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Atmospheric rivers (ARs) are long, narrow bands of warm and moist air that travel poleward from the midlatitudes. While Antarctic atmospheric rivers (ARs) occur only 1-3% of the time over the ice sheet, they are a significant contributor to Antarctic surface mass balance: they contribute 10% on average, and more than 20% locally, of Antarctic precipitation each year. Here we use an Antarctic-specific AR-detection algorithm to identify ARs in MERRA-2 and ERA5 reanalyses and the Community Earth System Model version 2 (CESM2). We use this algorithm to quantify the frequency, location, and precipitation attributed to Antarctic ARs for the period 1980-2014 and use these statistics to identify CESM2 biases relative to MERRA-2 and ERA5. We then apply the AR-detection algorithm to CESM2 for the future period (2015-2100) to examine how the frequency and intensity of ARs, AR-attributed total precipitation, and year-to-year variability in AR precipitation changes in the future under the SSP370 emissions scenario. Our results quantify past and future impacts of ARs on Antarctic annual precipitation, interannual variability, and trends, and ultimately provide an early assessment of future AR-driven changes in Antarctic surface mass balance.

The Antarctic Peninsula (AP) experienced a new extreme warm event and record high surface melt in February 2022, rivaling the recent temperature records from 2015 and 2020, and contributing to an alarming series of extreme warm events there. The northern/northwestern AP was directly impacted by an intense atmospheric river (AR) bringing anomalous heat and rainfall, while AR-enhanced foehn effect further warmed its northeastern side. The event was triggered by multiple large-scale atmospheric circulation patterns linking the AR formation to tropical convection anomalies and stationary Rossby waves, with anomalous Amundsen Sea low and record-breaking blocking high. The cascade of impacts culminated in widespread and intensive surface melt across the AP. The event was statistically attributed to global warming. Increasing frequency of such events can undermine the stability of the AP ice shelves, with multiple local to global impacts, including acceleration of the AP ice mass loss and changes in sensitive ecosystems.

The grain size of the superficial snow layer is a key determinant of the surface albedo in Antarctica. Its evolution is the result of multiple interacting processes, such as dry and wet metamorphism, melt, snow drift and precipitation. Among them, snow drift has the least known and least predictable impact. The goal of this study is to relate the variations of surface snow grain size to these processes in a windy location of the Antarctic coast. For this, we retrieved the daily grain size from 5 year-long in-situ observations of the spectral albedo recorded by a new multi-band albedometer, unique in terms of autonomy and described here for the first time. An uncertainty assessment and a comparison with satellite-retrieved grain size were carried out to verify the reliability of the instrument and an RMSE up to 0.16 mm on the observed grain size was found. By relating these in-situ measurements to timeseries of snow drift, surface temperature, snow surface height and snowfall, we established that the evolution of the grain size in the presence of snow drift is complex and follows two possible pathways: 1) A decrease in the grain size (about half of our measurements) resulting from the deposition of small grains advected by the wind. Surprisingly, this decreases is often (2/3 of the cases) associated with a decrease of the surface height, i.e. a net erosion over the drift episode, 2) an increase of the grain size (the other half) either due to the removal of the surface layer, or metamorphism. However, we note that this increase is often limited with respect to the increase predicted by a theoretical metamorphism model, suggesting that a concomitant deposition of small grains is likely. At last, we found that wind also completely impedes the deposition of snowfall during half of the observed precipitation events. When this happens, the grain size evolves as if precipitation was not occurring. As a result of all these processes, we conclude that the grain size in a windy area remains more stable than it would be in the absence of snow drift, hence limiting the variations of the albedo and of the radiative energy budget.

Sur une grande partie de l'Antarctique, le bilan de masse (c'est-à-dire de neige) de surface est dominé par quelques événements de précipitations extrêmes. Ces événements dépendent d'intrusions de masses d'air très humide associées à des phénomènes dénommés rivières atmosphériques en provenance de l'océan Austral. Ces rivières atmosphériques influencent fortement le climat ; pourtant, les caractéristiques, les mécanismes et les impacts associés restent mal connus en Antarctique. Nous résumons ici l'état des connaissances sur la mise en place de ces événements extrêmes et leurs impacts à la fois sur l'accumulation de neige, le réchauffement et la fonte en surface de la calotte. Over much of Antarctica, the surface mass balance (i.e. the resultant of snow fluxes at the surface of the ice sheet) is dominated by a few extreme precipitation events. It has recently been shown that these events are linked to intrusions of highly humid air masses related with atmospheric rivers traversing the Southern Ocean. These atmospheric rivers strongly influence the climate, yet their meteorological characterization and associated impacts remain poorly understood in Antarctica. We summarize here the latest research regarding the development of these extreme events and their impacts on snow accumulation, warming, and surface melt on the Antarctic ice sheet.

Stable water isotopes are a tracer of hydrological processes and a paleoclimate proxy from ice core records. The interpretation of the latter relies on fractionation processes throughout the hydrological cycle, from the evaporation over the ocean, during each precipitation event, and during post-deposition processes, in particular due to the exchanges between the snow and the moisture in the atmosphere. Thanks to new developments in infrared spectroscopy, it is now possible to monitor not only the snow isotopic composition but also the vapour continuously, and thus document exchanges between the snow and the vapour. On the East Antarctic Plateau, records of water vapour isotopic composition in Kohnen and Dome C during summer have revealed significant diurnal variability which can be used to address the exchange between surface snow and atmospheric water vapour as well as the stability of the atmospheric boundary layer. In this study, we present the first vapour monitoring on a transect across East Antarctica for a period of 3 months from November 2019 to February 2020 during the EAIIST traverse, covering more than 3600 km. In parallel, we also monitored the vapour isotopic composition at two stations: Dumont D’Urville (DDU), the starting point, and Dome C, half way through. Efforts on the calibration on each monitoring station, as well as cross-calibration of the different instruments offer a unique opportunity to compare both the spatial and temporal (diurnal variability or at the scale of several days) gradients of humidity, temperature and water vapour isotopic composition in East Antarctica during the summer season. With the use of the Modele Atmospherique Régional (MAR), we compare the variability measured in water vapour isotopic composition, temperature and humidity with the different systems (fixed or mobile location). Although further comparisons with the surface snow isotopic composition are required to quantify the impact of the snow-atmosphere exchanges on the local surface mass balance, these three simultaneous measurements of the vapour isotopic composition show the potential of using water stables isotopes to evaluate hydrological processes in East Antarctica.

