sea-ice cover and a range of ice types to reduce uncertainties in sea-ice thickness remote sensing. Our sea-ice density measurements are based on over 3000 km of high-resolution collocated airborne sea-ice and snow thickness and freeboard measurements in the western Arctic Ocean in 2017 and 2019. Sea-ice bulk density is derived assuming isostatic equilibrium for different ice types. Our results show higher average bulk densities for both first-year ice (FYI) and especially multi-year ice (MYI) compared to previous studies. In addition, we find a small difference between deformed and possibly unconsolidated FYI and younger MYI. We find a negative-exponential relationship between sea-ice bulk density and sea-ice freeboard and apply this parameterisation to one winter of monthly gridded CryoSat-2 sea-ice freeboard data. We discuss the suitability and the impact of the derived FYI and MYI bulk densities for sea-ice thickness retrievals and the uncertainty related to the indirect method of measuring sea-ice bulk density. The results suggest that retrieval algorithms be adapted to changes in sea-ice density and highlight the need of future studies to evaluate the impact of density parameterisation on the full sea-ice thickness data record.

Research areas: Cryospheric Indicators
Glacier surface speed variations on the Kenai Peninsula, Alaska, 2014–2019 (2022)

To characterize the spatiotemporal variations of glacier surface speed on the Kenai Peninsula, Alaska (∼3,900 km<SUP>2</SUP>), we derived 92 surface speed fields between October 2014 and December 2019 using intensity offset tracking on Sentinel-1 data. On average, speeds are 50% greater in spring (March-May) than the annual mean (69 m a<SUP>−1</SUP>) while winter speeds are close to the annual mean. While marine-terminating glaciers have their maximum speed near the terminus, both land- and lake-terminating glaciers flow fastest around the median glacier elevation. On average, the lake-terminating and tidewater glaciers flow 1.7 and 2.3 times faster than the land-terminating glaciers, respectively. Monthly variations over the 5-year period are strikingly synchronous regardless of terminus type suggesting that regional-scale meteorological drivers govern the temporal variability. Mean annual speeds fluctuate roughly ±10% of the period mean without an apparent trend. At lake-terminating Bear Glacier, a short-term tripling in ice speed in fall 2019 over the area below an ice-dammed lake coincides with an observed glacier lake outburst flood (GLOF). An earlier GLOF caused a persistent breach of the beach barrier between the proglacial lake and ocean which likely led to overall speed-up of the lower glacier part throughout 2019. A significant speedup was also observed at the lower part of the lake-terminating Ellsworth Glacier and attributed to rapid glacier retreat and lake expansion, probably further amplified by the terminus area becoming buoyant and a large tabular iceberg breaking off. Our results highlight the impact of GLOFs and proglacial characteristics in spatial and temporal glacier speed variations.

Research areas: Cryospheric Indicators