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Cycling data assimilation and forecast experiments in August 2016 together with a case study of an intense Arctic cyclone (AC16) are performed. Initial conditions from newly developed Multi-Resolution Incremental Four-Dimensional Variational (MRI-4DVAR) and Three-Dimensional Variational (3DVAR) data assimilation along with forecasts from the polar version of the Weather Research and Forecasting (Polar WRF) model, mimicking operational configurations, are applied. The tasks are to evaluate MRI-4DVAR performance during a 20-day cycling run, to investigate the impacts of initial conditions on the forecast skill of AC16, and to identify the factors impacting AC16's predictability. The results from the 20-day cycling period demonstrate the robustness and reliability of MRI-4DVAR for data assimilation and subsequent forecast skill. Multiple processes, including mergers of Arctic cyclones, mergers of vortices, vertical coupling between low-level and upper-level circulations, baroclinic processes and jet stream forcing, contributed to the generation and development of AC16. Compared to the initial conditions from 4DVAR, 3DVAR produced amplified polar vortices, stronger baroclinic instability, intensified upper-level jet streams and a stronger low-level frontal zone, causing the overdevelopment of AC16 in 3DVAR-based forecasts. For MRI-4DVAR, the accurate prediction of AC16 5-7 days ahead is likely due primarily to the more accurate representation of upper-level atmospheric fields, that was facilitated by better satellite radiance assimilation with MRI-4DVAR that also produced a balanced initial model state. It is concluded that the high-resolution Polar WRF which is optimized for Arctic conditions combined with 4DVAR facilitated the improved prediction of AC16 compared to the Global Forecast System (GFS) operational deterministic global forecast. [ABSTRACT FROM AUTHOR]

Radiosonde observations over Antarctica and the surrounding oceans were enhanced during the Year of Polar Prediction in the Southern Hemisphere (YOPP‐SH) summer Special Observing Period (SOP). Observing System Experiments (OSEs) were conducted in a continuous cycling framework using the Weather Research and Forecasting (WRF) Model and its data assimilation system. Routinely available observations were assimilated in the CTL (control) experiment, and special radiosonde observations from the YOPP‐SH SOP were additionally assimilated in the YOPP experiment. The results were compared to investigate the effects of additional radiosonde observations on analyses and forecasts over and around Antarctica. Verifications against ERA5 re‐analysis, radiosonde observations, and Automatic Weather Station (AWS) observations show overall positive effects of additional radiosonde observations. These positive effects are most noticeable in temperature at lower levels at earlier forecast lead times; afterward, wind forecast improvements at upper levels are the most noticeable. Although routine and special radiosonde observations are concentrated over the eastern and coastal regions of Antarctica (compared to the western and inland regions), the effects of the extra data spread in longitudinal and latitudinal directions; therefore, the effects on the forecasts are not limited to only the areas near the radiosonde observations. A case study reveals how cyclone forecasts are improved through the assimilation of the additional YOPP‐SH SOP radiosonde observations. This study provides insights into future observation strategies in Antarctica, such as horizontal/vertical observation locations, observation variables, and so forth to maximize effects of new observations on forecasts over Antarctica. [ABSTRACT FROM AUTHOR]

