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Atmospheric rivers (ARs) are the primary mechanism for transporting water vapor from low latitudes to polar regions, playing a significant role as drivers of extreme weather, such as heavy precipitation and heat waves in both the Arctic and Antarctica. With the rapidly growing interest in polar ARs during the past decade, it is imperative to establish an objective framework to quantify the strength and impact of these ARs for both scientific research and practical application. The AR scale introduced by Ralph et al. (2019) ranks ARs based on the duration of AR conditions and the intensity. However, the thresholds of integrated water vapor transport (IVT) used to rank ARs are selected based on the IVT climatology at middle latitudes. These thresholds are insufficient for polar regions due to the substantially lower temperature and moisture content. In this study, we analyze the IVT climatology in polar regions, focusing on the coasts of Antarctica and Greenland. Then we introduce an extended version of the AR scale tuned to polar regions by adding lower IVT thresholds of 100, 150, and 200 kg m-1 s-1 to the standard AR scale, which starts at 250 kg m-1 s-1. The polar AR scale is utilized to examine AR frequency, seasonality, trends, and associated precipitation and surface melt over the Antarctic and Greenland coasts. The polar AR scale better characterizes the strength and impacts of ARs in the Antarctic and Arctic regions, and has the potential to enhance communications across observation, research, and forecasts for polar regions. [ABSTRACT FROM AUTHOR]

Understanding the drivers of surface melting in West Antarctica is crucial for understanding future ice loss and global sea level rise. This study identifies atmospheric drivers of surface melt on West Antarctic ice shelves and ice sheet margins and relationships with tropical Pacific and high-latitude climate forcing using multidecadal reanalysis and satellite datasets. Physical drivers of ice melt are diagnosed by comparing satellite-observed melt patterns to anomalies of reanalysis near-surface air temperature, winds, and satellite-derived cloud cover, radiative fluxes, and sea ice concentration based on an Antarctic summer synoptic climatology spanning 1979–2017. Summer warming in West Antarctica is favored by Amundsen Sea (AS) blocking activity and a negative phase of the southern annular mode (SAM), which both correlate with El Niño conditions in the tropical Pacific Ocean. Extensive melt events on the Ross–Amundsen sector of the West Antarctic Ice Sheet (WAIS) are linked to persistent, intense AS blocking anticyclones, which force intrusions of marine air over the ice sheet. Surface melting is primarily driven by enhanced downwelling longwave radiation from clouds and a warm, moist atmosphere and by turbulent mixing of sensible heat to the surface by föhn winds. Since the late 1990s, concurrent with ocean-driven WAIS mass loss, summer surface melt occurrence has increased from the Amundsen Sea Embayment to the eastern Ross Ice Shelf. We link this change to increasing anticyclonic advection of marine air into West Antarctica, amplified by increasing air–sea fluxes associated with declining sea ice concentration in the coastal Ross–Amundsen Seas. [ABSTRACT FROM AUTHOR]

Low-level clouds are extensive in the Arctic and contribute to inadequately understood feedbacks within the changing regional climate. The simulation of low-level clouds, including mixed-phase clouds, over the Arctic Ocean during summer and autumn remains a challenge for both real-time weather forecasts and climate models. Here, improved cloud representations are sought with high-resolution mesoscale simulations of the August-September 2008 Arctic Summer Cloud Ocean Study (ASCOS) with the latest polar-optimized version (3.7.1) of the Weather Research and Forecasting (Polar WRF) Model with the advanced two-moment Morrison microphysics scheme. Simulations across several synoptic regimes for 10 August-3 September 2008 are performed with three domains including an outer domain at 27-km grid spacing and nested domains at 9-and 3-km spacing. These are realistic horizontal grid spacings for common mesoscale applications. The control simulation produces excessive cloud liquid water in low clouds resulting in a large deficit in modeled incident shortwave radiation at the surface. Incident longwave radiation is less sensitive. A change in the sea ice albedo toward the larger observed values during ASCOS resulted in somewhat more realistic simulations. More importantly, sensitivity tests show that a reduction in specified liquid cloud droplet number to very pristine conditions increases liquid precipitation, greatly reduces the excess in simulated low-level cloud liquid water, and improves the simulated incident shortwave and longwave radiation at the surface. [ABSTRACT FROM AUTHOR]

