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Increasing precipitation on the Antarctic Ice Sheet (AIS) in a warming climate has the potential to partially mitigate Antarctica’s contribution to sea level rise. We show that a simple, physically motivated change to the shallow convective cloud phase in the Community Earth System Model (CESM)—improving a long-standing bias in shortwave cloud forcing over the Southern Ocean—leads to an enhanced response of precipitation when the model is forced with realistic stratospheric ozone depletion, with other radiative forcing remaining constant. We analyze two ozone-forced ensemble experiments with the CESM version 1.1: one using the standard version of the model and the other using the cloud-modified version. The standard version exhibits a precipitation increase on the AIS of 34 gigatons year−1; the cloud-modified version shows an increase of 109 Gt year−1. The cloud-modified version shows a more robust, year-round poleward shift in the westerly jet and storm tracks, which brings more precipitation to the AIS, compared to the standard version. Greater surface warming and larger-amplitude stationary waves further increase the Antarctic precipitation response. The enhanced warming in the cloud-modified version is explained by larger positive shortwave cloud feedbacks, while the enhanced poleward jet shift is associated with a stronger meridional temperature gradient in the upper troposphere—lower stratosphere. These results illustrate (1) the sensitivity of forced changes in Antarctic precipitation to the mean state of a climate model and (2) the strong role of atmospheric dynamics in driving that forced precipitation response.

As most permanent observations in Antarctica started in the 1950s, understanding Antarctic climate variations throughout the twentieth century remains a challenge. To address this issue, the non-summer multi-decadal variability in pressure reconstructions poleward of 60°S is evaluated and assessed in conjunction with climate model simulations throughout the twentieth and early twenty-first centuries to understand historical atmospheric circulation variability over Antarctica. Austral autumn and winter seasons show broadly similar patterns, with negative anomalies in the early twentieth century (1905–1934), positive pressure anomalies in the middle twentieth century (1950–1980), and negative pressure anomalies in the most recent period (1984–2013), consistent with concurrent trends in the SAM index. In autumn, the anomalies are significant in the context of estimates of interannual variability and reconstruction uncertainty across most of the Antarctic continent, and the reconstructed patterns agree best with model-generated patterns when the simulation includes the forced response to tropical sea surface temperatures and external radiative forcing. In winter and spring, the reconstructed anomalies are less significant and are consistent with internal atmospheric variability alone. The specific role of tropical SST variability on pressure trends in these seasons is difficult to assess due to low reconstruction skill in the region of strongest tropical teleconnections, the large internal atmospheric variability, and uncertainty in the SST patterns themselves. Indirect estimates of pressure variability, whether through sea ice reconstructions, proxy records, or improved models and data assimilation schemes, will help to further constrain the magnitude of internal variability relative to the forced responses expected from SST trends and external radiative forcing.

During the late 20th 33 Century, the Antarctic atmospheric circulation has changed and significantly influenced the overall Antarctic climate, through processes including a poleward shift of the circumpolar westerlies. However, little is known about the full spatial pattern of atmospheric pressure over the Antarctic continent prior to 1979. Here we investigate surface pressure changes across the entire Antarctic continent back to 1905 by developing a new summer pressure reconstruction poleward of 60°S. We find that only across East Antarctica are the recent pressures significantly lower than pressures in the early 20th 40 century; we also discern periods of significant positive pressure trends in the early 20th 41 century across the coastal South Atlantic sector of Antarctica. Climate model simulations reveal that both tropical sea surface temperature variability and other radiative forcing mechanisms, in addition to ozone depletion, have played an important role in forcing the recent observed negative trends.