The Antarctica Peninsula (AP) has experienced more frequent and intense surface melting recently, jeopardizing the stability of ice shelves and ultimately leading to ice loss. Among the key phenomena that can initiate surface melting are atmospheric rivers (ARs) and leeside foehn; the combined impact of ARs and foehn led to moderate surface warming over the AP in December 2018 and record‐breaking surface melting in February 2022. Focusing on the more intense 2022 case, this study uses high‐resolution Polar WRF simulations with advanced model configurations, Reference Elevation Model of Antarctica topography, and observed surface albedo to better understand the relationship between ARs and foehn and their impacts on surface warming. With an intense AR (AR3) intrusion during the 2022 event, weak low‐level blocking and heavy orographic precipitation on the upwind side resulted in latent heat release, which led to a more deep‐foehn like case. On the leeside, sensible heat flux associated with the foehn magnitude was the major driver during the night and the secondary contributor during the day due to a stationary orographic gravity wave. Downward shortwave radiation was enhanced via cloud clearance and dominated surface melting during the daytime, especially after the peak of the AR/foehn events. However, due to the complex terrain of the AP, ARs can complicate the foehn event by transporting extra moisture to the leeside via gap flows. During the peak of the 2022 foehn warming, cloud formation on the leeside hampered the downward shortwave radiation and slightly increased the downward longwave radiation. Plain Language Summary: On the Antarctic Peninsula (AP), when ice shelves break up, glaciers flow faster from the land into the sea, leading to ice loss and increasing sea level rise. Surface warming is projected to double by 2050 over Antarctica and may have led to ice shelf collapse. Two phenomena that enhance surface warming are atmospheric rivers, (long corridors of moisture in the atmosphere) and leeside foehn effects (cooler and moist air advection on the upwind side that becomes warmer and drier when descending on the leeside). Here we study two combined atmospheric river and foehn events that led to surface warming on the AP, occurring in December 2018 and February 2022. The main warming mechanism in the northeastern AP during the nighttime was transfer of heat from the air to the ice surface (sensible heat flux), while the main mechanism during the daytime was intense sunlight, which was able to reach the surface because of clear skies on the opposite (lee) side caused by foehn. However, complicating the picture, there are gaps in the AP mountain range that let the atmospheric river through, allowing clouds to form on the other side, which then blocked some of the sunlight. Key Points: This study investigates the atmospheric river and foehn warming over the Antarctic Peninsula via observations and model simulationsAtmospheric rivers led to stronger precipitation on the upwind side and favored the foehn‐related sensible heat transfer on the leesideUnder the combined atmospheric rivers and foehn, shortwave radiation contributed the most to the ice surface warming, followed by sensible heat flux [ABSTRACT FROM AUTHOR]

Data impact experiments are conducted employing the Polar Weather Research and Forecasting (WRF) model during the YOPP‐SH summer special observing period (SOP) using the Antarctic Mesoscale Prediction System (AMPS) framework to determine the forecast impact of numerous additional radiosondes collected during the SOP. Hybrid variational‐ensemble three‐dimensional data assimilation is performed on model forecast domains over Antarctica and the Southern Ocean using all regular observations normally available (Experiment "NoSOP") and using the same set plus the extra soundings launched for the SOP (Experiment "SOP"). The SOP results show better near‐surface temperature and wind‐speed forecasts than the NoSOP results, primarily over West Antarctica. Radiosonde profiles confirm that temperature and wind‐speed forecasts are improved throughout the troposphere with the addition of the SOP radiosonde data, but the results for relative humidity are variable. Temperatures are improved at lower levels early in the forecasts, whereas wind speeds are better at higher levels later in the forecasts. An evaluation against the ERA5 global reanalysis that provides a much broader perspective reveals that the improved forecast skill for the SOP experiment persists up to 72 hours for temperature, wind speed, and relative humidity. The gains, however, are primarily confined to the Antarctic continent, consistent with the additional radiosonde spatial coverage being mainly poleward of 60°S. With extra radiosondes concentrated over the Antarctic Peninsula, SOP forecasts of the region downstream of the Peninsula were significantly improved compared to NoSOP forecasts. In addition, it is found that the assimilation of the additional radiosonde data can have a greater impact on the forecasts of strong cyclones, as shown for a major coastal cyclone affecting West Antarctica, with improvements in its magnitude and track. The results also suggest that increasing radiosonde launches at lower southern latitudes would improve forecasts over the Southern Ocean, especially during austral winter. [ABSTRACT FROM AUTHOR]