Two El Niño flavors have been defined based on whether warm sea surface temperature (SST) anomalies are located in the central or eastern tropical Pacific (CP or EP). This study further characterizes the impacts on atmospheric circulation in the high latitudes of the Southern Hemisphere associated with these types of El Niño events though a series of numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM). Comparing results with the Interim ECMWF Re-Analysis (ERA-Interim), CAM simulates well the known changes to blocking over Australia and a southward shift in the subtropical jet stream across the eastern Pacific basin during CP events. More importantly for the high southern latitudes, CAM simulates a westward shift in upper-level divergence in the tropical Pacific, which causes the Pacific-South American stationary wave pattern to shift toward the west across the entire South Pacific. These changes to the Rossby wave source region impact the South Pacific convergence zone and jet streams and weaken the high-latitude blocking that is typically present in the Amundsen-Bellingshausen Seas during EP events. Anticyclonic flow becomes established farther west in the south central Pacific, modifying high-latitude heat and momentum fluxes across the South Pacific and South Atlantic associated with the ENSO-Antarctic dipole. [ABSTRACT FROM AUTHOR]

A reconstruction of Antarctic monthly mean near-surface temperatures spanning 1958-2012 is presented. Its primary goal is to take advantage of a recently revised key temperature record from West Antarctica (Byrd) to shed further light on multidecadal temperature changes in this region. The spatial interpolation relies on a kriging technique aided by spatiotemporal temperature covariances derived from three global reanalyses [the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim), Modern-Era Retrospective Analysis for Research and Applications (MERRA), and Climate Forecast System Reanalysis (CFSR)]. For 1958-2012, the reconstruction yields statistically significant annual warming in the Antarctic Peninsula and virtually all of West Antarctica, but no significant temperature change in East Antarctica. Importantly, the warming is of comparable magnitude both in central West Antarctica and in most of the peninsula, rather than concentrated either in one or the other region as previous reconstructions have suggested. The Transantarctic Mountains act for the temperature trends, as a clear dividing line between East and West Antarctica, reflecting the topographic constraint on warm air advection from the Amundsen Sea basin. The reconstruction also serves to highlight spurious changes in the 1979-2009 time series of the three reanalyses that reduces the reliability of their trends, illustrating a long-standing issue in high southern latitudes. The study concludes with an examination of the influence of the southern annular mode (SAM) on Antarctic temperature trends. The results herein suggest that the trend of the SAM toward its positive phase in austral summer and fall since the 1950s has had a statistically significant cooling effect not only in East Antarctica (as already well documented) and but also (only in fall) in West Antarctica. [ABSTRACT FROM AUTHOR]

A reconstruction of Antarctic monthly mean near-surface temperatures spanning 1958-2012 is presented. Its primary goal is to take advantage of a recently revised key temperature record from West Antarctica (Byrd) to shed further light on multidecadal temperature changes in this region. The spatial interpolation relies on a kriging technique aided by spatiotemporal temperature covariances derived from three global reanalyses [the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim), Modern-Era Retrospective Analysis for Research and Applications (MERRA), and Climate Forecast System Reanalysis (CFSR)]. For 1958-2012, the reconstruction yields statistically significant annual warming in the Antarctic Peninsula and virtually all of West Antarctica, but no significant temperature change in East Antarctica. Importantly, the warming is of comparable magnitude both in central West Antarctica and in most of the peninsula, rather than concentrated either in one or the other region as previous reconstructions have suggested. The Transantarctic Mountains act for the temperature trends, as a clear dividing line between East and West Antarctica, reflecting the topographic constraint on warm air advection from the Amundsen Sea basin. The reconstruction also serves to highlight spurious changes in the 1979-2009 time series of the three reanalyses that reduces the reliability of their trends, illustrating a long-standing issue in high southern latitudes. The study concludes with an examination of the influence of the southern annular mode (SAM) on Antarctic temperature trends. The results herein suggest that the trend of the SAM toward its positive phase in austral summer and fall since the 1950s has had a statistically significant cooling effect not only in East Antarctica (as already well documented) and but also (only in fall) in West Antarctica. [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]

