Abstract:

Abstract. 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 <20 % 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.

Abstract: Sublimation of snow particles during transport has been recognized as an important ablation process on the Antarctic ice sheet. The resulting increase in moisture content and cooling of the ambient air are thermodynamic negative feedbacks that both contribute to increase the relative humidity of the air, inhibiting further sublimation when saturation is reached. This self-limiting effect and the associated development of saturated near-surface air layers in drifting snow conditions have mainly been described through modelling studies and a few field observations. A set of meteorological data, including drifting snow mass fluxes and vertical profiles of relative humidity, collected at site D17 in coastal Adélie Land (East Antarctica) during 2013 is used to study the relationship between saturation of the near-surface atmosphere and the occurrence of drifting snow in a katabatic wind region that is among the most prone to snow transport by wind. Atmospheric moistening by the sublimation of the windborne snow particles generally results in a strong increase in relative humidity with the magnitude of drifting snow and a decrease in its vertical gradient, suggesting that windborne-snow sublimation can be an important contributor to the local near-surface moisture budget. Despite a high incidence of drifting snow at the measurement location (60.1 % of the time), saturation, when attained, is however most often limited to a thin air layer below 1 m above ground. The development of a near-surface saturated air layer up to the highest measurement level of 5.5 m is observed in only 8.2 % of the drifting snow occurrences or 6.3 % of the time and mainly occurs in strong wind speed and drift conditions. This relatively rare occurrence of ambient saturation is explained by the likely existence of moisture-removal mechanisms inherent to the katabatic and turbulent nature of the boundary-layer flow that weaken the negative feedback of windborne-snow sublimation. Such mechanisms, potentially quite active in katabatic-generated windborne-snow layers all over Antarctica, may be very important in understanding the surface mass and atmospheric moisture budgets of the ice sheet by enhancing windborne-snow sublimation.

Abstract:

Abstract. Snow accumulation is the main positive component of the mass balance in Antarctica. In contrast to the major efforts deployed to estimate its overall value on a continental scale – to assess the contribution of the ice sheet to sea level rise – knowledge about the accumulation process itself is relatively poor, although many complex phenomena occur between snowfall and the definitive settling of the snow particles on the snowpack. Here we exploit a dataset of near-daily surface elevation maps recorded over 3 years at Dome C using an automatic laser scanner sampling 40–100 m² in area. We find that the averaged accumulation is relatively regular over the 3 years at a rate of +8.7 cm yr⁻¹. Despite this overall regularity, the surface changes very frequently (every 3 d on average) due to snow erosion and heterogeneous snow deposition that we call accumulation by “patches”. Most of these patches (60 %–85 %) are ephemeral but can survive a few weeks before being eroded. As a result, the surface is continuously rough (6–8 cm root-mean-square height) featuring meter-scale dunes aligned along the wind and larger, decameter-scale undulations. Additionally, we deduce the age of the snow present at a given time on the surface from elevation time series and find that snow age spans over more than a year. Some of the patches ultimately settle, leading to a heterogeneous internal structure which reflects the surface heterogeneity, with many snowfall events missing at a given point, whilst many others are overrepresented. These findings have important consequences for several research topics including surface mass balance, surface energy budget, photochemistry, snowpack evolution, and the interpretation of the signals archived in ice cores.

Abstract:

Abstract. We evaluate modelled Antarctic ice sheet (AIS) near-surface climate, surface mass balance (SMB) and surface energy balance (SEB) from the updated polar version of the regional atmospheric climate model, RACMO2 (1979–2016). The updated model, referred to as RACMO2.3p2, incorporates upper-air relaxation, a revised topography, tuned parameters in the cloud scheme to generate more precipitation towards the AIS interior and modified snow properties reducing drifting snow sublimation and increasing surface snowmelt.

Comparisons of RACMO2 model output with several independent observational data show that the existing biases in AIS temperature, radiative fluxes and SMB components are further reduced with respect to the previous model version. The model-integrated annual average SMB for the ice sheet including ice shelves (minus the Antarctic Peninsula, AP) now amounts to 2229Gty⁻¹, with an interannual variability of 109Gty⁻¹. The largest improvement is found in modelled surface snowmelt, which now compares well with satellite and weather station observations. For the high-resolution (∼5.5km) AP simulation, results remain comparable to earlier studies.