The climate of the polar regions is characterized by large fluctuations and has experienced dramatic changes over the past decades. In particular, the patterns of changes in sea ice and ice sheet mass are complex in the Southern Hemisphere. The Antarctic Ice Sheet has also lost mass in the past decades, especially in Western Antarctica, with a spectacular thinning and weakening of ice shelves, i.e., the floating extensions of the grounded ice sheet. Despite recent advances in observing and modelling the Antarctic climate, the mechanisms behind this long-term mass loss remain poorly understood because of the limited amount of observations and the large biases of climate models in polar regions, in concert with the large internal variability prevailing in the Antarctic. Among all the processes involved in the mass variability, changes in the general atmospheric circulation of the Southern Hemisphere may have played a substantial role. One of the most important atmospheric modes of climate variability in the Southern Ocean is the Southern Annular Mode (SAM), which represents the position and the strength of the westerly winds. During years with a positive SAM index, lower sea level pressure at high latitudes and higher sea level pressure at low latitudes occur, resulting in a stronger pressure gradient and intensified Westerlies. However, the current knowledge of the impact of these fluctuations of the Westerlies on the Antarctic cryosphere is still limited. Over the past few years, some efforts investigated the impact of the SAM on the Antarctic sea ice and the surface mass balance of the ice sheet from an atmosphere-only perspective. Recently, a few oceanic studies have focused on the local impact of SAM-related fluctuations on the ice-shelf basal melt in specific regions of Antarctica, particularly Western Antarctica. However, to our knowledge, there is no such study at the scale of the whole Antarctic continent. In this study, we performed idealized experiments with a pan-Antarctic regional ice-shelf cavity-resolving ocean - sea-ice model for different phases of the SAM. We show that positive (negative) phases lead to increased (decreased) upwelling and subsurface ocean temperature and salinity close to ice shelves. A one-standard-deviation increase of the SAM leads to a net basal mass loss of 40 Gt yr-1, with strong regional contrasts: increased melt in the Western Pacific and Amundsen-Bellingshausen sectors and the opposite response in the Ross sector. Taking these as a baseline sensitivity, we estimate last millennium and end-of-21st-century ice-shelf basal melt changes due to SAM of -60.7 Gt yr-1 and 1.8 to 26.8 Gt yr-1 (depending on the emission scenario considered), respectively, compared to the present.

The mass balance of Antarctica is sensitive to intrusions of extremely warm, moist airmasses from the mid-latitudes in the form of atmospheric rivers (ARs). These storms provide a sub-tropical link to the Antarctic continent and engender extreme atmospheric conditions that are largely consequential to surface melt, snowfall, and ice-shelf stability. Using an AR detection algorithm designed for polar regions, we characterize the AR life cycle and describe the atmospheric conditions conducive for ARs to reach the Antarctic continent.Despite their rarity of occurrence over Antarctica (maximum frequency of ~3 days per year over a given point), ARs have a relatively large impact on the surface melt processes in West Antarctica and snowfall patterns across the whole continent. During the summer season along the Antarctic Peninsula ice shelves, AR landfalls lead to conditions (i.e. extreme temperatures, rainfall, surface melt, sea-ice clearing, ocean swell enhancement), that act to destabilize the leeward ice shelves. Current research is exploring the origins of AR genesis and moisture pathways with a focus on the relationship between atmospheric blocking in the Southern Ocean and AR behavior over East Antarctica.When examining the life cycles of ARs and non-AR synoptic analogues occurring at Dumont d’Urville (DDU) Station, Antarctica, the AR events often have moisture sources further north in the Southern Ocean than the non-AR analogues. These more northern moisture sources correspond with enhanced latent heat release over anomalously warm sea surface temperatures in northern regions of the Southern Ocean which trigger Rossby wave propagation that enhances upper-level potential vorticity. A highly amplified wave pattern allows for intense poleward moisture transport towards DDU and downstream ridging from the AR position. Thus, any future changes in atmospheric blocking or tropical-polar teleconnections, which control AR behavior around Antarctica, along with further global warming, may have significant impacts on future mass balance projections and subsequent sea level changes.

Monitoring the surface albedo of snow is crucial for assessing the Earth energy budget in polar and high-mountain areas. Multiple algorithms are thus proposed to compute surface albedo from satellite observations. Still, satellite-retrieved information is limited by scarce calibration/validation sites in remote areas. Ground-based observations guarantee the continuity of measurements under cloudy weather and serve as a cal/val tool, but deployment of autonomous instruments is limited in remote cold areas. In this context, we introduce the Multiband albedometer: a new instrument that performs autonomous albedo measurements. It is battery-operated with low energy consumption, hence suited for long-term monitoring in polar regions. Measured snow spectral albedo and Specific Surface Area (SSA) time series are here used to validate the Pre-operational Sentinel-3 Snow and Ice products (SICE), derived from Copernicus Sentinel-3 Ocean and Land Color Instrument (OLCI) and yet not fully validated on snow. The Multiband albedometer includes two radiometric cosine detectors facing the sky and snow surface respectively. Each channel measures the incoming radiation in 7 selected bands in the visible to short-wave infrared spectrum. Processing includes corrections for the sensor dark current, the imperfect cosine response and cross-calibration of the two channels, the effect of surface slope and sensor tilt. Two multiband albedometers were installed close to the French station of Dumont d’Urville, in Adélie Land in East Antarctica, near the snow equilibrium line and in the upper accumulation area. Albedo and SSA time series were extracted from data collected during austral summers between January 2017 and January 2021. A new cloud mask developed from Sentinel-3 SLSTR products was used to validate the day-to-day weather and validate SICE at the same location. The good agreement between Sentinel-3 derived data and in-situ measured SSA highlights the potential of the Multiband albedometer for calibration and validation of satellite-retrieved snow albedo and SSA in remote regions.

Understanding the interannual variability of surface mass balance (SMB) and surface melting in Antarctica is key to quantify the signal-to-noise ratio in climate trends, identify opportunities for multi-year climate predictions and assess the ability of climate models to respond to climate variability. Here we simulate summer SMB and surface melting from 1979 to 2017 using the Regional Atmosphere Model (MAR) at 10 km resolution over the drainage basins of the Amundsen Sea glaciers in West Antarctica. Our simulations reproduce the mean present-day climate in terms of near-surface temperature (mean overestimation of 0.10 ∘C), near-surface wind speed (mean underestimation of 0.42 m s−1), and SMB (relative bias  % over Thwaites glacier). The simulated interannual variability of SMB and melting is also close to observation-based estimates. For all the Amundsen glacial drainage basins, the interannual variability of summer SMB and surface melting is driven by two distinct mechanisms: high summer SMB tends to occur when the Amundsen Sea Low (ASL) is shifted southward and westward, while high summer melt rates tend to occur when ASL is shallower (i.e. anticyclonic anomaly). Both mechanisms create a northerly flow anomaly that increases moisture convergence and cloud cover over the Amundsen Sea and therefore favors snowfall and downward longwave radiation over the ice sheet. The part of interannual summer SMB variance explained by the ASL longitudinal migrations increases westward and reaches 40 % for Getz. Interannual variation in the ASL relative central pressure is the largest driver of melt rate variability, with 11 % to 21 % of explained variance (increasing westward). While high summer SMB and melt rates are both favored by positive phases of El Niño–Southern Oscillation (ENSO), the Southern Oscillation Index (SOI) only explains 5 % to 16 % of SMB or melt rate interannual variance in our simulations, with moderate statistical significance. However, the part explained by SOI in the previous austral winter is greater, suggesting that at least a part of the ENSO–SMB and ENSO–melt relationships in summer is inherited from the previous austral winter. Possible mechanisms involve sea ice advection from the Ross Sea and intrusions of circumpolar deep water combined with melt-induced ocean overturning circulation in ice shelf cavities. Finally, we do not find any correlation with the Southern Annular Mode (SAM) in summer.