ABSTRACT The Adélie Land coastal region of Antarctica is one of the most prominent cyclogenesis regions in the Southern Hemisphere, and is adjacent to the continent's most intense katabatic wind regime. However, the physical mechanisms responsible for cyclogenesis are not known. A manual analysis of cyclogenesis for the 2003-2005 period using output from the Antarctic Mesoscale Prediction System (AMPS) identifies two primary patterns of cyclogenesis near the Adélie Land coast. For 'secondary development' cyclones, enhanced low-level cyclonic vorticity and baroclinicity result from the combination of an existing synoptic-scale cyclone to the west, coastal barrier winds and katabatic winds. 'Lee cyclogenesis' occurs near 152°E on the cyclonic-shear side of the Adélie Land katabatic jet, where a low-level warm potential temperature anomaly sets up a lee trough that becomes mobile with the arrival of upper-level synoptic-scale forcing. The representation of both 'secondary development' and 'lee cyclogenesis' cyclones in an automated cyclone-tracking scheme is explored, where it is found that the automated scheme overestimates cyclogenesis for this region. The location of the Antarctic coastal cyclogenesis maximum near Adélie Land is due to the unique juxtaposition of the extraordinary katabatic wind regime and dissipating synoptic-scale cyclones to the west. [ABSTRACT FROM AUTHOR]

Meteorological station records and regional climate model output are combined to develop a continuous 168-yr (1840–2007) spatial reconstruction of monthly, seasonal, and annual mean Greenland ice sheet near-surface air temperatures. Independent observations are used to assess and compensate for systematic errors in the model output. Uncertainty is quantified using residual nonsystematic error. Spatial and temporal temperature variability is investigated on seasonal and annual time scales. It is found that volcanic cooling episodes are concentrated in winter and along the western ice sheet slope. Interdecadal warming trends coincide with an absence of major volcanic eruptions. Year 2003 was the only year of 1840–2007 with a warm anomaly that exceeds three standard deviations from the 1951–80 base period. The annual whole ice sheet 1919–32 warming trend is 33% greater in magnitude than the 1994–2007 warming. The recent warming was, however, stronger along western Greenland in autumn and southern Greenland in winter. Spring trends marked the 1920s warming onset, while autumn leads the 1994–2007 warming. In contrast to the 1920s warming, the 1994–2007 warming has not surpassed the Northern Hemisphere anomaly. An additional 1.0°–1.5°C of annual mean warming would be needed for Greenland to be in phase with the Northern Hemispheric pattern. Thus, it is expected that the ice sheet melt rates and mass deficit will continue to grow in the early twenty-first century as Greenland’s climate catches up with the Northern Hemisphere warming trend and the Arctic climate warms according to global climate model predictions. [ABSTRACT FROM AUTHOR]

Abstract: A survey of the spatial and temporal behavior of the atmospheric general circulation as it relates to both polar regions is presented. The review is based on the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year reanalysis (ERA-40), updated using ECMWF operational analyses. The analysis spans 1960–2005 in the Northern Hemisphere, but is restricted to 1979–2005 in the Southern Hemisphere because of difficulties experienced by ERA-40 prior to the modern satellite era. The seasonal cycle of atmospheric circulation is illustrated by focusing on winter and summer. The huge circulation contrasts between the land-dominated Northern Hemisphere and the ocean-dominated Southern Hemisphere stand out. The intensification of the North Atlantic Oscillation/Northern Annular Mode and the Southern Annular Mode in DJF is highlighted and likely due to warming of the tropical Indian Ocean. The Arctic frontal zone during northern summer and the semi-annual oscillation throughout the year in the Southern Hemisphere are prominent features of the high latitude circulation in the respective hemispheres. Rotated principal component analysis (RPCA) is used to describe the primary modes of temporal variability affecting both polar regions, especially the links with the tropical forcing. The North Atlantic Oscillation is a key modulator of the atmospheric circulation in the North Atlantic sector, especially in winter, and is the dominant control on the moisture transport into the Arctic Basin. The Pacific-South American teleconnection patterns are primary factors in the high southern latitude circulation variability throughout the year, especially in the Pacific sector of Antarctica where the majority of moisture transport into the continent occurs. [Copyright &y& Elsevier]

