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David H. Bromwich
,
E. Richard Toracinta
,
Helin Wei
,
Robert J. Oglesby
,
James L. Fastook
, and
Terence J. Hughes

Abstract

Optimized regional climate simulations are conducted using the Polar MM5, a version of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), with a 60-km horizontal resolution domain over North America during the Last Glacial Maximum (LGM, 21 000 calendar years ago), when much of the continent was covered by the Laurentide Ice Sheet (LIS). The objective is to describe the LGM annual cycle at high spatial resolution with an emphasis on the winter atmospheric circulation. 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.

Polar MM5 produces a substantially different atmospheric response to the LGM boundary conditions than CCM3 and other recent GCM simulations. In particular, from November to April the upper-level flow is split around a blocking anticyclone over the LIS, with a northern branch over the Canadian Arctic and a southern branch impacting southern North America. The split flow pattern is most pronounced in January and transitions into a single, consolidated jet stream that migrates northward over the LIS during summer. Sensitivity experiments indicate that the winter split flow in Polar MM5 is primarily due to mechanical forcing by LIS, although model physics and resolution also contribute to the simulated flow configuration.

Polar MM5 LGM results are generally consistent with proxy climate estimates in the western United States, Alaska, and the Canadian Arctic and may help resolve some long-standing discrepancies between proxy data and previous simulations of the LGM climate.

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David H. Bromwich
,
Aric N. Rogers
,
Per Kållberg
,
Richard I. Cullather
,
James W. C. White
, and
Karl J. Kreutz

Abstract

The El Niño–Southern Oscillation (ENSO) signal in Antarctic precipitation is evaluated using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses and ECMWF 15-yr (1979–93) reanalyses. Operational and reanalysis datasets indicate that the ENSO teleconnection with Antarctic precipitation is manifested through a close positive correlation between the Southern Oscillation index and West Antarctic sector (75°–90°S, 120°W–180°) precipitation from the early 1980s to 1990, and a close negative correlation after 1990. However, a comparison between the operational analyses and reanalyses shows significant differences in net precipitation (PE) due to contrasts in the mean component of moisture flux convergence into the West Antarctic sector. These contrasts are primarily due to the mean winds, which differ significantly between the operational analyses and the reanalyses for the most reliable period of overlap (1985–93). Some of the differences in flow pattern are attributed to an error in the reanalysis assimilation of Vostok station data that suppresses the geopotential heights over East Antarctica. Reanalysis geopotential heights are also suppressed over the Southern Ocean, where there is a known cold bias below 300 hPa. Deficiencies in ECMWF reanalyses result in a weaker ENSO signal in Antarctic precipitation and cause them to miss the significant upward trend in precipitation found in recent operational analyses. Ice-core analyses reflect both an upward trend in ice accumulation and the ENSO teleconnection correlation pattern seen in the operational analyses. This study confirms the results of a previous study using ECMWF operational analyses that was the first to find a strong correlation pattern between the moisture budget over the West Antarctic sector and the Southern Oscillation index.

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Ryan L. Fogt
,
Judith Perlwitz
,
Andrew J. Monaghan
,
David H. Bromwich
,
Julie M. Jones
, and
Gareth J. Marshall

Abstract

This second paper examines the Southern Hemisphere annular mode (SAM) variability from reconstructions, observed indices, and simulations from 17 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) models from 1865 to 2005. Comparisons reveal the models do not fully simulate the duration of strong natural variability within the reconstructions during the 1930s and 1960s.

Seasonal indices are examined to understand the relative roles of forced and natural fluctuations. The models capture the recent (1957–2005) positive SAM trends in austral summer, which reconstructions indicate is the strongest trend during the last 150 yr; ozone depletion is the dominant mechanism driving these trends. In autumn, negative trends after 1930 in the reconstructions are stronger than the recent positive trend. Furthermore, model trends in autumn during 1957–2005 are the most different from observations. Both of these conditions suggest the recent autumn trend is most likely natural climate variability, with external forcing playing a secondary role. Many models also produce significant spring trends during this period not seen in observations. Although insignificant, these differences arise because of vastly different spatial structures in the Southern Hemisphere pressure trends. As the trend differences between models and observations in austral spring have been increasing over the last 30 yr, care must be exercised when examining the future SAM projections and their impacts in this season.