Supercooled water is common in the clouds near coastal Antarctica and occasionally occurs at temperatures at or below −30°C. Yet the ice physics in most regional and global numerical models will glaciate out these clouds. This presents a challenge for the simulation of highly supercooled clouds that were observed at McMurdo, Antarctica during the Atmospheric Radiation Measurement (ARM) West Antarctic Radiation Experiment (AWARE) project during 2015–2017. The polar optimized version of the Weather Research and Forecasting model (Polar WRF) with the recently developed two‐moment P3 microphysics scheme was used to simulate observed supercooled liquid water cases during March and November 2016. Nudging of the simulations to observed rawinsonde profiles and Antarctic automatic weather station observations provided increased realism and much greater cloud water amounts. Sensitivity tests that adjust the ice physics for extremely low ice nucleating particle (INP) concentrations decrease cloud ice and increases the cloud liquid water closer to observed amounts. In these tests, a liquid layer near cloud top is simulated, in agreement with observations. Accurate representation of INP concentrations appears to be critical for the simulation of coastal Antarctic clouds. Plain Language Summary: Liquid water is common in the clouds observed at McMurdo Station located on Ross Island, Antarctica. The liquid water occurs even when the temperature is well below freezing and ice might be expected from the common physics of clouds. The pristine atmosphere there has very few particles that are favorable for the formation of cloud ice. Models with cloud physics set for more populated regions of Earth therefore have difficulty simulating these cold liquid clouds. When we adjust our regional model Polar WRF for the pristine conditions, it improves the simulation of the liquid clouds. Also, our simulations can underrepresent the amount of water substance in vapor form in the coastal Antarctic atmosphere. This error appears to arise from the dry external forcing source we use for the regional simulations. When we adjust the simulations by gradually nudging toward balloon‐based observations at McMurdo and other regional observations the amount of vapor increases toward observed values. This has the beneficial result of improving the simulation of clouds. We believe that atmospheric models need to better represent the particulates in the atmosphere to improve the cloud simulations near Antarctica and elsewhere. Key Points: Data assimilation increases the modeled water vapor closer to observed amounts and improves Antarctic clouds in mesoscale simulationsAdding realistic ice nucleating particle (INP) concentrations in models improves simulated clouds in better agreement with observationsHighly supercooled liquid water extending to cloud tops observed at McMurdo can properly be simulated by specifying sparse amounts of INP [ABSTRACT FROM AUTHOR]

The impacts of initial conditions and model configuration on the simulations of polar lows (PLs) near Svalbard, the global "hotspot" for PL activity, were investigated using the Polar Weather Research and Forecasting (Polar WRF or PWRF) model and WRF three‐dimensional data assimilation (3DVAR). The well‐documented PL case presented by Sergeev et al., in 2017 from March 26, 2013 featuring in situ aircraft observations is revisited using PWRF and 3DVAR to identify the most suitable model configuration to simulate PLs. The simulation results using initial conditions from PWRF and 3DVAR cycling runs were compared with those of cold‐start experiments using reanalysis as the initial condition. For cycling runs, the impacts of initial conditions from synoptic observations together with satellite radiances were superior to the assimilation of synoptic data alone. Several additional experiments were undertaken to further refine the PWRF configuration by considering the horizontal resolution, analysis nudging, digital filter initialization, and two boundary‐layer and four microphysics schemes. Based on the chosen configuration of Polar WRF and 3DVAR from the case‐study, the monthly reanalysis mode with 3 hr time window cycling data assimilation and short‐term forecasts using PWRF alone were performed for March 2013 and the PLs that formed over the Nordic Seas were evaluated. Manual identification and verification of PLs were performed using thermal infrared satellite imagery and 10 m scatterometer winds. Overall, nine PLs were identified in addition to the case presented by Sergeev et al., in 2017. Both the case‐study and monthly simulation results show that the high‐resolution initial condition is the most important factor, but a reasonable forecast model configuration can help to improve the simulation performance. Satellite radiance data are very important for producing the high‐resolution initial conditions for Polar WRF to simulate PLs successfully; the best performance was obtained by assimilating both synoptic data and satellite radiances. [ABSTRACT FROM AUTHOR]