Regional climate model runs using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesocale Model modified for use in polar regions (Polar MM5), calibrated by independent in situ observations, demonstrate coherent regional patterns of Greenland ice sheet surface mass balance (SMB) change over a 17-yr period characterized by warming (1988–2004). Both accumulation and melt rates increased, partly counteracting each other for an overall negligible SMB trend. However, a 30% increase in meltwater runoff over this period suggests that the overall ice sheet mass balance has been increasingly negative, given observed meltwater-induced flow acceleration. SMB temporal variability of the whole ice sheet is best represented by ablation zone variability, suggesting that increased melting dominates over increased accumulation in a warming scenario. The melt season grew in duration over nearly the entire ablation zone by up to 40 days, 10 days on average. Accumulation area ratio decreased by 3%. Albedo reductions are apparent in five years of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived data (2000–04). The Advanced Very High Resolution Radiometer (AVHRR)-derived albedo changes (1988–99) were less consistent spatially. A conservative assumption as to glacier discharge and basal melting suggests an ice sheet mass loss over this period greater than 100 km3 yr-1, framing the Greenland ice sheet as the largest single glacial contributor to recent global sea level rise. Surface mass balance uncertainty, quantified from residual random error between model and independent observations, suggests two things: 1) changes smaller than approximately 200 km3 yr-1 would not satisfy conservative statistical significance thresholds (i.e., two standard deviations) and 2) although natural variability and model uncertainty were separated in this analysis, the magnitude of each were roughly equivalent. Therefore, improvements in model accuracy and analysis of longer periods (assuming larger changes) are both needed for definitive mass balance change assessments. [ABSTRACT FROM AUTHOR]

Many aspects of reanalysis data are of high quality over regions with sufficiently dense data, but the accuracy is uncertain over areas with sparse observations. NCEP–NCAR reanalysis (NNR) and ECMWF 15/40-Yr Re-Analysis (ERA-15 and ERA-40) variables are compared to two independent rawinsonde datasets from the periphery of the Arctic Ocean during the late 1980s and early 1990s: the Coordinated Eastern Arctic Research Experiment (CEAREX) and the Lead Experiment (LeadEx). The study is prompted by J. A. Francis who found that the NNR and ERA-15 upper-level winds are very different from those observed during these two field experiments. All three reanalyses display large biases in comparisons of the wind components and wind speeds with CEAREX observations, particularly above the 500-hPa level, but exhibit smaller discrepancies with respect to the LeadEx data, generally consistent with the previous findings of J. A. Francis. However, all three reanalyses well capture the wind variability during both experiment periods. For the geopotential height, temperature, and moisture fields, the reanalyses demonstrate close agreement with the CEAREX rawinsonde observations. From comparisons with surrounding fixed rawinsonde stations and examination of the average vertical wind speed shear, it is concluded that the CEAREX upper-level wind speeds (especially above the 500-hPa level) are erroneous and average about half of the actual values. Thus, this evaluation suggests that the three reanalyses perform reliably for tropospheric-state variables from the edge of the Arctic Ocean during the modern satellite era. [ABSTRACT FROM AUTHOR]

