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David H. Bromwich
,
Lesheng Bai
, and
Gudmundur G. Bjarnason
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Thomas R. Parish
,
David H. Bromwich
, and
Ren-Yow Tzeng

Abstract

The Antarctic topography and attendant katabatic wind regime appear to play a key role in the climate of the high southern latitudes. During the nonsummer months, persistent and often times intense katabatic winds occur in the lowest few hundred meters of the Antarctic atmosphere. These slope flows transport significant amounts of cold air northward and thereby modify the horizontal pressure field over the high southern latitudes. Three-year seasonal cycle numerical simulations using the NCAR Community Climate Model Version 1 (CCM1) with and without representation of the Antarctic orography were performed to explore the role of the elevated terrain and drainage flows on the distribution and evolution of the horizontal pressure field. The katabatic wind regime is an important part of a clearly defined mean meridional circulation in the high southern latitudes. The position and intensity of the attendant sea level low pressure belt appears to be tied to the Antarctic orography. The seasonal movement of mass in the high southern latitudes is therefore constrained by the presence of the Antarctic ice sheet. The semiannual oscillation of pressure over Antarctica and the high southern latitudes is well depicted in the CCMI only when the Antarctic orography is included.

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Thomas R. Parish
,
Yuhang Wang
, and
David H. Bromwich

Abstract

Pronounced seasonal variations in the surface pressure field are present over the Antarctic continent. Surface pressures over the ice sheet decrease during the austral autumn period January–April and increase during the austral springtime months September–December. The largest changes are found over the highest portions of the Antarctic ice sheets where seasonal surface pressure changes of up to 20 hPa are common. The outstanding feature of these surface pressure changes is that typically the isallobaric contours closely follow the Antarctic orography during both transition periods, suggesting a strong seasonal diabatic adjustment within the lower troposphere. During austral autumn, the pronounced cooling of the lower atmosphere adjacent to the ice sheets leads to an enhancement of the Antarctic katabatic wind regime and hence the lower branch of the mean meridional circulation over the high southern latitudes. The mass transport provided by these drainage flows is proposed as the mechanism behind the autumn pressure falls. Numerical simulations of the evolution of the Antarctic katabatic wind regime indicate that the radiative cooling of the sloping ice fields and attendant mass transport result in a modification of the temperature and pressure fields in the lower troposphere similar to what is seen during the early austral autumn period.

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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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Richard I. Cullather
,
David H. Bromwich
, and
Mark C. Serreze

Abstract

The atmospheric moisture budget is evaluated for the region 70°N to the North Pole using reanalysis datasets of the European Centre for Medium-Range Weather Forecasts (ECMWF; ERA: ECMWF Re-Analysis) and the collaborative effort of the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR). For the forecast fields of the reanalyses, the ERA annually averaged PE (precipitation minus evaporation/sublimation) field reproduces the major features of the basin perimeter as they are known, while the NCEP–NCAR reanalysis forecast fields contain a spurious wave pattern in both P and E. Comparisons between gauge data from Soviet drift camp stations and forecast P values of the reanalyses show reasonable agreement given the difficulties (i.e., gauge accuracy, translating location). When averaged for 70°–90°N, the ERA and NCEP–NCAR forecast PE are similar in the annual cycle. Average reanalysis forecast values of E for the north polar cap are found to be 40% or more too large based on comparisons using surface latent heat flux climatologies.

Differences between a synthesized average moisture flux across 70°N from rawinsonde data of the Historical Arctic Rawinsonde Archive (HARA) and the reanalysis data occur in the presence of rawinsonde network problems. It is concluded that critical deficiencies exist in the rawinsonde depiction of the summertime meridional moisture transport. However, it remains to be seen whether the rawinsonde estimate can be rectified with a different method. For 70°–90°N, annual moisture convergence (PE) values from the ERA and NCEP–NCAR are very similar; for both reanalyses, annual PE values obtained from forecast fields are much lower than those obtained from moisture flux convergence by about 60%, indicating severe nonclosure of the atmospheric moisture budget. The nonclosure primarily results from anomalously large forecast E values. In comparison with other studies, reanalyses moisture convergence values are much more reasonable. A synthesis of the reanalysis moisture convergence values and more recent studies yields a value of 18.9 ± 2.3 cm yr−1 for the north polar cap.