The West Antarctic climate is under the combined impact of synoptic and regional drivers. Regional factors have contributed to more frequent surface melting with a similar pattern recently, which accelerates ice loss and favors global sea‐level rise. Part I of this research identified and quantified the two leading drivers of Ross Ice Shelf (RIS) melting, viz. foehn effect and direct marine air advection, based on Polar WRF (PWRF) simulations. In this article (Part II), the impact of clouds and the pattern of surface energy balance (SEB) during melting are analyzed, as well as the relationship among these three factors. In general, net shortwave radiation dominates the surface melting with a daily mean value above 100 W·m−2. Foehn clearance and decreasing surface albedo respectively increase the downward shortwave radiation and increase the absorbed shortwave radiation, significantly contributing to surface melting in areas such as western Marie Byrd Land. Also, extensive downward longwave radiation caused by low‐level liquid cloud favors the melting expansion over the middle and coastal RIS. With significant moisture transport occurring over more than 40% of the time during the melting period, the impact from net radiation can be amplified. Moreover, frequent foehn cases can enhance the turbulent mixing on the leeside. With a Froude number (Fr) around 1 or slightly larger, fast downdrafts or reversed wind flows can let the warm foehn air penetrate down to the surface with up to 20 W·m−2 in sensible heat flux transfer to the ground. However, when the Froude number is close to infinity with breaking waves on the leeside, the contribution of turbulence to the surface warming is reduced. With better understanding of the regional factors for the surface melting, prediction of the future stability of West Antarctic ice shelves can be improved. [ABSTRACT FROM AUTHOR]

West Antarctica (WA), especially the Ross Ice Shelf (RIS), has experienced more frequent surface melting during the austral summer recently. The future is likely to see enhanced surface melting that will jeopardize the stability of ice shelves and cause ice loss. We investigate four major melt cases over the RIS via Polar Weather Research and Forecasting (WRF) simulations (4 km resolution) driven by European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5) reanalysis data and Moderate Resolution Imaging Spectroradiometer (MODIS) observed albedo. Direct warm air advection, recurring foehn effect, and cloud/upper warm air introduced radiative warming are the three major regional causes of surface melting over WA. In this paper, Part I, the first two factors are identified and quantified. The second paper, Part II, discusses the impact of clouds and summarizes all three factors from a surface energy balance perspective. With a high‐pressure ridge located westward towards the Sulzberger Ice Shelf (77° S, 148° W) and a low‐pressure center located between 165° and 180° W, warm marine air from the Ross Sea is advected towards the coastal RIS and leads to surface melting. When the high‐pressure ridge is located farther east towards Marie Byrd Land (120–150° W), the foehn effect can cause a 2–4°C increase in surface temperature on the leeside of the mountains. For three of four melt cases, more than 40% of the melting period experiences foehn warming. Isentropic drawdown is usually the dominant foehn mechanism and contributes up to a 14°C temperature increase, especially when strong low‐level blocking occurs on the upwind side. The thermodynamic mechanism can be important depending on the strength of moisture uptake and condensation on the windward side. Meanwhile, sensible heat flux contributes less to foehn warming, but still plays an important role in the melting. The prediction of future stability of the RIS should include foehn warming as a major driver. [ABSTRACT FROM AUTHOR]

