Recently, the climate version (HCLIM) of the regional numerical weather prediction model system ALADIN–HIRLAM of the ACCORD consortium, has been set up for the Arctic and Antarctic domains. Within the PolarRES project, HCLIM will be run, along with other regional climate models such as RACMO, MetUM, and MAR, to study the interactions between the atmosphere, oceans, and sea ice in the Arctic and Antarctic. For the Antarctic Peninsula, kilometre-scale horizontal resolution and non-hydrostatic model dynamics are essential to accurately resolve the complex topography and to capture small-scale processes such as the föhn winds that occur over ice shelves on the Antarctic Peninsula. 

Here, we present an analysis of the föhn event on 27 January 2011 over the Larsen C Ice Shelf, Antarctic Peninsula. The output of the non-hydrostatic HCLIM-AROME model, run at 2.5 km resolution, is evaluated against automatic weather station and radiosonde measurements and simulations of the non-hydrostatic regional climate model MetUM. We analyse the modelled air pressure, near-surface and tropospheric temperatures, wind speed and wind direction, and other atmospheric variables, demonstrating the strengths and weaknesses of the HCLIM-AROME model for this polar application. 

How to cite: Verro, K., van de Berg, W. J., Orr, A., Landgren, O., and van Ulft, B.: New non-hydrostatic polar regional climate model HCLIM-AROME: analysis of the föhn event on 27 January 2011 over the Larsen C Ice Shelf, Antarctic Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13864, https://doi.org/10.5194/egusphere-egu23-13864, 2023.

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.

How to cite: Maclennan, M., Winters, A., Shields, C., Wille, J., Baiman, R., Barthelemy, L., and Favier, V.: Antarctic Atmospheric Rivers in the Past and Future Climates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-119, https://doi.org/10.5194/egusphere-egu23-119, 2023.

The McMurdo Dry Valleys (MDVs) in Antarctica have a unique environment classified as a hyper-arid desert with glacier runoff being the main source of liquid water. Previous studies have identified winds as the controlling factor of the climate in this region and especially the occurrence of foehn induced warming. Episodic foehn warming during the austral summer can contribute to above freezing temperatures sustained for multiple days. Years with extreme glacial runoff leading to flooding have been correlated with a higher occurrence of foehn induced warming events. Understanding the temporal availability of meltwater caused by extreme meteorological events is highly important since it is a dependant variable to the functioning of the area’s fragile ecosystem. Synoptic scale circulations in the surrounding Ross Sea Region are a driving factor for the occurrence of foehn warming in the MDVs with the local mesoscale meteorology modulating the spatiotemporal variability of the foehn-induced near-surface warming. AntAir ICE, a newly developed daily mean near surface air temperature dataset with a spatial grid resolution of 1 km² has proven capable of capturing these mesoscale temperature variabilities for multiple seasons within the complex topography of the MDVs.

A case study on the 2^nd of January 2020 where the maximum temperature measured in a Lake Vanda automatic weather station was above +9 degrees Celsius with multiple valleys experiencing foehn induced warming, displayed a clear warming signal for the MDVs in AntAir ICE. The atmospheric dynamic analysis from the numerical weather prediction model the Antarctic Mesoscale Prediction System (AMPS) indicated a clear foehn signature. This event was linked to a meso-low located in the Ross Sea which was detected in the climate re-analysis ERA5 mean sea level pressure dataset. By confidently identifying these warming events within the MDVs where there is a relatively high availability of Automatic Weather Stations and AMPS predictions, has allowed for further exploration of extreme sustained warming and potentially foehn induced warming along the terrestrial coastal margin of Antarctica. Using AntAir ICE, warming events during the austral summer season from November to February for the period 2003 to 2021 with sustained daily mean temperatures above freezing for multiple days have been identified for the Ross Sea Region. This study aims at capturing the mesoscale meteorological and climatological variability for multiple seasons within the Ross Sea Region, while linking these extreme warming events to larger scale circulation patterns can allow for understanding local extreme events in context of shifting large scale circulation drivers.