25 pages; International audience; Here, we define weather regimes in the East Antarctica—Southern Ocean sector based on daily anomalies of 700 hPa geopotential height derived from ERA5 reanalysis during 1979–2018. Most regimes and their preferred transitions depict synoptic-scale disturbances propagating eastwards off the Antarctic coastline. While regime sequences are generally short, their interannual variability is strongly driven by the polarity of the Southern Annular Mode (SAM). Regime occurrences are then intersected with atmospheric rivers (ARs) detected over the same region and period. ARs are equiprobable throughout the year, but clearly concentrate during regimes associated with a strong atmospheric ridges/blockings on the eastern part of the domain, which act to channel meridional advection of heat and moisture from the lower latitudes towards Antarctica. Both regimes and ARs significantly shape climate variability in Antarctica. Regimes favorable to AR occurrences are associated with anomalously warm and humid conditions in coastal Antarctica and, to a lesser extent, the hinterland parts of the Antarctic plateau. These anomalies are strongly enhanced during AR events, with warmer anomalies and dramatically amplified snowfall amounts. Large-scale conditions favoring AR development are finally explored. They show weak dependency to the SAM, but particularly strong atmospheric ridges/blockings over the Southern Ocean appear as the most favorable pattern, in which ARs can be embedded, and to which they contribute.

The disintegration of the ice shelves along the Antarctic Peninsula have spurred much discussion on the various processes leading to their eventual dramatic collapse, but without a consensus on an atmospheric forcing that could connect these processes. Here, using an atmospheric river (AR) detection algorithm along with a regional climate model and satellite observations, we show that particularly intense ARs have a ~40% probability of inducing extreme events of temperature, surface melt, sea-ice disintegration, or large swells; all processes proven to induce ice-shelf destabilization. This was observed during the collapses of the Larsen A, B, and overall, 60% of calving events triggered by ARs from 2000-2020. The loss of the buttressing effect from these ice shelves leads to further continental ice loss and subsequent sea-level rise. Understanding how ARs connect various disparate processes cited in ice-shelf collapse theories is essential for identifying other at-risk ice shelves like the Larsen C.

The detailed meteorological and drifting snow data proposed in this dataset were acquired in the framework of an extensive drifting snow observation campaign that was initiated in January 2010 on the marginal slopes of Adelie Land, East Antarctica. With strong, frequent and persistent katabatic winds occurring year long, the measurement area is among the most prone to snow transport by wind on the ice sheet. Available data collected almost continuously at a half-hourly resolution at two locations 100 km apart include 2-m wind speed and direction, 2-m air temperature, 2-m air relative humidity with respect to ice and drifting snow mass fluxes. The data files contain the following variables and units: YYYY: year; MM: month; DD: day;hh: hour; mm: minute;U2m: 2-m wind speed(m/s): zU: actual height of the anemometer (m); T2m: 2-m air temperature (° C); RH2m: 2-m air relative humidity (% w.r.i.);zT:actual height of the thermo-hygrometer (m);FC1: drifting snow flux from lower level(g m-2 s-1);FC2: drifting snow flux from upperlevel(g m-2 s-1); zFC1:actual top height above ground of thelower FlowCapt sensor (m).

Dumont d'Urville station, located on the East coast of Antarctica in Adelie Land, is in one of the windiest coastal region on Earth, due to katabatic winds downslope from the East Antarctic ice sheet. In summer, the season of interest in this study, coastal weather is characterized by well-marked diel cycles in temperature and wind patterns. Our study aims at exploring the added value of water vapor stable isotopes in coastal Adelie Land to provide new information on the local atmospheric water cycle and climate. An important application is the interpretation of water isotopic profiles in snow and ice cores recently drilled in Adelie Land. We present the first continuous measurements of δ 18 O and d-excess in water vapor over Adelie Land. During our measurements period (26/12/2016 to 03/02/2017), we observed clear diel cycles in terms of temperature, humidity and isotopic composition. The cycles in isotopic composition are particularly large given the muted variations in temperature when compared to other Antarctic sites where similar monitoring have been performed. Based on data analyses and simulations obtained with the regional MAR model on the coastal Adelie Land, we suggest that the driver for δ 18 O and d-excess diel variability in summer at Dumont d'Urville is the variation of the strength of the wind coming from the continent: the periods with strong wind are associated with the arrival of relatively dry air with water vapor associated with low δ 18 O and high d-excess from the Antarctic plateau. Finally, in addition to the interpretation of snow and ice core isotopic profiles in the coastal regions, our study has implications for the evaluation of atmospheric models equipped with water isotopes.

A new 21.3 m firn core was drilled in 2015 at a coastal Antarctic high-accumulation site in Adélie Land (66.78∘ S; 139.56∘ E, 602 m a.s.l.), named Terre Adélie 192A (TA192A). The mean isotopic values (-19.3‰±3.1 ‰ for δ18O and 5.4 ‰±2.2 ‰ for deuterium excess) are consistent with other coastal Antarctic values. No significant isotope–temperature relationship can be evidenced at any timescale. This rules out a simple interpretation in terms of local temperature. An observed asymmetry in the δ18O seasonal cycle may be explained by the precipitation of air masses coming from the eastern and western sectors in autumn and winter, recorded in the d-excess signal showing outstanding values in austral spring versus autumn. Significant positive trends are observed in the annual d-excess record and local sea ice extent (135–145∘ E) over the period 1998–2014. However, process studies focusing on resulting isotopic compositions and particularly the deuterium excess–δ18O relationship, evidenced as a potential fingerprint of moisture origins, as well as the collection of more isotopic measurements in Adélie Land are needed for an accurate interpretation of our signals.

The Antarctic ice sheet mass balance is a major component of the sea level budget and results from the difference of two fluxes of a similar magnitude: ice flow discharging in the ocean and net snow accumulation on the ice sheet surface, i.e. the surface mass balance (SMB). Separately modelling ice dynamics and SMB is the only way to project future trends. In addition, mass balance studies frequently use regional climate models (RCMs) outputs as an alternative to observed fields because SMB observations are particularly scarce on the ice sheet. Here we evaluate new simulations of the polar RCM MAR forced by three reanalyses, ERA-Interim, JRA-55, and MERRA-2, for the period 1979–2015, and we compare MAR results to the last outputs of the RCM RACMO2 forced by ERA-Interim. We show that MAR and RACMO2 perform similarly well in simulating coast-to-plateau SMB gradients, and we find no significant differences in their simulated SMB when integrated over the ice sheet or its major basins. More importantly, we outline and quantify missing or underestimated processes in both RCMs. Along stake transects, we show that both models accumulate too much snow on crests, and not enough snow in valleys, as a result of drifting snow transport fluxes not included in MAR and probably underestimated in RACMO2 by a factor of 3. Our results tend to confirm that drifting snow transport and sublimation fluxes are much larger than previous model-based estimates and need to be better resolved and constrained in climate models. Sublimation of precipitating particles in low-level atmospheric layers is responsible for the significantly lower snowfall rates in MAR than in RACMO2 in katabatic channels at the ice sheet margins. Atmospheric sublimation in MAR represents 363 Gt yr−1 over the grounded ice sheet for the year 2015, which is 16 % of the simulated snowfall loaded at the ground. This estimate is consistent with a recent study based on precipitation radar observations and is more than twice as much as simulated in RACMO2 because of different time residence of precipitating particles in the atmosphere. The remaining spatial differences in snowfall between MAR and RACMO2 are attributed to differences in advection of precipitation with snowfall particles being likely advected too far inland in MAR.