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Keith M. Hines
,
David H. Bromwich
,
Lesheng Bai
,
Cecilia M. Bitz
,
Jordan G. Powers
, and
Kevin W. Manning

Abstract

The Polar Weather Research and Forecasting Model (Polar WRF), a polar-optimized version of the WRF Model, is developed and made available to the community by Ohio State University’s Polar Meteorology Group (PMG) as a code supplement to the WRF release from the National Center for Atmospheric Research (NCAR). While annual NCAR official releases contain polar modifications, the PMG provides very recent updates to users. PMG supplement versions up to WRF version 3.4 include modified Noah land surface model sea ice representation, allowing the specification of variable sea ice thickness and snow depth over sea ice rather than the default 3-m thickness and 0.05-m snow depth. Starting with WRF V3.5, these options are implemented by NCAR into the standard WRF release. Gridded distributions of Arctic ice thickness and snow depth over sea ice have recently become available. Their impacts are tested with PMG’s WRF V3.5-based Polar WRF in two case studies. First, 20-km-resolution model results for January 1998 are compared with observations during the Surface Heat Budget of the Arctic Ocean project. Polar WRF using analyzed thickness and snow depth fields appears to simulate January 1998 slightly better than WRF without polar settings selected. Sensitivity tests show that the simulated impacts of realistic variability in sea ice thickness and snow depth on near-surface temperature is several degrees. The 40-km resolution simulations of a second case study covering Europe and the Arctic Ocean demonstrate remote impacts of Arctic sea ice thickness on midlatitude synoptic meteorology that develop within 2 weeks during a winter 2012 blocking event.

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Andrew J. Monaghan
,
David H. Bromwich
,
He-Lin Wei
,
Arthur M. Cayette
,
Jordan G. Powers
,
Ying-Hwa Kuo
, and
Matthew A. Lazzara

Abstract

In late April 2001, an unprecedented late-season flight to Amundsen–Scott South Pole Station was made in the evacuation of Dr. Ronald Shemenski, a medical doctor seriously ill with pancreatitis. This case study analyzes the performance of four of the numerical weather prediction models that aided meteorologists in forecasting weather throughout the operation: 1) the Antarctic Mesoscale Prediction System (AMPS) Polar MM5 (fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model), 2) the National Centers for Environmental Prediction Aviation Model (AVN), 3) the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast model, and 4) the NCAR Global MM5. To identify specific strengths and weaknesses, key variables for each model are statistically analyzed for all forecasts initialized between 21 and 25 April for several points over West Antarctica at the surface and at 500- and 700-hPa levels. The ECMWF model performs with the highest overall skill, generally having the lowest bias and rms errors and highest correlations for the examined fields. The AMPS Polar MM5 exhibits the next best skill, followed by AVN and Global MM5. For the surface variables, all of the models show high skill in predicting surface pressure but demonstrate modest skill in predicting temperature, wind speed, and wind direction. In the free atmosphere, the models show high skill in forecasting geopotential height, considerable skill in predicting temperature and wind direction, and good skill in predicting wind speed. In general, the models produce very useful forecasts in the free atmosphere, but substantial efforts are still needed to improve the surface prediction. The spatial resolution of each model exerts an important influence on forecast accuracy, especially in the complex topography of the Antarctic coastal regions. The initial and boundary conditions for the AMPS Polar MM5 exert a significant influence on forecasts.

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Jason E. Box
,
Noel Cressie
,
David H. Bromwich
,
Ji-Hoon Jung
,
Michiel van den Broeke
,
J. H. van Angelen
,
Richard R. Forster
,
Clement Miège
,
Ellen Mosley-Thompson
,
Bo Vinther
, and
Joseph R. McConnell

Abstract

Ice core data are combined with Regional Atmospheric Climate Model version 2 (RACMO2) output (1958–2010) to develop a reconstruction of Greenland ice sheet net snow accumulation rate, Ât (G), spanning the years 1600–2009. Regression parameters from regional climate model (RCM) output regressed on 86 ice cores are used with available cores in a given year resulting in the reconstructed values. Each core site’s residual variance is used to inversely weight the cores’ respective contributions. The interannual amplitude of the reconstructed accumulation rate is damped by the regressions and is thus calibrated to match that of the RCM data. Uncertainty and significance of changes is measured using statistical models.