How to cite: Bendix Nielsen, E., Katurji, M., Zawar-Reza, P., and Meyer, H.: Extreme temperature events for the past 19 years in the McMurdo Dry Valleys, Antarctica linked to mesoscale meteorological variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8805, https://doi.org/10.5194/egusphere-egu23-8805, 2023.

The quantity and characteristics of atmospheric rivers over Antarctica, which import heat and moisture towards the continent, are a major source of uncertainty in future sea level rise estimates. We employ a new variable-resolution grid over Antarctica, using CESM2 (VR-CESM2), which balances the extensibility of a GCM with the high computational costs of a high-resolution climate model. This setup uses observed sea surface temperature and sea ice concentration, implements moisture-tagging (linking precipitation to a moisture source region on the globe), and produces high spatial and temporal resolution atmosphere and ice sheet surface outputs, which can be used to detect atmospheric rivers and to estimate their impact.

As a baseline for experiments testing the relative importance of large-scale drivers, we first quantify, over an idealized 10-year period, the global sources of moisture and the portion of total precipitation that reaches the ice sheet during large-scale vs atmospheric river events (and their associated synoptic characteristics). Beyond this baseline, we will use this setup to perform initial test scenarios assessing the relative impact of reduced sea ice combined with enhanced ocean heat at lower latitudes.

How to cite: Datta, R., Herrington, A., Trusel, L., Schneider, D., Nusbaumer, J., and Yin, Z.: Global Sources of Moisture for Atmospheric Rivers over Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10042, https://doi.org/10.5194/egusphere-egu23-10042, 2023.

Polynyas are recurrent areas of open water or thin ice within the ice pack, which alter the local heat and moisture exchange and high-latitude atmosphere-ocean circulation interannual variability. They are differentiated as coastal (latent heat) or open-ocean (sensible heat) polynya according to their forming location. Especially, coastal polynyas are critical sources of dense water and the formation of Antarctic Bottom Water (AABW) following the brine enrichment of surface waters during sea-ice formation, and easily influenced by the local atmosphere conditions. However, few studies have examined the atmospheric response of open-ocean polynyas on the coastal polynyas given the fact that open-ocean polynyas have capability to re-adjust mesoscale atmosphere circulation. To better understand the surrounding impact of large open-ocean polynya events, output from CMIP6 historical experiment synoptic scale EC-Earth3 is adopted. Our results show an increasing coastal polynya frequency and extent accompanying with more active open-ocean polynya years in the Weddell Sea. The results are explained by near-surface wind speed differences in the coastal regions, which are found statistically significant between more and less active open-ocean polynya years. Furthermore, those intensifications of winds are found in days where easterly-dominated winds north-westerly to north-easterly and easterly to south-easterly cross the open-ocean polynya. Increased near-surface air temperatures as well as a deepening in sea level pressure are also observed during the years with more active open-ocean polynya events. The findings contribute to a better understanding of coastal polynya opening processes, as well as how we might expect to see the different type of polynya interact by their influence and dependence on surrounding atmospheric conditions.

How to cite: Gunnarsson, J., Zhou, L., and Heuzé, C.: The Atmospheric effects of Southern Ocean open-ocean polynyas onto coastal polynyas in EC-Earth3, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2311, https://doi.org/10.5194/egusphere-egu23-2311, 2023.

The current period of Arctic amplification has been characterized by a pronounced reduction in high-latitude snow and ice cover that is reflective of rapidly changing thermodynamic environment. Given this change in the local background conditions, it is not surprising that the Greenland Ice Sheet (GrIS) has undergone drastic surface mass loss since the turn of the century; however, research has shown that the recent acceleration of runoff from the GrIS is strongly linked to a shift in the large-scale atmospheric circulation over the same period that has brought more frequent and intense bouts of summer Greenland blocking. While this atmospheric dynamical change may merely be a manifestation of internal variability, there is growing evidence that widespread changes in surface cover and near-surface thermal gradients under Arctic amplification may favor persistent extremes such as the episodes of Greenland blocking that have encouraged melt of the ice sheet.