The Kerguelen Archipelago (49°S, 69°E) is an excellent location for the study of multi-millennial glacier fluctuations, since it is the largest still glaciated emerged area (552 km2 in 2001) in the sub-Antarctic sector of the Indian Ocean, where many glacio-geomorphological formations such as moraines may be dated. To investigate the so-far little-known Late Glacial and the Holocene glacier fluctuations in Kerguelen, we apply cosmogenic nuclide dating of moraines in 3 glacial valleys: Val Travers valley, Ampere glacier valley and Arago glacier valley. We use in situ 36Cl dating of the basaltic moraine boulders at the first two sites, and 10Be dating of the quartz-bearing syenite boulders at the third site. The new 36Cl and 10Be exposure ages provide time constraints over the last 17,000 years. A glacial advance was highlighted during the Late Glacial at 14.4 ± 1.4 ka ago, probably linked to the Antarctic Cold Reversal event. These results are consistent with those previously obtained on the archipelago (Jomelli et al., 2017, 2018; Charton et al., 2020) and more generally those from other the sub-Antarctic regions (e.g. Sagredo et al., 2018). This suggests that all glaciers at this latitude were broadly sensitive to this specific climatic signal. No Early nor Mid Holocene advances were evidenced in Kerguelen glacier evolution during the Holocene due to missing moraines that may have formed in these specific periods. Radiocarbon-dated peat, published in the 1990s, provides evidence of less extensive glacier extents during the Early Holocene than during the Late Holocene (Frenot et al., 1997). Finally, glaciers seem to have re-advanced only during the Late Holocene, especially within the last millennium, at ⁓1 ka, ⁓620 years and ⁓390 years (Verfaillie et al., submitted). A comparison of this new dataset with the available 10Be ages from other sub-Antarctic regions allows for the identification of 3 different glacier evolution patterns during the Holocene. The glacial fluctuations experienced by Kerguelen glaciers seems particularly uncommon, and are likely due to its singular location in the Southern Indian Ocean. Finally, climatic factors that may explain the Kerguelen glacier evolution (temperature, precipitation) are discussed. To this end, we investigate the chronology of glacier advance/retreat periods with (i) the variation in atmospheric temperatures recorded in ice cores in Antarctica and (ii) the variation in precipitation (Southern Westerly Winds, Southern Annular Mode).Charton et al., 2020 : Ant. Sci. 1-13Frenot et al., 1997 : C.R. Acad. Sci. Paris Life Sciences 320, 567-573Jomelli et al., 2017 : Quat. Sci. Rev. 162, 128-144Jomelli et al., 2018 : Quat. Sci. Rev. 183, 110-123Sagredo et al., 2018 : Quat Sci. Rev. 188, 160-166Verfaillie et al., submitted

The strength and the location of Southern Hemisphere winds (SHW) define an annular mode (SAM), a major component of interannual climatic variability in the Southern Hemisphere. SAM has a significant impact on mid-latitude westerly winds, but also acts on the meridional moisture transport over the Southern Ocean (for example, in the case of atmospheric rivers), with dramatic consequences on the cryosphere at high latitudes. Assessment of past, present and future changes in the SAM is essential for understanding climate variations and impacts at high latitudes. To date, Holocene proxy-based reconstructions of SAM are limited to South America, Australia/New Zealand, and Antarctica. In opposite, the paucity of SAM-related records for the Southern Indian Ocean presently limits our understanding of the spatial and temporal extent of SHW behavior in this region.To this aim, we present a series of 30-m long marine records retrieved from a fjord fed by glacial melt of the Ampere glacier belonging to the Cook Ice Cap in the Kerguelen Archipelago (49˚20’S, 69˚20’E). A new chronological framework, based on Bayesian modelling of 50 radiocarbon ages along with 137Cs and 210Pb measures, allows reconstructing 4 kyrs of sediment discharge related to glacier variability. Sedimentological and geochemical analyses from XRF and GEOTEK core scanners highlight (i) a regional tephra at 950 cal BP; (ii) regular occurrences of floods during the LIA; and (iii) a background sedimentation related to glacial flour inputs through hypo- and hyperpycnal flows favoring very high sedimentation rates (1-2 cm.a-1) in the fjord. Phases of glacier advances and retreats linked to moisture transport by SHW are reflected by fluctuations in sedimentological and geochemical signals, and correlated with moraines dating on land. Over the past 4 kyrs, four cycles of glacier advances/retreats can be evidenced, reflecting wet/dry periods in response to shifts in the position and changes in magnitude of the SHW, associated with moisture transport and precipitation in the Southern Indian Ocean. On centennial timescales, wet/dry periods inferred from Kerguelen are in-phase with Holocene SAM-related records from South America and Tasmania over the last 2 kyrs, suggesting the long-term glacier dynamic at Kerguelen is also related to a centennial expression of SAM.Acknowledgments : Marion Dufresne Crew, ARTEMIS program, UGA-ARCA.

Daily weather regimes are defined around the Kerguelen Islands (Southern Ocean) based on daily 500 hPa geopotential height anomalies derived from the ERA5 ensemble reanalysis over the period 1979-2018. Ten regimes are retained as significant. Their occurrences are highly consistent across reanalysis ensemble members. Regimes show weak seasonality and non-significant long-term trends in their occurrences. Their sequences are usually short (1-3 days), with extreme persistence values above 10 days. Seasonal regime frequency is mostly driven by the phase of the Southern Annular Mode over Antarctica, mid-latitude dynamics over the Southern Ocean like the Pacific South American mode, and to a lesser extent, tropical variability, with significant but weaker relationships with El Niño Southern Oscillation. At the local scale over the Kerguelen Islands, regimes have a strong influence on measured atmospheric and oceanic variables, including minimum and maximum air temperature, mostly driven by horizontal advections, sea water temperature recorded 5 m below the surface, wind speed and sea level pressure. Relationships are weaker for precipitation amounts. Regimes also modify regional contrasts between observational sites in Kerguelen, highlighting strong exposure contrasts. The regimes allow improving our understanding of weather and climate variability and interactions in this region; they will be used in future work to assess past and projected long-term circulation changes in the southern mid-latitudes.

International audience; The Antarctic ice sheet (AIS) is classified as a polar desert where, similar to other deserts around the world, the annual precipitation is dependent on a few episodic precipitation events. Recent research has highlighted that certain regions of the AIS receive 40%-60% of their total annual precipitation from the largest 10% of daily precipitation events (Turner et al., 2019). There is a high coast-inland snowfall gradient, as most

The Cook Ice Cap (CIC) on the sub-Antarctic Kerguelen Islands recently experienced extremely negative surface mass balance. Further deglaciation could have important impacts on endemic and invasive fauna and flora. To put this exceptional glacier evolution into a multi-centennial-scale context, we refined the evolution of the CIC over the last millennium, investigated the associated climate conditions and explored its potential evolution by 2100 ce. A glaciological model, constrained by cosmic ray exposure dating of moraines, historical documents and recent direct mass balance observations, was used to simulate the ice-cap extents during different phases of advance and retreat between the last millennium and 2100 ce. Cosmogenic dating suggests glacial advance around the early Little Ice Age (LIA), consistent with findings from other sub-Antarctic studies, and the rather cold and humid conditions brought about by the negative phase of the Southern Annular Mode (SAM). This study contributes to our currently limited understanding of palaeoclimate for the early LIA in the southern Indian Ocean. Glaciological modelling and observations confirm the recent decrease in CIC extent linked to the intensification of the SAM. Although affected by large uncertainties, future simulations suggest a complete disappearance of CIC by the end of the century.