Regional climate simulations are conducted using the Polar fifth-generation Pennsylvania State University (PSU)–NCAR Mesoscale Model (MM5) with a 60-km horizontal resolution domain over North America to explore the summer climate of the Last Glacial Maximum (LGM: 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). Output from a tailored NCAR Community Climate Model version 3 (CCM3) simulation of the LGM climate is used to provide the initial and lateral boundary conditions for Polar MM5. LGM boundary conditions include continental ice sheets, appropriate orbital forcing, reduced CO2 concentration, paleovegetation, modified sea surface temperatures, and lowered sea level. The simulated LGM summer climate is characterized by a pronounced low-level thermal gradient along the southern margin of the LIS resulting from the juxtaposition of the cold ice sheet and adjacent warm ice-free land surface. This sharp thermal gradient anchors the midtropospheric jet stream and facilitates the development of synoptic cyclones that track over the ice sheet, some of which produce copious liquid precipitation along and south of the LIS terminus. Precipitation on the southern margin is orographically enhanced as moist southerly low-level flow (resembling a contemporary Great Plains low-level jet configuration) in advance of the cyclone is drawn up the ice sheet slope. Composites of wet and dry periods on the LIS southern margin illustrate two distinctly different atmospheric flow regimes. Given the episodic nature of the summer rain events, it may be possible to reconcile the model depiction of wet conditions on the LIS southern margin during the LGM summer with the widely accepted interpretation of aridity across the Great Plains based on geological proxy evidence. [ABSTRACT FROM AUTHOR]

To evaluate and improve the treatment of clouds and radiation by the climate models of the National Center for Atmospheric Research (NCAR), simulations by the NCAR Community Climate Model version 3 (CCM3), as well as the recently released Community Atmosphere Model version 2 (CAM2), are examined. The Rasch and Kristjánsson prognostic cloud condensate scheme, which is now the standard scheme for CAM2, is included in a version of CCM3 and evaluated. Furthermore, the Rapid Radiative Transfer Model (RRTM), which alleviates the deficit in downward clear-sky longwave radiation, is also included in a version of CCM3. The new radiation scheme in CAM2 also alleviates the clear-sky longwave bias, although RRTM is not included. The impact of the changes is especially large over the interior of Antarctica. The changes induced by the introduction of the prognostic cloud scheme are found to have a much larger impact on the CCM3 simulations than do those from the introduction of RRTM. The introduction of the prognostic cloud scheme increases cloud emissivity in the upper troposphere, reduces cloud emissivity in the lower troposphere, and results in a better vertical distribution of cloud radiative properties over interior Antarctica. The climate simulations have a very large cold bias in the stratosphere, especially during summer. There are significant deficiencies in the simulation of Antarctic cloud radiative effects. The optical thickness of Antarctic clouds appears to be excessive. This contributes to a warm bias in surface temperature during winter and a deficit in downward shortwave radiation during summer. Some biases for Antarctica are larger for CCM3 with the prognostic cloud condensate scheme than with the standard diagnostic clouds. When the mixing ratio threshold for autoconversion from suspended ice cloud to falling precipitation is reduced toward a more realistic value, the Antarctic clouds are thinned and some of the biases are reduced. To improve the surface energy balance, not only must the radiative effects of clouds be improved, it is also necessary to improve the representation of sensible heat flux. Insufficient vertical resolution of the frequently very shallow, very stable surface boundary layer apparently contributes to an excessive heat flux from the atmosphere to the surface during winter. The representations of Antarctic clouds and radiation by the new NCAR CAM2 are not clearly improved compared to those of the earlier CCM3. For example, the surface albedo over Antarctica is descreased in CAM2 and Community Climate System Model version 2 (CCSM2) simulations in comparison to CCM3 simulations, contributing to a summer warm bias in tropospheric temperature for the former. [ABSTRACT FROM AUTHOR]

The impacts of the El Niño–Southern Oscillation (ENSO) on the Antarctic region are of special importance in evaluating the variability and change of the climate system in high southern latitudes. In this study, the ENSO signal in modeled precipitation over West Antarctica since 1979 is evaluated using forecast precipitation from several meteorological analyses and reanalyses. Additionally, a dynamical retrieval method (DRM) for precipitation is applied. Over the last two decades, the Southern Oscillation index (SOI) has an overall anticorrelation with precipitation over the West Antarctic sector bounded by 75°–90°S, 120°W–180° while it is positively correlated with precipitation over the South Atlantic sector bounded by 65°–75°S, 30°–60°W. Decadal variations are found as the relationship between the SOI and West Antarctic precipitation is stronger in the 1990s than that in the 1980s. The polar front jet stream, West Antarctic precipitation, and the SOI show a well-ordered correspondence during the 1990s as the jet zonal speed is negatively correlated to the SOI and positively correlated to West Antarctic precipitation. These relationships are weaker during the 1980s, consistent with the change in sign of the correlation between the SOI and West Antarctic precipitation. The decadal variations are apparently related to changes in the quasi-stationary eddies that determine the local onshore and offshore flow over West Antarctica. [ABSTRACT FROM AUTHOR]