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Ren-Yow Tzengo
,
David H. Bromwich
, and
Thomas R. Parish

Abstract

Five-year seasonal cycle output produced by the NCAR Community Climate Model Version 1 (CCM 1) with R15 resolution is used to evaluate the ability of the model to simulate the present-day climate of Antarctica. The model results are compared with observed horizontal syntheses and point data.

Katabatic winds, surface temperatures over the continent, the circumpolar trough, the vertical motion field, the split jet stream over the Pacific Ocean, and the snowfall accumulation are analyzed. The results show that the CCM1 with R15 resolution can well simulate to some extent the dynamics of Antarctic climate not only for the synoptic scale, but also for some mesoscale features (mesoscale cyclogenesis). This is reflected in the zonal-mean pattern of vertical motion by the presence of two convergence centers. The finding suggests that the CCM1 might also capture the split jet stream over New Zealand in winter, but the evidence is mixed. This is inferred to be due to inadequate simulation of the thermal forcing over high southern latitudes. The CCM1 can also capture the phase and amplitude of the annual and semiannual variation of temperature, sea level pressure, and zonally averaged zonal (E-W) wind. That the CCM1 can simulate some characteristics of the semiannual variation may be due to the improved radiation treatment compared to the earlier CCM0.

The most dramatic shortcomings were associated with the model's anomalously large precipitation amounts at high latitudes, which result from the scheme to suppress negative moisture values. The simulations of cloudiness and the atmospheric heat balance are adversely affected. A greatly refined moisture budget scheme is needed to eliminate these problems and may allow the split jet-stream feature over the New Zealand area in winter to be accurately reproduced. A coupled mesoscale-CCM1 model may be needed to adequately simulate the feedback from mesoscale cyclones to synoptic-scale weather systems, and the katabatic wind circulation.

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David H. Bromwich
,
Ren-Yow Tzeng
, and
Thomas R. Parish

Abstract

The NCAR CCM1's simulation of the modern arctic climate is evaluated by comparing a five-year seasonal cycle simulation with the ECMWF global analyses. The sea level pressure (SLP), storm tracks, vertical cross section of height, 500-hPa height, total energy budget, and moisture budget are analyzed to investigate the biases in the simulated arctic climate.

The results show that the model simulates anomalously low SLP, too much storm activity, and anomalously strong baroclinicity to the west of Greenland and vice versa to the east of Greenland. This bias is mainly attributed to the model's topographic representation of Greenland. First, the broadened Greenland topography in the model distorts the path of cyclone waves over the North Atlantic Ocean. Second, the model oversimulates the ridge over Greenland, which intensifies its blocking effect and steers the cyclone waves clockwise around it and hence produces an artificial “circum-Greenland” trough. These biases are significantly alleviated when the horizontal resolution increases to T42.

Over the Arctic basin, the model simulates large amounts of low-level (stratus) clouds in winter and almost no stratus in summer, which is opposite to the observations. This bias is mainly due to the location of the simulated SLP features and the negative anomaly of storm activity, which prevent the transport of moisture into this region during summer but favor this transport in winter.

The moisture budget analysis shows that the model's net annual precipitation ([P - E]) between 70°N and the North Pole is 6.6 times larger than the observations and the model transports six times more moisture into this region. The bias in the advection term is attributed to the positive moisture fixer scheme and the distorted flow pattern. However, the excessive moisture transport into the Arctic basin does not solely result from the advection term. The contribution by the moisture fixer is as large as from advection. By contrast, the semi-Lagrangian transport scheme used in the CCM2 significantly improves the moisture simulation for this region; however, globally the error is as serious as for the positive moisture fixer scheme.

Finally, because the model has such serious problems in simulating the present arctic climate, its simulations of past and future climate change for this region are questionable.

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Aaron B. Wilson
,
David H. Bromwich
, and
Keith M. Hines

Abstract

Numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM) are conducted based on tropical forcing of El Niño flavors. Though these events occur on a continuum, two general types are simulated based on sea surface temperature anomalies located in the central (CP) or eastern (EP) tropical Pacific. The goal is to assess whether CAM adequately represents the transient eddy dynamics associated with each of these El Niño flavors under different southern annular mode (SAM) regimes. CAM captures well the wide spatial and temporal variability associated with the SAM but only accurately simulates the impacts on atmospheric circulation in the high southern latitudes when the observed SAM phase is matched by the model. Composites of in-phase (El Niño–SAM−) and out-of-phase (El Niño–SAM+) events confirm a seasonal preference for in-phase (out of phase) events during December–February (DJF) [June–August (JJA)]. Modeled in-phase events for both EP (during DJF) and CP (during JJA) conditions support observations of anomalous equatorward momentum flux on the equatorward side of the eddy-driven jet, shifting this jet equatorward and consistent with the low phase of the SAM. Out-of-phase composites show that the El Niño–associated teleconnection to the high southern latitudes is strongly modulated by the SAM, as a strong eddy-driven jet is well maintained by high-latitude transient eddy convergence despite the tropical forcing. A regional perspective confirms that this interaction takes place primarily over the Pacific Ocean, with high-latitude circulation variability being a product of both tropical and high-latitude forcing.