Stable water isotopes are effective hydrological tracers due to fractionation processes throughout the water cycle, and thus, the stable isotopes from ice cores can serve as valuable proxies for past changes in the climate and local environment of polar regions. Proper interpretation of these isotopes requires to understand the influence of each potential fractionating process, such as initial evaporation over the ocean and precipitation events, but also the effects of post-depositional exchange between snow and moisture in the atmosphere. Thanks to new developments in infrared spectroscopy, it is now possible to continuously monitor the isotopic composition of atmospheric water vapor in coordination with discrete snow sampling. This allows us to readily document the isotopic and mass exchanges between snow and vapor as well as the stability of the atmospheric boundary layer, as has recently been shown on the East Antarctic Plateau at Kohnen (Ritter et al., TC, 2016) and Dome C (Casado et al., ACP, 2016) stations where substantial diurnal isotopic variations have been recorded.In this study, we present the first vapor monitoring of an East Antarctic transect that covered more than 3600 km over a period of 3 months from November 2019 to February 2020 as part of the EAIIST mission. The isotopic record therefore describes the evolution from typical coastal values to highly depleted values deep inside the continent on the high-altitude plateau. In parallel, we also monitored the vapor isotopic composition at two stations: the coastal starting point of Dumont D’Urville (DDU) and the plateau halfway point of Dome C. Two automatic weather stations (at Paleo and Megadunes sites) were also installed in a previously unexplored region of the East Antarctic plateau that was covered by this transect. This suite of cross-calibrated vapor isotope observations and weather stations, coupled with Modele Atmospherique Régional (MAR) climate modeling, offers a unique opportunity to compare the spatial and temporal gradients of humidity, temperature, and water vapor isotopic composition in East Antarctica during the summer season, and to estimate how the water vapour isotope measurements at Dome C and DDU are representative of the conditions in East Antarctica. The quantitative agreement between the EAIIST record and those recorded at DDU and Dome C stations at the times the raid was nearby, gives confidence in the quality of the results acquired on this traverse. Although further comparisons with the surface snow isotopic composition are required to quantify the impact of the snow-atmosphere exchanges on the local surface mass balance, these initial results of vapor isotopic composition show the potential of using water stables isotopes to evaluate hydrological processes in East Antarctica and better reconstruct past climate changes through ice cores.

On December 19-21, 2018, an atmospheric river hit the French-Italian Concordia station, located at Dome C, East Antarctic Plateau, 3 269 m above sea level. It induced an extreme surface warming (+ 15°C in 3 days), combined with high specific humidity (multiplied by 3 in 3 days) and a strong isotopic anomaly in water vapor (+ 15 ‰ for δ18O). The isotopic composition of water vapor monitored during the event may be explained by (1) the isotopic signature of long-range water transport, and by (2) local moisture uptake during the event. In this study we quantify the influence of each of these processes.To estimate the isotopic composition of water vapor advected by long-range transport, we perform back-trajectories with the FLEXible PARTicle dispersion model FLEXPART. We retrieve meteorological conditions along different trajectories between the moisture uptake area and Concordia, and use them to compute isotopic fractionation during transport with the mixed cloud isotope model MCIM. While intermediate conditions along the trajectory do not seem to have a major impact on the final isotopic composition (less than 0.1 ‰), the latter appears sensitive to surface conditions (temperature, pressure and relative humidity) in the moisture uptake area (±5.1 ‰). As the event is characterized by the presence of liquid water clouds above Concordia, we show additional sensitivity tests exploring the impact of mixed phase clouds on the water vapor isotopic composition.Finally, we perform a water vapor mass budget in the boundary layer using observations and simulations from the regional atmospheric model MAR, ran with and without drifting snow. The presence of mixed-phase clouds during the event induced a significant increase in downward longwave radiative fluxes, which led to high turbulent mixing in the boundary layer and to heavy drifting snow (white-out conditions). Using MAR simulations, we show that a significant part of the atmospheric water vapor originates from sublimation of drifting snow particles removed from the snowpack. Consequently, the isotopic signal monitored in water vapor during this atmospheric river event reflects both long-range moisture advection and interactions between the boundary layer and the snowpack. Only specific meteorological conditions driven by the atmospheric river, and their associated intense poleward moisture transport, can explain these strong interactions.

Debris-covered glaciers constitute a substantial part of the worldwide cryosphere (Scherler et al. 2018). However, their long-term response to multi-millennial climate variability has rarely been studied, in particular in the Southern Hemisphere. The presence of both debris-covered and debris-free glaciers on Kerguelen Archipelago (49°S, 69°E) offers therefore an excellent opportunity to investigate and compare long-term evolution of these two types of glaciers. To do so, we used the cosmogenic 36Cl surface dating method on moraine boulders that allows to establish temporal constraints of glacier oscillation. We provide here the first Late Glacial and Holocene glacier chronology of a still active debris-covered glacier from the archipelago: the Gentil Glacier. Results show that the Gentil Glacier advanced once at ~14.3 ka, i.e. during the Late Glacial (19.0 – 11.6 ka), and re-advanced during the Late Holocene at ~2.6 ka (Charton et al., 2020). Both debris-covered and debris-free glaciers experienced a broadly synchronous advance during the Late Glacial, that may be assigned to the Antarctic Cold Reversal event (14.5 – 12.9 ka) (Jomelli et al., 2017; 2018). This suggests that both types (debris-covered and debris-free) of glaciers at Kerguelen were sensitive to large amplitude temperature fluctuations recorded in Antarctic ice cores (WAIS divide Project Members, 2013), associated with increased precipitations (Van der Putten, 2015). However, during the Late Holocene, the advance at about ~2.6 ka was not observed on other glaciers and seems to be a specific response of the debris-covered Gentil Glacier, either related to distinct ice dynamics or an individual response to precipitation changes. Charton et al., 2020 : Ant. Sci. 1-13Jomelli et al., 2017 : Quat. Sci. Rev. 162, 128-144Jomelli et al., 2018 : Quat. Sci. Rev. 183, 110-123Scherler et al., 2018 : GRL. 45, 11,798-11,805Van der Putten et al., 2015 : Quat. Sci. Rev. 122, 142-157WAIS Divide Project Members, 2013: Nature. 500, 440-444