Recent work suggests that natural variability has played a significant role in the increase of Antarctic sea ice extent during 1979–2013. The ice extent has responded strongly to atmospheric circulation changes, including a deepened Amundsen Sea Low (ASL), which in part has been driven by tropical variability. Nonetheless, this increase has occurred in the context of externally forced climate change, and it has been difficult to reconcile observed and modeled Antarctic sea ice trends. To understand observed-model disparities, this work defines the internally driven and radiatively forced patterns of Antarctic sea ice change and exposes potential model biases using results from two sets of historical experiments of a coupled climate model compared with observations. One ensemble is constrained only by external factors such as greenhouse gases and stratospheric ozone, while the other explicitly accounts for the influence of tropical variability by specifying observed SST anomalies in the eastern tropical Pacific. The latter experiment reproduces the deepening of the ASL, which drives an increase in regional ice extent due to enhanced ice motion and sea surface cooling. However, the overall sea ice trend in every ensemble member of both experiments is characterized by ice loss and is dominated by the forced pattern, as given by the ensemble-mean of the first experiment. This pervasive ice loss is associated with a strong warming of the ocean mixed layer, suggesting that the ocean model does not locally store or export anomalous heat efficiently enough to maintain a surface environment conducive to sea ice expansion. The pervasive upper-ocean warming, not seen in observations, likely reflects ocean mean-state biases.

This study examines the biases, intermodel spread, and intermodel range of surface air temperature (SAT) across the Antarctic ice sheet and Southern Ocean in 26 structurally different climate models. Over the ocean (40°–60°S), an ensemble-mean warm bias peaks in late austral summer concurrently with the peak in the intermodel range of SAT. This warm bias lags a spring–summer positive bias in net surface radiation due to weak shortwave cloud forcing and is gradually reduced during autumn and winter. For the ice sheet, inconsistencies among reanalyses and observational datasets give low confidence in the ensemble-mean bias of SAT, but a small summer warm bias is suggested in comparison with nonreanalysis SAT data. The ensemble mean hides a large intermodel range of SAT, which peaks during the summer insolation maximum. In summer on the ice sheet, the SAT intermodel spread is largely associated with the surface albedo. In winter, models universally exhibit a too-strong deficit in net surface radiation related to the downward longwave radiation, implying that the lower atmosphere is too stable. This radiation deficit is balanced by the transfer of sensible heat toward the surface (which largely explains the intermodel spread in SAT) and by a subsurface heat flux. The winter bias in downward longwave radiation is due to the longwave cloud radiative effect, which the ensemble mean underestimates by a factor of 2. The implications of these results for improving climate simulations over Antarctica and the Southern Ocean are discussed.

Westerly wind trends at 850 hPa over the Southern Ocean during 1979–2011 exhibit strong regional and seasonal asymmetries. On an annual basis, trends in the Pacific sector (40°–60°S, 70°–160°W) are 3 times larger than zonal-mean trends related to the increase in the southern annular mode (SAM). Seasonally, the SAM-related trend is largest in austral summer, and many studies have linked this trend with stratospheric ozone depletion. In contrast, the Pacific sector trends are largest in austral autumn. It is proposed that these asymmetries can be explained by a combination of tropical teleconnections and polar ozone depletion. Six ensembles of transient atmospheric model experiments, each forced with different combinations of time-dependent radiative forcings and SSTs, support this idea. In summer, the model simulates a positive SAM-like pattern, to which ozone depletion and tropical SSTs (which contain signatures of internal variability and warming from greenhouse gasses) contribute. In autumn, the ensemble-mean response consists of stronger westerlies over the Pacific sector, explained by a Rossby wave originating from the central equatorial Pacific. While these responses resemble observations, attribution is complicated by intrinsic atmospheric variability. In the experiments forced only with tropical SSTs, individual ensemble members exhibit wind trend patterns that mimic the forced response to ozone. When the analysis presented herein is applied to 1960–2000, the primary period of ozone loss, ozone depletion largely explains the model’s SAM-like zonal wind trend. The time-varying importance of these different drivers has implications for relating the historical experiments of free-running, coupled models to observations.