The rarity of reports in the literature of brief and spatially limited observations of drizzle at temperatures below −20 °C suggest that riming and other temperature‐dependent cloud microphysical processes such as heterogeneous ice nucleation and ice crystal depositional growth prevent drizzle persistence in cold environments. In this study, we report on a persistent drizzle event observed by ground‐based remote sensing measurements at McMurdo Station, Antarctica. The temperatures in the drizzle‐producing cloud were below −25 °C and the drizzle persisted for a period exceeding 7.5 hr. Using ground‐based, satellite, and reanalysis data, we conclude that drizzle was likely present in parts of a widespread cloud field, which stretched more than ~1,000 km along the Ross Ice Shelf coast. Parameter space sensitivity tests using two‐moment bulk microphysics in large eddy simulations constrained by the observations suggest that activated ice freezing nuclei and accumulation‐mode aerosol number concentrations aloft during this persistent drizzle period were likely on the order of 0.2 L−1 and 20 cm−3, respectively. In such constrained simulations, the drizzle moisture flux through cloud base exceeds that of ice. The simulations also indicate that drizzle can lead to the formation of multiple peaks in cloud water content profiles. This study suggests that persistent drizzle at these low temperatures may be common at the low aerosol concentrations typical of the Antarctic and Southern Ocean atmospheres. Key Points: Drizzle was observed over McMurdo at cloud T <‐25 degrees celsius for more than 7.5 hr in the presence of ice nucleation and growthAtmospheric conditions necessary for the observed drizzle are evaluated using an LES model with two‐moment microphysicsDrizzle at cloud T <‐25 degrees celsius may be common at the low aerosol concentrations typical of the Antarctic and the Southern Ocean atmospheres [ABSTRACT FROM AUTHOR]

The Ross Ice Shelf (RIS) buttresses ice streams from the Antarctic continent and restrains the grounded ice sheet from flowing into the ocean, which is important for the stability of the ice sheet. In recent decades, West Antarctic ice shelves, including the RIS, have experienced more frequent surface melting during summer. We investigated the role of warm, descending föhn winds in a major melt event that occurred on the RIS in January 2016. Only a few summer melt events of this magnitude have been observed since 1979. Backward trajectories from the area of earliest melting were constructed using the Antarctic Mesoscale Prediction System to investigate the dominant mechanisms at the beginning of the melt event, mainly from 10 to 13 January. Analysis was conducted over two distinct areas. The föhn effect contributed around 2–4 °C to the surface temperature increase over the coastal mountains of Marie Byrd Land and around 1 °C over the much lower Edward VII Peninsula. Most of the föhn warming was caused by isentropic drawdown of air aloft. On 10 January, the second‐most important contributor for both mountain ranges was the thermodynamic mechanism. On 11 January, the second‐most important mechanism was the sensible and radiative heat flux. This study contributes to a better understanding of surface melt events over the RIS and benefits research associated with the stability of West Antarctic ice shelves. The presence of föhn warming during the prominent 2016 melt event over the Ross Ice Shelf is investigated primarily for 10 and 11 January. Through trajectory analysis, detailed quantification of the various physical mechanisms contributing to the föhn warming is provided, particularly in the Siple Dome area. This study provides a better understanding of the climate of the Ross Ice Shelf, which is important to predict its future response to climate change. [ABSTRACT FROM AUTHOR]

Satellite data and a 35-year hindcast of the Amundsen Sea Embayment summer climate using the Weather Research and Forecasting model are used to understand how regional and large-scale atmospheric variability affects thinning of ice shelves in this sector of West Antarctica by melting from above and below (linked to intrusions of warm water caused by anomalous westerlies over the continental shelf edge). El Niño episodes are associated with an increase in surface melt but do not have a statistically significant impact on westerly winds over the continental shelf edge. The location of the Amundsen Sea Low and the polarity of the Southern Annular Mode (SAM) have negligible impact on surface melting, although a positive SAM and eastward shift of the Amundsen Sea Low cause anomalous westerlies over the continental shelf edge. The projected future increase in El Niño episodes and positive SAM could therefore increase the risk of disintegration of West Antarctic ice shelves. [ABSTRACT FROM AUTHOR]