The climate of polar regions is characterized by large fluctuations and has experienced dramatic changes over the past decades. In the high latitudes of the Southern Hemisphere, the patterns of changes in sea ice and ice sheet mass, in particular, are more complex than for the Northern Hemisphere. Some regions have warmed less than the global average with some sea-ice advance, in particular in the Ross Sea, while other regions such as the Bellingshausen Sea have warmed significantly and displayed sea-ice loss. The Antarctic Ice Sheet has also lost mass in the past decades, with a spectacular thinning and weakening of ice shelves, i.e., the floating extensions of the grounded ice sheet. Despite recent advances in observing and modelling the Antarctic climate, the mechanisms at the origin of those trends are very uncertain because of the limited amount of observations and the large biases of climate models in polar regions, in concert with the large internal variability prevailing in the Antarctic. One of the most important atmospheric modes of climate variability in the Southern Ocean is the Southern Annular Mode (SAM), which represents the position and the strength of the westerly winds. During years with a positive SAM index, lower sea level pressure at high latitudes and higher sea level pressure at low latitudes occur, resulting in a stronger pressure gradient and intensified Westerlies. However, the current knowledge of the impact of these fluctuations of the Westerlies on the Southern Ocean and Antarctic cryosphere is still limited. Some efforts have been devoted over the past few years to the impact of the SAM on the Antarctic sea ice and the surface mass balance of the ice sheet from an atmospheric-specific perspective. Recently, a few studies have focused on the local impact on ice-shelf basal melt in specific regions of Antarctica. However, to our knowledge, there is no such study of the impact of the SAM on ice-shelf basal melt at the pan-Antarctic scale. In this communication, we will address this issue by using simulations performed with the regional ocean and sea-ice model NEMO-LIM3.6 at a spatial resolution of 0.25° forced by the ERA5 reanalysis over the period 1979-2018 CE. The impact of both the annular and the non-annular components of the SAM on ice-shelf basal melt will be assessed through regressions and correlations between the seasonal or annual averages of the SAM index and the ice-shelf basal melt.

ASTER Team: Georges Aumaître, Didier L. Bourlès and Karim Keddadouche; International audience; Debris-covered glaciers constitute a large part of the world's cryosphere. However, little is known about their long-term response to multi-millennial climate variability, in particular in the Southern Hemisphere. Here, we provide first insights into the response of a debris-covered glacier to multi-millennial climate variability in the sub-Antarctic Kerguelen Archipelago, which can be compared to that of recently investigated debris-free glaciers. We focus on the Gentil Glacier and present thirteen new 36 Cl cosmic-ray exposure ages from moraine boulders. The Gentil Glacier experienced at least two glacial advancesthe first one during the Late Glacial (19.0-11.6 ka) at ~14.3 ka and the second one during the Late Holocene at ~2.6 ka. Both debris-covered and debris-free glaciers advanced broadly synchronously during the Late Glacial, most probably during the Antarctic Cold Reversal event (14.5-12.9 ka). This suggests that both glacier types at Kerguelen were sensitive to abrupt temperature changes recorded in Antarctic ice cores, associated with increased moisture. However, during the late Holocene, the advance at about ~2.6 ka was not observed in other glaciers and seems to be an original feature of the debris-covered Gentil Glacier, either related to distinct dynamics or distinct sensitivity to precipitation changes.

Paleoclimate archives, such as high-resolution ice core records, provide a means to investigate past climate variability. Until recently, the Law Dome (Dome Summit South site) ice core record remained one of few millennial-length high-resolution coastal records in East Antarctica. A new ice core drilled in 2017/2018 at Mount Brown South, approximately 1000 km west of Law Dome, provides an additional high-resolution record that will likely span the last millennium in the Indian Ocean sector of East Antarctica. Here, we compare snow accumulation rates and sea salt concentrations in the upper portion (∼ 20 m) of three Mount Brown South ice cores and an updated Law Dome record over the period 1975–2016. Annual sea salt concentrations from the Mount Brown South site record preserve a stronger signal for the El Niño–Southern Oscillation (ENSO; austral winter and spring, r = 0.533, p < 0.001, Multivariate El Niño Index) compared to a previously defined Law Dome record of summer sea salt concentrations (November–February, r = 0.398, p = 0.010, Southern Oscillation Index). The Mount Brown South site record and Law Dome record preserve inverse signals for the ENSO, possibly due to longitudinal variability in meridional transport in the southern Indian Ocean, although further analysis is needed to confirm this. We suggest that ENSO-related sea surface temperature anomalies in the equatorial Pacific drive atmospheric teleconnections in the southern mid-latitudes. These anomalies are associated with a weakening (strengthening) of regional westerly winds to the north of Mount Brown South that correspond to years of low (high) sea salt deposition at Mount Brown South during La Niña (El Niño) events. The extended Mount Brown South annual sea salt record (when complete) may offer a new proxy record for reconstructions of the ENSO over the recent millennium, along with improved understanding of regional atmospheric variability in the southern Indian Ocean, in addition to that derived from Law Dome.

Paleoclimate archives, such as high-resolution ice core records, provide a means to investigate long-term (multi-centennial) climate variability. Until recently, the Law Dome (Dome Summit South) ice core record remained one of few long-term high-resolution records in East Antarctica. A new ice core drilled in 2017/2018 at Mount Brown South, approximately 1000 km west of Law Dome, provides an additional high-resolution record that will likely span the last millennium in the Indian Ocean sector of East Antarctica. Here, we compare snowfall accumulation rates and sea salt concentrations in the upper portion (~21 m) of the Mount Brown South record, and an updated Law Dome record over the period 1975–2016. Annual sea salt concentrations from the Mount Brown South record preserves a stronger signal for the El Niño-Southern Oscillation (ENSO; in austral winter and spring, r = 0.521, p r = −0.387, p = 0.018, Niño 3.4). The Mount Brown South and Law Dome ice cores record inverse signals for the ENSO, suggesting the occurrence of distinct moisture and aerosol intrusions. We suggest that ENSO-related sea surface temperature anomalies in the equatorial Pacific drive atmospheric teleconnections in the southern mid-latitudes. These anomalies are associated with a weakening (strengthening) of regional westerly winds to the north of Mount Brown South that corresponds to years of low (high) sea salt deposition at Mount Brown South during La Niña (El Niño) events. The Mount Brown South annual sea salt record when complete will offer a new proxy record for reconstructions of the ENSO over the recent millennium, along with improved understanding of regional atmospheric variability in the southern Indian Ocean in addition to that derived from Law Dome.

In this study, we use run-time bias-correction to correct for ARPEGE atmospheric model systematic errors on large-scale atmospheric circulation. The bias correction terms are built using the climatological mean of the adjustment terms on tendency errors in an ARPEGE simulation relaxed towards ERA-Interim reanalyses. The improvements with respect to the AMIP-style uncorrected control run for the general atmospheric circulation in the Southern Hemisphere are significant for mean state and daily variability. Comparisons for the Antarctic Ice Sheet with the polar-oriented regional atmospheric models MAR and RACMO2 and in-situ observations also suggest substantial bias reduction for near-surface temperature and precipitation in coastal areas. Applying the method to climate projections for the late 21st century (2071–2100) leads to large differences in the projected changes of the atmospheric circulation in the Southern high latitudes and of the Antarctic surface climate. The projected poleward shift and strengthening of the southern westerly winds are greatly reduced. These changes result in a significant 0.7 to 0.9 K additional warming and a 6 to 9 % additional increase in precipitation over the grounded ice sheet. The sensitivity of precipitation increase to temperature (+7.7 and +9 %.K−1) found is also higher than previous estimates. Highest additional warming rates are found over East Antarctica in summer. In winter, there is a dipole of weaker warming and weaker precipitation increase over West Antarctica, contrasted by a stronger warming and a concomitant stronger precipitation increase from Victoria to Adélie Land, associated with a weaker intensification of the Amundsen Sea Low.