Here, we explore whether the change in summer atmospheric circulation over Greenland may be a dynamical response to Arctic amplification and attendant snow cover loss. Our results suggests that low North American spring snow cover and a weakened meridional temperature gradient combine to encourage the high-amplitude Omega blocking patterns that we show to have driven the recent trend in summer Greenland blocking. We show that this delayed response to anomalous spring snow cover follows from the snow-hydrological effect, whereby low spring snow cover causes early depletion of soil moisture and anomalously warm surface temperature over eastern North America. The consequent stationary Rossby wave response enforces an anomalous anticyclone, centered over Baffin Bay, that resembles that of high-amplitude Omega blocks and the atmospheric conditions which have promoted melt of the northern GrIS. Together, these results provide evidence that Arctic amplification, and thus anthropogenic climate change, has contributed to recent atmospheric dynamical forcing of GrIS surface mass loss. However, regardless of how strong this link between climate change and atmospheric circulation over Greenland may be, the change in the local thermodynamic environment under Arctic amplification represents a far more robust climate change signal. We also examine the thermodynamic contribution to GrIS surface mass loss using the regional climate model, Modèle Atmosphérique Régional (MAR) associated with blocking circulation. MAR output of surface temperature, meltwater production, and runoff are used to assess the differential impact of blocking events across the ice sheet.

How to cite: Preece, J., Mote, T., and Wachowicz, L.: Examining Atmospheric Dynamical Forcing of Greenland Ice Sheet Surface Mass Loss Within the Context of Arctic Amplification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10541, https://doi.org/10.5194/egusphere-egu23-10541, 2023.

Greenland Ice Sheet (GrIS) snow melting rates have drastically increased since the 1990s, with relevant implications in the entire ecosystem. According to climate projections, extreme weather events will potentially increase in the coming decades over the GrIS. Thus, it is necessary to analyze the past temporal evolution of GrIS extreme melting patterns, as well as their climate drivers. This work analyzes the GrIS summer extreme snow melting spatiotemporal evolution and trends (1990 to 2021). Further, we determine the contribution of synoptic weather types that drive extreme snow melting events. Results evidence that the frequency, magnitude, and the relative contribution of extreme snow melting to the total summer snow melting differs depending on the GrIS sector. Maximum extreme snow melting days per season are observed in western GrIS, whereas minimums are observed in northern sectors. The average extreme snow melting during summer is non-statistically significant increasing in the entire GrIS, which is consistent with the increase of the average snow melting for the same temporal period. Extreme snow melting days as well as the contribution of extreme snow melting to the total snow melting per season show an upward trend, except in the central and northern zones. The analysis of twenty summer circulation weather types reveals that extreme snow melting episodes for most of the GrIS sectors are mainly explained by a few synoptic systems; characterized by a high-pressure system located in central, southern, and eastern GrIS. During these synoptic episodes, stable weather conditions prevail, and the energy available for snow melting is mainly controlled by positive shortwave radiation heat fluxes leading to positive 850 hPa air temperature anomalies. Results presented in this work are relevant for a better understanding of extreme weather events over GrIS within a changing climate context.

How to cite: Bonsoms, J., Oliva, M., and López-Moreno, J. I.: Evaluation of Greenland extreme snow melting patterns and their synoptic drivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-814, https://doi.org/10.5194/egusphere-egu23-814, 2023.