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David H. Bromwich
,
Julien P. Nicolas
, and
Andrew J. Monaghan

Abstract

This study evaluates the temporal variability of the Antarctic surface mass balance, approximated as precipitation minus evaporation (PE), and Southern Ocean precipitation in five global reanalyses during 1989–2009. The datasets consist of the NCEP/U.S. Department of Energy (DOE) Atmospheric Model Intercomparison Project 2 reanalysis (NCEP-2), the Japan Meteorological Agency (JMA) 25-year Reanalysis (JRA-25), ECMWF Interim Re-Analysis (ERA-Interim), NASA Modern Era Retrospective-Analysis for Research and Application (MERRA), and the Climate Forecast System Reanalysis (CFSR). Reanalyses are known to be prone to spurious trends and inhomogeneities caused by changes in the observing system, especially in the data-sparse high southern latitudes. The period of study has seen a dramatic increase in the amount of satellite observations used for data assimilation.

The large positive and statistically significant trends in mean Antarctic PE and mean Southern Ocean precipitation in NCEP-2, JRA-25, and MERRA are found to be largely spurious. The origin of these artifacts varies between reanalyses. Notably, a precipitation jump in MERRA in the late 1990s coincides with the start of the assimilation of radiances from the Advanced Microwave Sounding Unit (AMSU). ERA-Interim and CFSR do not exhibit any significant trends. However, the potential impact of the assimilation of rain-affected radiances in ERA-Interim and inhomogeneities in CFSR pressure fields over Antarctica cast some doubt on the reliability of these two datasets.

The authors conclude that ERA-Interim likely offers the most realistic depiction of precipitation changes in high southern latitudes during 1989–2009. The range of the trends in Antarctic PE among the reanalyses is equivalent to 1 mm of sea level over 21 years, which highlights the improvements still needed in reanalysis simulations to better assess the contribution of Antarctica to sea level rise. Finally, this work argues for continuing cautious use of reanalysis datasets for climate change assessment.

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David H. Bromwich
,
Andrew J. Monaghan
, and
Zhichang Guo

Abstract

The Polar fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) is employed to examine the El Niño–Southern Oscillation (ENSO) modulation of Antarctic climate for July 1996–June 1999, which is shown to be stronger than for the mean modulation from 1979 to 1999 and appears to be largely due to an eastward shift and enhancement of convection in the tropical Pacific Ocean. This study provides a more comprehensive assessment than can be achieved with observational datasets by using a regional atmospheric model adapted for high-latitude applications (Polar MM5). The most pronounced ENSO response is observed over the Ross Ice Shelf–Marie Byrd Land and over the Weddell Sea–Ronne/Filchner Ice Shelf. In addition to having the largest climate variability associated with ENSO, these two regions exhibit anomalies of opposite sign throughout the study period, which supports and extends similar findings by other investigators. The dipole structure is observed in surface temperature, meridional winds, cloud fraction, and precipitation. The ENSO-related variability is primarily controlled by the large-scale circulation anomalies surrounding the continent, which are consistent throughout the troposphere. When comparing the El Niño/La Niña phases of this late 1990s ENSO cycle, the circulation anomalies are nearly mirror images over the entire Antarctic, indicating their significant modulation by ENSO. Large temperature anomalies, especially in autumn, are prominent over the major ice shelves. This is most likely due to their relatively low elevation with respect to the continental interior making them more sensitive to shifts in synoptic forcing offshore of Antarctica, especially during months with considerable open water. The Polar MM5 simulations are in broad agreement with observational data, and the simulated precipitation closely follows the European Centre for Medium-Range Weather Forecasts Tropical Ocean–Global Atmosphere precipitation trends over the study period. The collective findings of this work suggest the Polar MM5 is capturing ENSO-related atmospheric variability with good skill and may be a useful tool for future climate studies.

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