The northern Patagonian Icefield (NPI) is the second largest ice mass in Patagonia (3740 km²). Estimation of recent volume changes confirm an acceleration of ice loss in the last decades compared to the mean mass loss since the Little Ice Age. However, Icefield-wide responses at shorter time scales (5-25 yrs) are still poorly documented and not well understood.We compare five digital elevation models (DEM) acquired between 1975 and 2016 over the NPI: SPOT6 and SPOT7 DEMs for year 2016, SPOT5-HRS DEMs for 2012 and 2005, the Shuttle Radar Topography Mission DEM (SRTM) for year 2000 and the earlier Chilean military institute cartography (IGM) derived from aerial photographs acquired in 1975. We derive cefield-wide mass balances during four different time periods (1975-2000, 2000-2005, 2005-2012, 2012-2016). Our results suggest an acceleration of mass loss from 1975 to 2016. Although error bars are large, we suggest a shift from moderately negative icefield-wide mass balance rates before 2000 (of the order of -0.6 m w.e. yr-1), towards larger mass losses during the first decade of the 21st century(of the order of -0.8 m w.e. yr-1) and even more negative value from 2012 to 2016 (-1.2 ± 0.2 m w.e. yr-1).But these results must be considered cautiously. The 1975-2000 map of elevation change shows a thickening rate of over 1 m/yr which are not supported by image analysis. We stress the need to construct a revised 1975 NPI topography in order to document the NPI mass balance observations back to 1975 with improved confidence.

Due to the increased ability of the air to hold moisture with temperature, precipitation in Antarctica is expected to increase significantly over the next century. This process will undoubtedly lead to an increase in snow accumulation on the continent, thereby partially mitigating future sea level rise. However, recent observed trends in temperature and snow accumulation in Antarctica are insignificant and paradoxical. The role played by moisture intrusions in temperature and precipitation variability could partly explain this phenomenon. These events are known to transport warm air masses from the oceans to the Antarctic continent, but only individual and unrelated intrusion events have been described so far. In an earlier study of the Dronning Maud Land region, the description of atmospheric rivers (ARs) provided an innovative view to describe high intensity moisture intrusions. In this thesis, we develop an atmospheric river detection algorithm adapted to the polar regions, in order to create a climatology of these events at the Antarctic scale. Using the outputs of the regional climate model, MAR (Regional Atmospheric Model), we evaluate the impacts of atmospheric rivers on the melting in West Antarctic, ice-shelf stability on the Antarctic Peninsula ice shelves, and on snowfall across the Antarctic ice sheet.Atmospheric rivers are infrequent events with coastal areas of Antarctica experiencing AR conditions around only three days per year on average and this value is even lower in Antarctic interior. However, ARs control surface melting processes on West Antarctica and precipitation variability on East Antarctica. In the west, ARs entering the interior of the continent cause positive anomalies in downward longwave radiation via highly liquid-laden clouds and wind, resulting in a marked foehn effect along leeward slopes. Between 1979 and 2017, rivers were thus associated with about 40% of the summer melt on the Ross Ice Shelf (nearly 100% at higher altitudes on Marie Byrd Land) and 40-80% of the winter melt along the ice shelves of the Antarctic Peninsula. In summer, these rivers also contribute to the melting of the Larsen ice shelves located east of the Peninsula. Their direct contribution to cumulative melting is more limited than in winter because the intense solar radiation in summer allows daily melting to occur. However, ARs caused 60-80% of the most intense melt/runoff events as well as high temperature extremes. This melting is linked to the Foehn effect and the positive longwave radiative fluxes anomalies over the eastern Peninsula during AR passages. The melt water accumulates in lakes and crevasses on the ice shelf surface, a preliminary step in ice shelf disintegration by hydraulic fracturing. ARs also push sea ice away from the coast, allowing swells to hit and apply strain to the ice-shelf margins. ARs can thus trigger the final disintegration of ice shelves. In particular, ARs were present during the disintegration of the Larsen A ice shelf in late January 1995 and the Larsen B ice shelf in late February/early March 2002. Overall, since 2000, 12 of the last 20 calving or collapse events along the Larsen Ice Shelves have been preceded (within 5 days) by the arrival of an AR. Finally, atmospheric rivers are responsible for the majority of the most intense precipitation in Antarctica. In eastern Antarctica, 20-30% of the snow accumulation occurred during of AR landfalls. Although this value remains modest, we observe that AR activity controls the trends and interannual variability of snowfall in this part of the continent. This control even extends to most of the ice sheet between 1980 and 2018.Ultimately, ARs play an important role in the Antarctic surface mass balance. Therefore, a change in atmospheric blocking conditions around Antarctica during the 21st century would lead to changes in the Antarctic surface mass balance. Such changes are currently ignored in climate change impact projections.

Recent research shows increasing decadal ice mass losses from the Greenland and Antarctic Ice Sheets and more generally from glaciers worldwide in the light of continued global warming. Here, in an update of our previous ISMASS paper (Hanna et al. 2013), we review recent observational estimates of ice sheet and glacier mass balance, and their related uncertainties, first briefly considering relevant monitoring methods. Focusing on the response to climate change during 1992–2018, and especially the post-IPCC AR5 period, we discuss recent changes in the relative contributions of ice sheets and glaciers to sea-level change. We assess recent advances in understanding of the relative importance of surface mass balance and ice dynamics in overall ice-sheet mass change. We also consider recent improvements in ice-sheet modelling, highlighting data-model linkages and the use of updated observational datasets in ice-sheet models. Finally, by identifying key deficiencies in the observations and models that hamper current understanding and limit reliability of future ice-sheet projections, we make recommendations to the research community for reducing these knowledge gaps. Our synthesis aims to provide a critical and timely review of the current state of the science in advance of the next Intergovernmental Panel on Climate Change Assessment Report that is due in 2021., SCOPUS: re.j, info:eu-repo/semantics/published

Andean glaciers are among the fastest shrinking and largest contributors to sea level rise on Earth. They also represent crucial water resources in many tropical and semi-arid mountain catchments. Yet the magnitude of the recent ice loss is still debated. Here we present Andean glacier mass changes (from 10° N to 56° S) between 2000 and 2018 using time series of digital elevation models derived from ASTER stereo images. The total mass change over this period was −22.9 ± 5.9 Gt yr−1 (−0.72 ± 0.22 m w.e. yr−1 (m w.e., metres of water equivalent)), with the most negative mass balances in the Patagonian Andes (−0.78 ± 0.25 m w.e. yr−1) and the Tropical Andes (−0.42 ± 0.24 m w.e. yr−1), compared to relatively moderate losses (−0.28 ± 0.18 m w.e. yr−1) in the Dry Andes. Subperiod analysis (2000–2009 versus 2009–2018) revealed a steady mass loss in the tropics and south of 45° S. Conversely, a shift from a slightly positive to a strongly negative mass balance was measured between 26 and 45° S. In the latter region, the drastic glacier loss in recent years coincides with the extremely dry conditions since 2010 and partially helped to mitigate the negative hydrological impacts of this severe and sustained drought. These results provide a comprehensive, high-resolution and multidecadal data set of recent Andes-wide glacier mass changes that constitutes a relevant basis for the calibration and validation of hydrological and glaciological models intended to project future glacier changes and their hydrological impacts. Glaciers in the Andes have lost about 23 Gt of mass per year between 2000 and 2018, with the fastest loss in Patagonia, according to time series of digital elevation models that are based on ASTER stereo images.