The updated model provides a new, high-resolution data set of the contemporary near-surface climate and SMB of the AIS; this model version will be used for future climate scenario projections in a forthcoming study.

Abstract: Accurate Assessment Of Anthropogenic Carbon Dioxide (Co2) Emissions And Their Redistribution Among The Atmosphere, Ocean, And Terrestrial Biosphere – The “Global Carbon Budget” – Is Important To Better Understand The Global Carbon Cycle, Support The Development Of Climate Policies, And Project Future Climate Change. Here We Describe Data Sets And Methodology To Quantify The Five Major Components Of The Global Carbon Budget And Their Uncertainties. Fossil Co2 Emissions (Eff) Are Based On Energy Statistics And Cement Production Data, While Emissions From Land Use Change (Eluc), Mainly Deforestation, Are Based On Land Use And Land Use Change Data And Bookkeeping Models. Atmospheric Co2 Concentration Is Measured Directly And Its Growth Rate (Gatm) Is Computed From The Annual Changes In Concentration. The Ocean Co2 Sink (Socean) And Terrestrial Co2 Sink (Sland) Are Estimated With Global Process Models Constrained By Observations. The Resulting Carbon Budget Imbalance (Bim), The Difference Between The Estimated Total Emissions And The Estimated Changes In The Atmosphere, Ocean, And Terrestrial Biosphere, Is A Measure Of Imperfect Data And Understanding Of The Contemporary Carbon Cycle. All Uncertainties Are Reported As ±1σ. For The Last Decade Available (2009–2018), Eff Was 9.5±0.5 Gtc Yr−1, Eluc 1.5±0.7 Gtc Yr−1, Gatm 4.9±0.02 Gtc Yr−1 (2.3±0.01 Ppm Yr−1), Socean 2.5±0.6 Gtc Yr−1, And Sland 3.2±0.6 Gtc Yr−1, With A Budget Imbalance Bim Of 0.4 Gtc Yr−1 Indicating Overestimated Emissions And/or Underestimated Sinks. For The Year 2018 Alone, The Growth In Eff Was About 2.1 % And Fossil Emissions Increased To 10.0±0.5 Gtc Yr−1, Reaching 10 Gtc Yr−1 For The First Time In History, Eluc Was 1.5±0.7 Gtc Yr−1, For Total Anthropogenic Co2 Emissions Of 11.5±0.9 Gtc Yr−1 (42.5±3.3 Gtco2). Also For 2018, Gatm Was 5.1±0.2 Gtc Yr−1 (2.4±0.1 Ppm Yr−1), Socean Was 2.6±0.6 Gtc Yr−1, And Sland Was 3.5±0.7 Gtc Yr−1, With A Bim Of 0.3 Gtc. The Global Atmospheric Co2 Concentration Reached 407.38±0.1 Ppm Averaged Over 2018. For 2019, Preliminary Data For The First 6–10 Months Indicate A Reduced Growth In Eff Of +0.6 % (Range Of −0.2 % To 1.5 %) Based On National Emissions Projections For China, The Usa, The Eu, And India And Projections Of Gross Domestic Product Corrected For Recent Changes In The Carbon Intensity Of The Economy For The Rest Of The World. Overall, The Mean And Trend In The Five Components Of The Global Carbon Budget Are Consistently Estimated Over The Period 1959–2018, But Discrepancies Of Up To 1 Gtc Yr−1 Persist For The Representation Of Semi-decadal Variability In Co2 Fluxes. A Detailed Comparison Among Individual Estimates And The Introduction Of A Broad Range Of Observations Shows (1) No Consensus In The Mean And Trend In Land Use Change Emissions Over The Last Decade, (2) A Persistent Low Agreement Between The Different Methods On The Magnitude Of The Land Co2 Flux In The Northern Extra-tropics, And (3) An Apparent Underestimation Of The Co2 Variability By Ocean Models Outside The Tropics. This Living Data Update Documents Changes In The Methods And Data Sets Used In This New Global Carbon Budget And The Progress In Understanding Of The Global Carbon Cycle Compared With Previous Publications Of This Data Set (Le Quéré Et Al., 2018a, B, 2016, 2015a, B, 2014, 2013). The Data Generated By This Work Are Available At Https://doi.org/10.18160/gcp-2019 (Friedlingstein Et Al., 2019).