The air temperature lapse rate (TLR) plays an important role in estimating ice and snow melt in high mountain regions. The TLR can vary depending on several factors, including the topography of the catchments and the microclimate. TLR calculations are typically not precise in the Himalayan glacierised regions due to a lack of in-situ observation of meteorological parameters. Therefore, a dense in-situ monitoring network over a high altitudinal gradient is needed to estimate the TLR accurately. We have obtained in-situ measurements of air temperature data from five automatic weather stations (AWS) installed at the best possible locations in the Chandra basin catchment of the semi-arid zone of the western Himalaya from October 2019 to September 2022. The altitudinal range for air temperature measurement varied between ~4000 and 5000 m a.s.l. We utilise the air temperature data to estimate the TLR by regressing the temperature with the corresponding elevations.
Comparing all the estimated TLR, the mean annual value (4.9°C/km) was significantly lower than the standard environmental lapse rate (6.5 °C/km) with substantial seasonality. The maximum TLR (~6.8 °C/km) during the summer is likely due to the high-altitude range and thin air and the presence of cold air pools at higher altitudes. However, the significantly lower TLR (~1.9 °C/km) during winters is likely due to the low air temperature and high moisture content in the region due to western disturbance. Further, we observed strong diurnal variations of TLR, which was highest during the daytime and lowest at night. This study highlighted that the TLR was potentially influenced by the local topography, particularly with higher lapse rates at higher elevations. TLR vary topographically and temporally significantly in the Chandra basin, indicating that the air temperature in this region is more sensitive to climatic variations. The findings of this study will play an important role in glacio-hydrological models, where TLR is one of the essential inputs.

How to cite: Oulkar, S. N., Sharma, P., Pratap, B., Patel, L., Laha, S., and Thamban, M.: Spatio-temporal variability of air temperature lapse rate in the glacierised catchment of the Chandra basin, western Himalaya using in-situ measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3192, https://doi.org/10.5194/egusphere-egu23-3192, 2023.

The Arctic climate system has been suggested to be ‘en route’ to a new state with seasonally ice-free conditions expected within two-three decades under high-emissions scenarios. Here we show the prospect of its delayed emergence stemming from a consideration of observed and modelled Arctic cryosphere sensitivity to atmospheric circulation changes. While the observed Arctic warming contains a substantial contribution from large-scale circulation, it is not reflected in the modelled forced response. Numerical model simulations with the CESM2 with an active Greenland ice sheet model (CISM2), where model winds are nudged towards the observed state, advocate for the need to have a circulation-based model sensitivity evaluation metric. Hence a recalibration is proposed by matching the warming signals free of atmospheric circulation impacts in observations and models over 1979-2020. This constraint yields a ~decade delay in the projected timing of the first seasonally sea-ice free Arctic and widespread Greenland melting. Accounting for the role of large-scale atmospheric forcing in Arctic climate change offers new perspectives of estimating Arctic sea- and land-ice sensitivity to anthropogenic forcing and understanding the recently emerging issue of some CMIP6 climate models being ‘too hot’.

How to cite: Topal, D. and Ding, Q.: Atmospheric circulation-constrained model sensitivity recalibrates Arctic climate projections, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3971, https://doi.org/10.5194/egusphere-egu23-3971, 2023.

Changes in the Greenland ice sheet (GrIS) firn layer may impact its ability to retain meltwater. These changes also need to be accounted for when converting measured ice sheet volume changes to mass changes. With a firn model (IMAU-FDM v1.2G) forced by a regional climate model (RACMO2.3p2), we investigate how the GrIS firn layer depth and pore space have evolved since 1958 in response to variability in the large-scale atmospheric circulation. On interannual timescales, the firn layer’s depth and pore space shows a spatially heterogeneous response to variability in the North Atlantic Oscillation (NAO). Notably, a stronger NAO following the record warm summer of 2012 led the firn layer in the south and east of the ice sheet to regain thickness and pore space after a period of thinning and reduced pore space. The main driving forces behind these changes vary between GrIS sectors: in the southwest, a decrease in melt dominates, whereas in the east an increase in snow accumulation dominates. However, these trends are not uniform across the GrIS, and over the same period, the firn in the northwest continued to lose pore space. The NAO is also stronger in winter than in summer and we observe that this impacts the seasonal cycle of the firn. In the wet southeastern GrIS, most of the snow accumulates during the winter, when firn compaction is slow, amplifying the seasonal cycle in firn depth and pore space. The opposite occurs in other regions, where snowfall peaks in summer or autumn, at the same time as densification and melt, damping the seasonal oscillations in the firn thickness and pore space.

How to cite: Brils, M., Kuipers Munneke, P., and van den Broeke, M.: Spatial response of Greenland’s firn layer to NAO variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13291, https://doi.org/10.5194/egusphere-egu23-13291, 2023.