Global climate simulations are conducted to examine the sensitivity of the Last Glacial Maximum (LGM) climate to prescribed sea surface temperatures (SSTs) that are modified from the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) study. Based on the consensus from various LGM proxy data, the SSTs are cooled by 4°C uniformly in the Tropics (30°N–30°S) relative to CLIMAP, and the high-latitude sea ice extent is reduced. Compared to results from a simulation with CLIMAP SSTs, the modified LGM SSTs cause significant opposing changes in the hemispheric and regional-scale atmospheric circulation, which are most pronounced in the winter hemisphere. For instance, there is significant weakening of the midlatitude circulation and reduction of 500-hPa eddy kinetic energy and midlatitude precipitation resulting from the decreased meridional temperature gradient in the modified SST simulation. In contrast, reduced sea ice extent during the boreal winter causes increased regional baroclinicity and intensified atmospheric circulation in the western North Pacific and the North Atlantic. Cooled tropical SSTs also increase the land–ocean temperature contrast, which strengthens the Asian summer monsoon circulation. Both LGM simulations produce enhanced low-level convergence and increased precipitation along the South Pacific convergence zone (SPCZ) relative to present day, despite the cooler LGM climate. The SPCZ orientation and intensity are closely linked to the distribution of South Pacific SSTs. Comparison of surface temperature estimates from land- and ocean-based proxy data with model output suggests that uniform cooling of the tropical SSTs and modification of the high-latitude sea ice extent may be sufficient to accurately simulate the first-order characteristics of the LGM climate. [ABSTRACT FROM AUTHOR]

Presents a mesoscale simulation model of the atmospheric state and katabatic winds around Greenland. Verification of the simulation model; Description of the model; Comparison of the verification results with observations from automatic weather stations and instrumented aircrafts.

Examines the role of Antarctic topography and katabatic wind regime in the climate of the high southern latitudes. Occurrence of katabatic winds; Use of three-year seasonal cycle numerical simulations in the exploration of the role of elevated terrain and drainage flows on the distribution and evolution of the horizontal pressure field.

Surface pressure decreases of up to 20 hPa occurred over much of the Antarctic continent during a 4-day midwinter period of 1988. The widespread change in the pressure field accompanied intense cyclonic activity to the north of the ice sheet. The equatorward mass transport across the Antarctic coastline resulted in a redistribution of atmospheric pressure that extended to the subtropics of the Southern Hemisphere. Most of the mass flux from Antarctica was the result of low-level processes and appears tied to the katabatic wind circulation. The observed surface pressure decrease over the continent reflects a perturbation of the mean meridional cir-culation between Antarctica and the subpolar latitudes by synoptic-scale processes. Zonally averaged circulations over Antarctica were examined using output from the European Centre for Medium-Range Weather Forecasts model. Results suggest that only a poorly defined return branch of the meridional circulation exists in the middle and upper troposphere. This southward-directed flow does not compensate for the northward mass transport provided by the katabatic wind outflow in the lower atmosphere. Isallobaric contours over the Antarctic ice sheet roughly match the area of the large-scale drainage catchment associated with katabatic wind transport through the Ross Sea sector. An intense extratropical cyclone was present in the circumpolar oceanic belt to the north of the continent. The horizontal pressure gradient associated with the cyclone prompted enhanced drainage off the high interior plateau. The resulting katabatic flow issued from the continent through a narrow corridor across the Ross Ice Shelf and out over the Southern Ocean. [ABSTRACT FROM AUTHOR]

