David H. Bromwich

A Reconciled Estimate of Ice-Sheet Mass Balance

Warming and Melting Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly—in some cases, not even agreeing about whether there is a net loss or a net gain—making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise. The mass balance of the polar ice sheets is estimated by combining the results of existing independent techniques. We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth’s polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.

Strong Trends in the Skill of the ERA-40 and NCEP–NCAR Reanalyses in the High and Midlatitudes of the Southern Hemisphere, 1958–2001*

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 observatio...

Greenland Ice Sheet Surface Mass Balance Variability (1988–2004) from Calibrated Polar MM5 Output*

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 km 3 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 km 3 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.

A Review of Antarctic Surface Snow Isotopic Composition: Observations, Atmospheric Circulation, and Isotopic Modeling*

A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

State of the Antarctic and Southern Ocean climate system

This paper reviews developments in our understanding of the state of the Antarctic and Southern Ocean climate, and its relation to the global climate system over the last few millennia. Climate over this and earlier periods has not been stable, as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in Antarctic ice core proxies for past climate. Two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies (also known as the Southern Annular Mode) between ~6000 and 5000 years ago and since 1200-1000 years ago. Following the last of these is a period of major trans-Antarctic reorganization of atmospheric circulation and temperature between AD1700 and 1850. The two earlier Antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the North Atlantic, and the end of the more recent event is coincident with reorganization of atmospheric circulation in the North Pacific. Improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols. Special attention is given to the climate of the past 200 years, which was recorded by a network of recently available shallow firn cores, and to that of the past 50 years, which was monitored by the continuous instrumental record. Significant regional climate changes have taken place in the Antarctic during the past 50 years. Atmospheric temperatures have increased markedly over the Antarctic Peninsula, linked to nearby ocean warming and intensification of the circumpolar westerlies. Glaciers are retreating on the Peninsula, in Patagonia, on the sub-Antarctic islands, and in West Antarctica adjacent to the Peninsula. The penetration of marine air masses has become more pronounced over parts of West Antarctica. Above the surface, the Antarctic troposphere has warmed during winter while the stratosphere has cooled year-round. The upper kilometer of the circumpolar Southern Ocean has warmed, Antarctic Bottom Water across a wide sector off East Antarctica has freshened, and the densest bottom water in the Weddell Sea has warmed. In contrast to these regional climate changes, over most of Antarctica near-surface temperature and snowfall have not increased significantly during at least the past 50 years, and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years. Furthermore, the total sea ice cover around Antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s, despite large but compensating regional changes. The inhomogeneity of Antarctic climate in space and time implies that recent Antarctic climate changes are due on the one hand to a combination of strong multi-decadal variability and anthropogenic effects and, as demonstrated by the paleoclimate record, on the other hand to multi-decadal to millennial scale and longer natural variability forced through changes in orbital insolation, greenhouse gases, solar variability, ice dynamics, and aerosols. Model projections suggest that over the 21st century the Antarctic interior will warm by 3.4° ± 1oC, and sea ice extent will decrease by ~30%. Ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level. Considering the potentially major impacts of a warming climate on Antarctica, vigorous efforts are needed to better understand all aspects of the highly coupled Antarctic climate system as well as its influence on the Earths climate and oceans.

Mesoscale Modeling of Katabatic Winds over Greenland with the Polar MM5

Abstract Verification of two months, April and May 1997, of 48-h mesoscale model simulations of the atmospheric state around Greenland are presented. The simulations are performed with a modified version of The Pennsylvania State University–National Center for Atmospheric Research fifth-generation Mesoscale Model (MM5), referred to as the Polar MM5. Global atmospheric analyses as well as automatic weather station and instrumented aircraft observations from Greenland are used to verify the forecast atmospheric state. The model is found to reproduce the observed atmospheric state with a high degree of realism. Monthly mean values of the near-surface temperature and wind speed predicted by the Polar MM5 differ from the observations by less than 1 K and 1 m s−1, respectively, at most sites considered. In addition, the model is able to simulate a realistic diurnal cycle for the surface variables, as well as capturing the large-scale, synoptically forced changes in these variables. Comparisons of modeled profil...

Evaluation of Polar MM5 simulations of Greenland's atmospheric circulation

A complete annual cycle over the Greenland ice sheet is simulated with the Polar MM5, a mesoscale model optimized for use over extensive ice sheets. These simulations are compiled from a series of short duration (48 hour), forecast mode, simulations. The model output is compared to observations primarily from the Greenland Climate Network automatic weather station (AWS) array. The model simulations show a high degree of skill for all variables evaluated with the AWS data (pressure, temperature, water vapor mixing ratio, wind speed and direction, downwelling shortwave radiation, and net radiation) for all seasons, although the use of a fixed albedo in the Polar MM5 leads to large errors in the simulated net radiation budget over melting ice surfaces during the summer months. The modeled precipitation distribution agrees with available observations in the interior of the ice sheet but is excessive along the steep margins of the island. A discussion of possible future applications of the Polar MM5 is presented.

Continental-scale simulation of the Antarctic katabatic wind regime

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 som...

Interannual variations in Antarctic precipitation related to El Niño-Southern Oscillation

The accurate estimation of Antarctic precipitation variability is an essential component in understanding global sea level fluctuations; direct measurement techniques, however, are replete with practical difficulties. In this study, net precipitation (precipitation minus sublimation) for the Antarctic continent is computed for 1980–1994 using operational numerical analyses obtained from the ECMWF (European Centre for Medium-Range Weather Forecasts). The resulting estimations reveal a strong interannual variability for the Antarctic continent, implying a ±1.2 − 1.5 mm yr−1 maximum range in the Antarctic eustatic change contribution. In particular, variability for the South Pacific sector (120°W–180°W) is shown to be correlated with the El Nino-Southern Oscillation (ENSO) phenomenon for 1980–1990. The relation becomes anticorrelated after 1990, associated with a strong East Antarctic ridging pattern that coincides with the start of the prolonged series of warm events of the early 1990s. This result is relevant to other studies relating ENSO variability to high southern latitudes, and a more elaborate picture of this teleconnection pattern is presented. Comparisons of sea level pressure values using available ship observations show good agreement and offer a confirmation of the analyses in this data-sparse region. Additionally, a comparison of results with values obtained from the precipitation fields of the NCEP/NCAR (NCEP: National Centers for Environmental Prediction; NCAR: National Center for Atmospheric Research) reanalysis project are discussed.

Historical SAM Variability. Part II: Twentieth-Century Variability and Trends from Reconstructions, Observations, and the IPCC AR4 Models*

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.