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Ryan L. Fogt
and
David H. Bromwich

Abstract

Decadal variability of the El Niño–Southern Oscillation (ENSO) teleconnection to the high-latitude South Pacific is examined by correlating the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr Re-Analysis (ERA-40) and observations with the Southern Oscillation index (SOI) over the last two decades. There is a distinct annual contrast between the 1980s and the 1990s, with the strong teleconnection in the 1990s being explained by an enhanced response during austral spring. Geopotential height anomaly composites constructed during the peak ENSO seasons also demonstrate the decadal variability.

Empirical orthogonal function (EOF) analysis reveals that the 1980s September–November (SON) teleconnection is weak due to the interference between the Pacific–South American (PSA) pattern associated with ENSO and the Southern Annular Mode (SAM). An in-phase relationship between these two modes during SON in the 1990s amplifies the height and pressure anomalies in the South Pacific, producing the strong teleconnections seen in the correlation and composite analyses. The in-phase relationship between the tropical and high-latitude forcing also exists in December–February (DJF) during the 1980s and 1990s.

These results suggest that natural climate variability plays an important role in the variability of SAM, in agreement with a growing body of literature. Additionally, the significantly positive correlation between ENSO and SAM only during times of strong teleconnection suggests that both the Tropics and the high latitudes need to work together in order for ENSO to strongly influence Antarctic climate.

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Thomas R. Parish
and
David H. Bromwich

Abstract

Katabatic winds are a common feature of the lower Antarctic atmosphere. Although these drainage flows are quite shallow, there is increasing evidence that the low-level circulations are an important component in establishing large-scale tropospheric motions in the high southern latitudes. Three-dimensional numerical simulations of the Antarctic katabatic wind regime and attendant tropospheric circulations have been conducted over the entire continent to depict the topographically forced drainage patterns in the near-surface layer of the atmosphere. Results of the simulation enable a mapping of katabatic wind potential and identification of coastal regions which may experience anomalously intense katabatic winds. A large upper-level cyclonic circulation forms rapidly in response to the evolving katabatic wind structure in the lower atmosphere, suggesting that the drainage circulations are an important component in prescribing the resulting resulting circumpolar vortex. These results imply that some representation of the Antarctic katabatic wind regime is necessary in general circulation models in order to properly simulate the large-scale circulations about the continent.

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Julien P. Nicolas
and
David H. Bromwich

Abstract

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.

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Francis O. Otieno
and
David H. Bromwich

Abstract

The role of atmospheric circulations, yielding extremely cold summer and wet winter seasons, in the development of perennial snow cover over the inception region of the Laurentide Ice Sheet is investigated using the Community Land Model, version 3 (CLM3) with bias-corrected 40-yr ECMWF Re-Analysis (ERA-40) idealized atmospheric forcing. Potential contribution of changes in frequency of these extremes under contemporary and Eemian (116 kyr BP) conditions is also examined by adjusting the atmospheric forcing.

The results confirm that colder atmospheric temperatures during the melt season are more important than extreme amounts of winter snowfall. Increases in frequency of extremely cold and persistent summer air temperatures in the contemporary climate do not produce perennial snow. An additional cooling of 4°C together with adjustments for Eemian incident radiation is required for perennial snow to start growing around Hudson Bay. Deeper snow is found over the Labrador–Ungava area, close to the North Atlantic Ocean moisture sources, compared to the Keewatin area. These areas are in agreement with the locations of the Laurentide Ice Sheet domes found from free gravity analysis.

Starting from the warm present-day atmosphere a 25% decrease in summer insolation is required for CLM3 to develop perennial snow. This suggests that cooling resulting from modest decreases in local insolation in response to Milankovitch radiation forcing was insufficient for inception at 116 kyr BP. Remote cooling or local feedbacks that amplify the impact of the modest insolation reductions are required. A large-scale atmospheric cooling appears to have played a decisive role in inception.

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Julien P. Nicolas
and
David H. Bromwich

Abstract

High-resolution numerical weather forecasts from the Antarctic Mesoscale Prediction System (AMPS) archive are used to investigate the climate of West Antarctica (WA) during 2006–07. A comparison with observations from West Antarctic automatic weather stations confirms the skill of the model at simulating near-surface variables. AMPS cloud cover is also compared with estimates of monthly cloud fractions over Antarctica derived from spaceborne lidar measurements, revealing close agreement between both datasets. Comparison with 20-yr averages from the Interim ECMWF Re-Analysis (ERA-Interim) dataset demonstrates that the 2006–07 time period as a whole is reflective of the West Antarctic climate from the last two decades. On the 2006–07 annual means computed from AMPS forecasts, the most salient feature is a tongue-shaped pattern of higher cloudiness, accumulation, and 2-m potential temperature stretching over central WA. This feature is caused by repeated intrusions of marine air inland linked to the sustained cyclonic activity in the Ross and western Amundsen Seas. It is further enhanced by the ice sheet’s topography and by the mid–low-tropospheric wind flow on either side of the central ice divide. Low pressures centered over the Ross Sea (as opposed to the Bellingshausen Sea) are found to be most effective in conveying heat and moisture into WA. This study offers a perspective on how recent and projected changes in cyclonic activity in the South Pacific sector of the Southern Ocean may affect the climate and surface mass balance of WA.