During the late twentieth century, the Antarctic atmospheric circulation has changed and significantly influenced the overall Antarctic climate, through processes including a poleward shift of the circumpolar westerlies. However, little is known about the full spatial pattern of atmospheric pressure over the Antarctic continent prior to 1979. Here we investigate surface pressure changes across the entire Antarctic continent back to 1905 by developing a new summer pressure reconstruction poleward of 60°S. We find that only across East Antarctica are the recent pressures significantly lower than pressures in the early twentieth century; we also discern periods of significant positive pressure trends in the early twentieth century across the coastal South Atlantic sector of Antarctica. Climate model simulations reveal that both tropical sea surface temperature variability and other radiative forcing mechanisms, in addition to ozone depletion, have played an important role in forcing the recent observed negative trends. [ABSTRACT FROM AUTHOR]

Southern Greenland is home to a number of weather systems characterized by high speed low-level winds that are the result of topographic flow distortion. These systems include tip jets, barrier winds and katabatic flows. Global atmospheric reanalyses have proven to be important tools in furthering our understanding of these systems and their role in the climate system. However, there is evidence that their mesoscale structure may be poorly resolved in these global products. Here output from the regional Arctic System Reanalysis (ASRv1-30 km and ASRv2-15 km grid resolutions) are compared to the global ERA-Interim Reanalysis (ERA-I-80 km grid resolution), focusing on their ability to represent winds in the vicinity of southern Greenland. Comparisons are made to observations from surface and upper-air stations, as well as from research aircraft flights during the Greenland Flow Distortion Experiment (GFDex). The ERA-I reanalysis has a tendency to underestimate high wind speeds and overestimate low wind speeds, which is reduced in ASRv1 and nearly eliminated in ASRv2. In addition, there is generally a systematic reduction in the root-mean-square error between the observed and the reanalysis wind speeds from ERA-I to ASRv1 to ASRv2, the exception being low-level marine winds where the correspondence is similar in all reanalyses. Case-studies reveal that mesoscale spatial features of the wind field are better captured in ASRv2 as compared to the ERA-I or ASRv1. These results confirm that a horizontal grid size on the order of 15 km is needed to characterize the impact that Greenland's topography has on the regional wind field and climate. However even at this resolution, there are still features of the wind field that are under-resolved. [ABSTRACT FROM AUTHOR]

The Arctic System Reanalysis version 1 (ASRv1), a high-resolution regional assimilation of model output, observations and satellite data across the mid- and high latitudes of the Northern Hemisphere, and the global European Centre for Medium Range Forecasting Interim Reanalysis (ERAI) are compared with atmospheric observations for the period December 2006 to November 2007. Results throughout the troposphere show observations to be well assimilated in the ASRv1, as monthly and annual near-surface (upper-level) temperature, dew-point (relative humidity), pressure (geopotential height) and wind-speed biases compared with surface stations and radiosondes are very small. These results are similar to the ERAI, although wind-speed biases are significantly smaller in the ASRv1. Despite the ASRv1's use of a 3D-variational (Var) assimilation compared with the ERAI's 4D-Var, similar results suggest that a regional approachwith higher-resolution terrain and a detailed land-surface description forced by a global reanalysismay improve the assimilation of observations and help offset temporal information lost by the 3D-Var compared with the 4D-Var. However, the ASRv1 forecast field results compared with the ERAI are mixed. The ASRv1 and ERAI show negative precipitation biases during cool months compared with gauge observations, and too much precipitation falls in the ASRv1 during summer in the midlatitudes. Stations north of 60°N demonstrate smaller precipitation biases in the ASRv1 than the ERAI except during the summer, when the ASRv1 is very dry. Short-wave radiation compared with observations is much too large in the ASRv1, and both reanalyses show long-wave radiation deficits during most months. These results point to inadequacies in model physics in the ASRv1 (e.g. convective and radiation schemes) that will continue to be refined in subsequent versions of the ASR. [ABSTRACT FROM AUTHOR]