Present-day Arctic and Antarctic climate of the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3) is presented. The CCM3 simulation is from a prescribed and interannually varying sea surface temperature integration from January 1979 through August 1993. Observations from a variety of sources, including the European Centre for Medium-Range Weather Forecasts analyses, rawinsonde, and surface station data, are used for validation of CCM3's polar climate during this period. Overall, CCM3 can simulate many important polar climatic features and in general is an incremental improvement over CCM2. The 500-hPa polar vortex minima are too deep by 50--100 m and too zonally symmetric. The Arctic sea level pressure maximum is displaced poleward, while the Icelandic region minimum is extended toward Europe, and the Aleutian region minimum is extended toward Asia. The Antarctic circumpolar trough of low sea level pressure is slightly north of the observed position and is 2--3 hPa too low. Antarctic katabatic winds are similar to observations in magnitude and regional variation. The Antarctic surface wind stress is estimated to be 30%--50% too strong in some regions. Polar tropospheric temperatures are 2&deg--4&degC colder than observations, mostly in the summer season. Low-level winter inversions over the Arctic Ocean are only 3&deg--4&degC, rather than the observed 10&degC. In the Antarctic midcontinent they are around 25&deg--30&degC (about 5&deg stronger than observed) and continue to be stronger than observed along the coast. Although water vapor column is uniformly low by 10%--20% compared to analyses in both polar regions, the regional patterns of minima over Greenland and the East Antarctic plateau are well represented. Annual 70&deg to pole CCM3 values are 5.8 kg m[sup-2] for the Arctic and 1.7 kg m[sup-2] for the Antarctic. The regional distribution of precipitation minus evaporation compares reasonably with analyses. (...). [ABSTRACT FROM AUTHOR]

Presents observations of a cyclonic activity near McMurdo Station, Antarctica. Origins of katabatic airstreams; Wind dam provided by the Transantarctic Mountains; Wind pressure types documented.

A sensitivity study to evaluate the impact upon regional and hemispheric climate caused by changing the optical properties of clouds over the Antarctic continent is conducted with the NCAR Community Model version 2 (CCM2). Sensitivity runs are performed in which radiation interacts with ice clouds with particle sizes of 10 and 40 mm rather than with the standard 10- mm water clouds. The experiments are carried out for perpetual January conditions with the diurnal cycle considered. The effects of these cloud changes on the Antarctic radiation budget are examined by considering cloud forcing at the top of the atmosphere and net radiation at the surface. Changes of the cloud radiative properties to those of 10- mm ice clouds over Antarctica have significant impacts on regional climate: temperature increases throughout the Antarctic troposphere by 18--28C and total cloud fraction over Antarctica is smaller than that of the control at low levels but is larger than that of the control in the mid-to upper troposphere. As a result of Antarctic warming and changes in the north--south temperature gradient, the drainage flows at the surface as well as the meridional mass circulation are weakened. Similarly, the circumpolar trough weakens significantly by 4--8 hPa and moves northward by about 48--58 latitude. This regional mass field adjustment halves the strength of the simulated surface westerly winds. As a result of indirect thermodynamic and dynamic effects, significant changes are observed in the zonal mean circulation and eddies in the middle latitudes. In fact, the simulated impacts of the Antarctic cloud radiative alteration are not confined to the Southern Hemisphere. The meridional mean mass flux, zonal wind, and latent heat release exhibit statis-tically significant changes in the Tropics and even extratropics of the Northern Hemisphere. The simulation with radiative properties of 40- mm ice clouds produces colder surface temperatures over... [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one by D. H. Browich on his examination on the issue of how well the contemporary atmospheric circulation can be described and another one by J. S. Tilley on the improved initial conditions for forecast modes via data assimilation.

Presents weather pressure changes in Antarctica. February to April as the pressure change season; Period fluctuations; Illustrations of zonally averaged mean wind components.

A correction to the article "High-Resolution Regional Climate Simulations over Iceland Using Polar MMS" that was previously published in the Advances in High Latitude Numerical Weather Prediction special section of "Monthly Weather Review" is presented.