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Bruce P. Briegleb
and
David H. Bromwich

Abstract

Present-day Arctic and Antarctic radiation budgets of the National Center for Atmospheric Research Community Climate Model version 3 (CCM3) are presented. The CCM3 simulation is from a prescribed and interannually varying sea surface temperature integration from January 1979 through August 1993. Earth Radiation Budget Experiment (ERBE) data from 1985 through 1989 are used for validation of top-of-atmosphere (TOA) absorbed shortwave radiation (ASR) and outgoing longwave radiation (OLR). Summer ASR in both polar regions is less than the observations by about 20 W m−2. While the annual mean OLR in both polar regions is only 2–3 W m−2 less than the ERBE data, the seasonal amplitude in OLR of 40 W m−2 is smaller than the observed of 55–60 W m−2. The annual polar TOA radiation balance is smaller than observations by 5–10 W m−2. Compared to selected model and observational surface data, downward shortwave (SW) is too small by 50–70 W m−2 and downward longwave (LW) too large by 10–30 W m−2. Surface downward LW in clear atmospheres is too small by 10–20 W m−2. The absence of sea-ice melt ponds results in 10–20 W m−2 too much SW absorption during early summer and from 20 to 40 W m−2 too little during late summer. Summer cloud covers are reasonably well simulated, but winter low cloud cover is too high by 0.5–0.7 compared to surface cloud observations. Comparison with limited satellite and in situ observations indicates cloud water path (CWP) is too high by about a factor of 2. While cloud particle sizes are approximately in the range of observed values, regional variation between maritime and continental droplet sizes is too strong over coastlines. Despite several improvements in CCM3 radiation physics, the accuracy of polar TOA annual radiation balance is degraded against the ERBE data compared to CCM2. Improvement in CCM3 polar radiation budgets will require improved simulation of CWP, clear sky LW, and sea ice albedo.

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Bruce P. Briegleb
and
David H. Bromwich

Abstract

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°–4°C colder than observations, mostly in the summer season. Low-level winter inversions over the Arctic Ocean are only 3°–4°C, rather than the observed 10°C. In the Antarctic midcontinent they are around 25°–30°C (about 5° 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° to pole CCM3 values are 5.8 kg m−2 for the Arctic and 1.7 kg m−2 for the Antarctic. The regional distribution of precipitation minus evaporation compares reasonably with analyses. The annual 70° to pole values are 18.1 cm yr−1, which are close to the most recent observational estimates of 16 to 18 cm yr−1 in the Arctic and 18.4 ± 3.7 cm yr−1 in the Antarctic. In both polar regions, summer surface energy budgets are estimated to be low by roughly 20 W m−2.

Suggestions as to causes of simulation deficiencies are 1) polar heat sinks that are too strong; 2) inadequate representation of sea-ice–atmosphere heat exchange, due to lack of fractional coverage of sea ice of variable thickness; 3) effects of low horizontal resolution; and 4) biased extrapolar influence.

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David H. Bromwich
and
Ryan L. Fogt

Abstract

The European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (NCEP1) data are compared with Antarctic and other mid- to high-latitude station observations for the complete years of overlap, 1958–2001. Overall, it appears that ERA-40 more closely follows the observations; however, a more detailed look at the presatellite era reveals many shortcomings in ERA-40, particularly in the austral winter.

By calculating statistics in 5-yr moving windows for June–July–August (JJA), it is shown that ERA-40 correlations with observed MSLP and surface (2 m) temperatures are low and even negative during the mid-1960s. A significant trend in skill in ERA-40 is observed in conjunction with the assimilation of satellite data during winter, eventually reaching a high level of skill after 1978 that is superior to NCEP1. NCEP1 shows consistency in its correlation with observations throughout time in this season; however, the biases in the NCEP1 MSLP fields decrease significantly with time. Similar problems are also found in the 500-hPa geopotential height fields above the direct influences of the mountainous topography. The height differences between ERA-40 and NCEP1 over the South Pacific are substantial before the modern satellite era throughout the depth of the troposphere. The ability for ERA-40 to be more strongly constrained by the satellite data compared to NCEP1, which is largely constrained by the station observational network, suggests that the differing assimilation schemes between ERA-40 and NCEP1 lead to the large discrepancies seen here. Thus, both reanalyses must be used with caution over high southern latitudes during the nonsummer months prior to the assimilation of satellite sounding data.

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David L. Williamson
,
David H. Bromwich
, and
Ren-Yow Tzeng

Abstract

No abstract available

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Keith M. Hines
,
David H. Bromwich
, and
Gareth J. Marshall

Abstract

An examination of 50 years of the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis from 1949 to 1998 reveals that significant spurious trends occur in the surface pressure field. Long-term surface pressure reductions are apparent south of 45°S. The largest trend in surface pressure is near 65°S where an approximately steady long-term pressure reduction of about 0.20 hPa yr−1 (10 hPa in 50 yr) is located. The negative pressure trend represents a gradual reduction in a positive bias for the reanalysis. Observations at Antarctic stations do not support this long-term trend, although short-term interannual variations are reasonably well captured after about 1970. The negative pressure tendency near 65°S continues well into the 1990s although a reasonable number of stations between 65° and 70°S began taking observations along the coast of east Antarctica during the 1950s and 1960s. Few Antarctic observations, however, are used by the reanalysis until about 1968, and the quality of the pressure field for the reanalysis appears poor in high southern latitudes prior to then. The trend in high southern latitudes appears to be a component of global temporal variations in the reanalysis, some of which are supported by observations but others are not.

In the Southern Hemisphere, the sea level pressure difference between 40° and 60°S, an indicator of westerly wind intensity, increases approximately from 20 hPa in the early 1950s to 25 hPa in the early 1970s and 28 hPa in recent years. The relatively high density of observing stations along the Antarctic Peninsula, however, results in an approximately steady local surface pressure after the pressure fell about 4 hPa during the late 1950s. Based upon these findings, researchers should account for jumps and long-term trends when making use of the NCEP–NCAR reanalysis.

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