Foehn winds are a prominent feature of the McMurdo Dry Valleys (MDVs) climate, and are responsible for periods of strong winds and warming. The foehn mechanism determined from a case study presented in earlier work is shown here to be robust for a set of the MDVs summer foehn events over the 1994-2009 period using output from the Polar Weather Research and Forecasting Model (Polar WRF). Gap flow south of the MDVs is evidenced by the positive relationship between the pressure gradient and near-surface wind speed along the gap. Subsequently, mountain waves are generated and result in adiabatic warming and the downward transport of warm air into the MDVs, and differences in mountain wave characteristics depend on the ambient wind direction and the degree of flow nonlinearity. Pressure-driven channelling then brings warm foehn air downvalley. Although a large range of synoptic-scale circulation patterns can drive foehn events, the warmest foehn events are typically associated with blocking highs over the Australian sector of the Southern Ocean, leading to warm air advection over continental Antarctica. The episodic nature of foehn events, and the tenuous connections between such events and interannual modes of climate variability, suggests that intraseasonal variability may be more important for determining their frequency and magnitude. The extraordinarily warm austral summer of 2001/2002 across Antarctica shows that advection of warm maritime air into the continental interior and strong flow aloft result in warm foehn conditions and significant melt for the MDVs. [ABSTRACT FROM AUTHOR]

Foehn events over the McMurdo Dry Valleys (MDVs), the largest ice-free region of Antarctica, promote glacial melt that supports biological activity in the lakes, streams, rocks and soils. Although MDVs foehn events are known to depend upon the synoptic-scale circulation, the physical processes responsible for foehn events are unknown. A polar-optimized version of the Weather Research and Forecasting model (Polar WRF) is used for a case study of a representative summer foehn event from 29 December 2006 to 1 January 2007 in order to identify and explain the MDVs foehn mechanism. Pressure differences across an elevated mountain gap upstream of the MDVs provide forcing for southerly flow into the western, upvalley entrance of the MDVs. Complex terrain over the elevated gap and the MDVs leads to mountain wave effects such as leeside acceleration, hydraulic jumps, wave breaking and critical layers. These mountain wave effects depend on the ambient (geostrophic) wind direction. Pressure-driven channelling then brings the warm, dry foehn air downvalley to eastern MDV sites. Brief easterly intrusions of maritime air into the eastern MDVs during foehn events previously have been attributed to either a sea-breeze effect in summer or local cold-pooling effects in winter. In this particular case, the easterly intrusions result from blocking effects of nearby Ross Island and the adjacent Antarctic coast. Temperature variability during the summer foehn event, which is important for meltwater production and biological activity when it exceeds 0°C, primarily depends on the source airmass rather than differences in foehn dynamics. [ABSTRACT FROM AUTHOR]

Warm, dry and gusty foehn winds are frequently experienced in the McMurdo Dry Valleys (MDVs), Antarctica; however, their significance in the region's climate is unknown. Foehn events in the MDVs are caused by topographic modification of southwesterly airflow which is related to the occurrence of synoptic-scale cyclones in the Amundsen/Ross Sea region. The intra- and interannual frequency and intensity of foehn events therefore varies in response to the position and frequency of cyclones in this region that are believed to be strongly influenced by the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Here, we present a 20-year climatology of foehn winds from observational records in the MDVs. The SAM is found to significantly influence foehn wind frequency during the Antarctic summer and autumn months, whereas ENSO only holds significant correlations with winter air temperatures in the MDVs. The positive relationship between the SAM and the foehn wind regime in summer is particularly significant as foehn winds frequently cause summer temperatures to rise above 0 °C leading to extensive melt and thaw in MDVs. Foehn winds are a major climatological feature of the MDVs with their frequency and duration affecting the region's temperature records and their trends. Accordingly, analysis of the region's weather and climate records and predictions of future impacts of climate change on the MDVs is incomplete without consideration of foehn winds and their influence. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]