Recent major melting events in West Antarctica have raised concerns about a potential hydrofracturing and ice shelf instability. These events often share common forcings of surface melt-like anomalous radiative fluxes, turbulent heat fluxes and fohn winds. Using an atmospheric river detection algorithm developed for Antarctica together with surface melt datasets, we produced a climatology of atmospheric river-related surface melting around Antarctica and show that atmospheric rivers are associated with a large percentage of these surface melt events. Despite their rarity (around 12 events per year in West Antarctica), atmospheric rivers are associated with around 40% of the total summer meltwater generated across the Ross Ice Shelf to nearly 100% in the higher elevation Marie Byrd Land and 40–80% of the total winter meltwater generated on the Wilkins, Bach, George IV and Larsen B and C ice shelves. These events were all related to high-pressure blocking ridges that directed anomalous poleward moisture transport towards the continent. Major melt events in the West Antarctic Ice Sheet only occur about a couple times per decade, but a 1–2 °C warming and continued increase in atmospheric river activity could increase the melt frequency with consequences for ice shelf stability. Atmospheric rivers associated with blocking events are related to a large fraction of the surface ice melt events in West Antarctica, suggest observation-based analyses of atmospheric dynamics and West Antarctic surface melt.

We review recent literature on atmospheric, surface ocean and sea-ice observations and modeling results in the Antarctic sector and relate the observed climatic trends with the potential changes in the surface mass balance (SMB) of the ice sheet since 1900. Estimates of regional scale SMB distribution and trends remain subject to large uncertainties. Approaches combining and comparing multiple satellite and model-based assessments of ice sheet mass balance aim at reducing these knowledge gaps. During the last decades, significant changes in atmospheric circulation occurred around Antarctica, due to the exceptional positive trend in the Southern Annular Mode and to the climate variability observed in the tropical Pacific at the end of the twentieth century. Even though climate over the East Antarctic Ice-Sheet remained quite stable, a warming and precipitation increase was observed over the West Antarctic Ice-Sheet and over the West Antarctic Peninsula (AP) during the twentieth century. However, the high regional climate variability overwhelms climate changes associated to human drivers of global temperature changes, as reflected by a slight recent decadal cooling trend over the AP. Climate models still fail to accurately reproduce the multi-decadal SMB trends at a regional scale, and progress has to be achieved in reproducing atmospheric circulation changes related to complex ocean/ice/atmosphere interactions. Complex processes are also still insufficiently considered, such as (1) specific polar atmospheric processes (clouds, drifting snow, and stable boundary layer physics), (2) surface firn physics involved in the surface drag variations, or in firn air depletion and albedo feedbacks. Finally, progress in reducing the uncertainties relative to projections of the future SMB of Antarctica will largely depend on climate model capability to correctly consider teleconnections with low and mid-latitudes, and on the ability to correct them for biases, taking into account the coupling between ocean, ice, and atmosphere in high southern latitudes.

The sensitivity of tropical glaciers to paleoclimatic conditions that prevailed during the Antarctic cold reversal (ACR, ca. 14.5-12.9 ka) has been the subject of a wide debate. In 2014 a paper suggested that tropical glaciers responded very sensitively to the changing climate during the ACR (Jomelli et al., 2014). In this study, we reexamine the conclusions from this study by recalculating previous chronologies based on 226 10Be and 14 3He ages respectively, and using the most up-to date production rates for these cosmogenic nuclides in the Tropical Andes. 53 moraines from 25 glaciers were selected from the previous analysis provided by Jomelli et al. (2014) located in Colombia, Peru and Bolivia. We then focused on two distinct calculations. First we considered the oldest moraine and its uncertainty for every glacier representing the preserved evidence of the maximum glacier extents during the last deglaciation period, and binned the results into 5 distinct periods encompassing the Antarctic cold reversal and Younger Dryas (YD) chronozones: pre-ACR, ACR, ACR-YD, YD and post-YD respectively. Results revealed a predominance of pre-ACR and ACR ages, accounting for 60% of the glaciers. Second we counted the number of moraines per glacier according to the different groups. 21 moraines (40%) of the selected glaciers belong to the pre-ACR-ACR chronozones while 3 moraines only (5%) were dated to the YD and YD-Holocene groups. The rest was assigned to the ACR-YD. These results suggest that moraine records are a very good proxy to document the ACR signal in the Tropical Andes.

A 22.4 m-long shallow firn core was extracted during the 2006/2007 field season from coastal Adélie Land. Annual layer counting based on subannual analyses of δ18O and major chemical components was combined with 5 reference years associated with nuclear tests and non-retreat of summer sea ice to build the initial ice-core chronology (1946–2006), stressing uncertain counting for 8 years. We focus here on the resulting δ18O and accumulation records. With an average value of 21.8 ± 6.9 cm w.e. yr−1, local accumulation shows multi-decadal variations peaking in the 1980s, but no long-term trend. Similar results are obtained for δ18O, also characterised by a remarkably low and variable amplitude of the seasonal cycle. The ice-core records are compared with regional records of temperature, stake area accumulation measurements and variations in sea-ice extent, and outputs from two models nudged to ERA (European Reanalysis) atmospheric reanalyses: the high-resolution atmospheric general circulation model (AGCM), including stable water isotopes ECHAM5-wiso (European Centre Hamburg model), and the regional atmospheric model Modèle Atmosphérique Régional (AR). A significant linear correlation is identified between decadal variations in δ18O and regional temperature. No significant relationship appears with regional sea-ice extent. A weak and significant correlation appears with Dumont d'Urville wind speed, increasing after 1979. The model-data comparison highlights the inadequacy of ECHAM5-wiso simulations prior to 1979, possibly due to the lack of data assimilation to constrain atmospheric reanalyses. Systematic biases are identified in the ECHAM5-wiso simulation, such as an overestimation of the mean accumulation rate and its interannual variability, a strong cold bias and an underestimation of the mean δ18O value and its interannual variability. As a result, relationships between simulated δ18O and temperature are weaker than observed. Such systematic precipitation and temperature biases are not displayed by MAR, suggesting that the model resolution plays a key role along the Antarctic ice sheet coastal topography. Interannual variations in ECHAM5-wiso temperature and precipitation accurately capture signals from meteorological data and stake observations and are used to refine the initial ice-core chronology within 2 years. After this adjustment, remarkable positive (negative) δ18O anomalies are identified in the ice-core record and the ECHAM5-wiso simulation in 1986 and 2002 (1998–1999), respectively. Despite uncertainties associated with post-deposition processes and signal-to-noise issues, in one single coastal ice-core record, we conclude that the S1C1 core can correctly capture major annual anomalies in δ18O as well as multi-decadal variations. These findings highlight the importance of improving the network of coastal high-resolution ice-core records, and stress the skills and limitations of atmospheric models for accumulation and δ18O in coastal Antarctic areas. This is particularly important for the overall East Antarctic ice sheet mass balance.