A 12% or 86 Gt yr−1 increase in ice sheet accumulation rate is found from the end of the Little Ice Age in ~1840 to the last decade of the reconstruction. This 1840–1996 trend is 30% higher than that of 1600–2009, suggesting an accelerating accumulation rate. The correlation of Ât (G) with the average surface air temperature in the Northern Hemisphere (SATNH t ) remains positive through time, while the correlation of Ât (G) with local near-surface air temperatures or North Atlantic sea surface temperatures is inconsistent, suggesting a hemispheric-scale climate connection. An annual sensitivity of Ât (G) to SATNH t of 6.8% K−1 or 51 Gt K−1 is found.

The reconstuction, Ât (G), correlates consistently highly with the North Atlantic Oscillation index. However, at the 11-yr time scale, the sign of this correlation flips four times in the 1870–2005 period.

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Jason E. Box
,
David H. Bromwich
,
Bruce A. Veenhuis
,
Le-Sheng Bai
,
Julienne C. Stroeve
,
Jeffrey C. Rogers
,
Konrad Steffen
,
T. Haran
, and
Sheng-Hung Wang

Abstract

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.

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Andrew Orr
,
Pranab Deb
,
Kyle R. Clem
,
Ella Gilbert
,
David H. Bromwich
,
Fredrik Boberg
,
Steve Colwell
,
Nicolaj Hansen
,
Matthew A. Lazzara
,
Priscilla A. Mooney
,
Ruth Mottram
,
Masashi Niwano
,
Tony Phillips
,
Denys Pishniak
,
Carleen H. Reijmer
,
Willem Jan van de Berg
,
Stuart Webster
, and
Xun Zou

Abstract

We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq; %) and intensity (MPI-int; K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23%–35%, MPI-int 1.2–2.1 K), lowest (2%–3%, <0 K) for the Ronne–Filchner and Ross ice shelves, and around 10%–24% and 0.6–1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3–4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

Open access
Márcio Rocha Francelino
,
Carlos Schaefer
,
Maria de Los Milagros Skansi
,
Steve Colwell
,
David H. Bromwich
,
Phil Jones
,
John C. King
,
Matthew A. Lazzara
,
James Renwick
,
Susan Solomon
,
Manola Brunet
, and
Randall S. Cerveny

ABSTRACT

Two reports of Antarctic region potential new record high temperature observations (18.3°C, 6 February 2020 at Esperanza station and 20.8°C, 9 February 2020 at a Brazilian automated permafrost monitoring station on Seymour Island) were evaluated by a World Meteorological Organization (WMO) panel of atmospheric scientists. The latter figure was reported as 20.75°C in the media. The panel considered the synoptic situation and instrumental setups. It determined that a large high pressure system over the area created föhn conditions and resulted in local warming for both situations. Examination of the data and metadata of the Esperanza station observation revealed no major concerns. However, analysis of data and metadata of the Seymour Island permafrost monitoring station indicated that an improvised radiation shield led to a demonstrable thermal bias error for the temperature sensor. Consequently, the WMO has accepted the 18.3°C value for 1200 LST 6 February 2020 (1500 UTC 6 February 2020) at the Argentine Esperanza station as the new “Antarctic region (continental, including mainland and surrounding islands) highest temperature recorded observation” but rejected the 20.8°C observation at the Brazilian automated Seymour Island permafrost monitoring station as biased. The committee strongly emphasizes the permafrost monitoring station was not badly designed for its purpose, but the project investigators were forced to improvise a nonoptimal radiation shield after losing the original covering. Second, with regard to media dissemination of this type of information, the committee urges increased caution in early announcements as many media outlets often tend to sensationalize and mischaracterize potential records.

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Helge F. Goessling
,
Thomas Jung
,
Stefanie Klebe
,
Jenny Baeseman
,
Peter Bauer
,
Peter Chen
,
Matthieu Chevallier
,
Randall Dole
,
Neil Gordon
,
Paolo Ruti
,
Alice Bradley
,
David H. Bromwich
,
Barbara Casati
,
Dmitry Chechin
,
Jonathan J. Day
,
François Massonnet
,
Brian Mills
,
Ian Renfrew
,
Gregory Smith
, and
Renee Tatusko
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