This study compares the distribution of surface climate trends over the Southern Ocean in austral summer between 1979–2011 and 1950–1978, using a wide variety of data sets including uninterpolated gridded marine archives, land station data, reanalysis, and satellite products. Apart from the Antarctic Peninsula and adjacent regions, sea surface temperatures and surface air temperatures decreased during 1979–2011, consistent with the expansion of Antarctic sea ice. In contrast, the Southern Ocean and coastal Antarctica warmed during 1950–1978. Sea level pressure (SLP) and zonal wind trends provide additional evidence for a sign reversal between the two periods, with cooling (warming) accompanied by stronger (weaker) westerlies and lower (higher) SLP at polar latitudes in the early (late) period. Such physically consistent trends across a range of independently measured parameters provide robust evidence for multidecadal climate variability over the Southern Ocean and place the recent Antarctic sea ice trends into a broader context.

High vessel attenuation and high contrast-to-noise ratio (CNR) are prerequisites for high diagnostic confidence in CT pulmonary angiography (CTPA). This study evaluated the impact of calculated monoenergetic dual-energy (DE) CTPA datasets on vessel attenuation and CNR.50 Patients (24 men, mean age 68 ± 14 years) who underwent DE-CTPA were retrospectively included in this study. The 80 and 140-kV DE polyenergetic image data were used to calculate virtual monoenergetic image datasets in 10 kiloelectron volt (keV) increments from 40 to 120 keV. Vessel and soft tissue attenuation and image noise were measured in various regions of interest and the CNR was subsequently calculated. Differences in vessel attenuation and CNR were compared between the different monoenergetic datasets. The best monoenergetic dataset was then compared to the standard 120-kV polyenergetic dataset.Vessel attenuation and CNR of 70-keV CTPA datasets were superior to all other monoenergetic image datasets (all p0.05). 70-keV monoenergetic datasets provided a statistically significant 12% increase in mean vessel attenuation compared to standard 120-kV polyenergetic datasets (384 ± 117 HU vs. 342 ± 106 HU, respectively; p0.0001) and a statistically significant 18% increase in mean CNR (29 ± 13 vs. 24 ± 11 respectively; p0.0001).Virtual 70-keV monoenergetic CTPA image datasets significantly increase vessel attenuation and CNR of DE-CTPA studies, suggesting that clinical application of low-keV monoenergetic reconstructions may allow a decrease in the amount of iodinated contrast required for adequate image quality in DE-CTPA examinations.

To compare objective image quality indices in dual-energy CT angiography (DE-CTA) studies of the abdomen and lower extremity using conventional polyenergetic images (PEIs) and virtual monoenergetic images (MEIs) at different kiloelectron volt (keV) levels.We retrospectively evaluated 68 dual-source DE-CTA studies. 50 patients (42 men, 71 ± 10 years) underwent abdominal DE-CTA. 18 patients (13 men, 67 ± 10 years) underwent lower extremity DE-CTA. MEIs from 40 to 120 keV were reconstructed. Signal intensity, noise, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were assessed in infrarenal aorta, superior mesenteric, external iliac, femoral, popliteal, and lower leg arteries. Comparisons between MEIs and PEIs were performed with Dunnett's test.222 arteries were evaluated. In abdominal arteries 70 keV MEIs showed statistically equal signal intensity, noise and CNR levels (+13%; +31%, -14% on average; all p0.05) compared to PEIs; SNR was equal or slightly impaired (-7% on average; p0.001-1.00). In lower extremity arteries 60 keV MEIs resulted in significantly higher signal intensity and CNR (+54%; +54% on average; all p0.05) compared to PEIs at equal noise levels (+18% on average; all p0.05) and equal or higher SNR (+49% on average; p0.01-0.35).Low-keV MEIs lead to equal or higher signal intensity and CNR compared to PEIs. In lower extremity DE-CTA, additional reconstruction of low-keV MEIs at 60 keV might increase diagnostic confidence.

West Antarctic temperature and pressure observations begin onlyin 1957, and reliable satellite observations of Antarctic sea icedate to 1979. Comprehensive observations of glacier dynamicsin the most rapidly changing areas were initiated in the 1990s.Borehole temperature data from the WAIS, although confirmingthe recent rapid rise in temperature, do not resolve decadal-scalevariability in the past

The D/H isotope ratio is used to attribute boundary layer humidity changes to the set of contributing fluxes for a case following a snowstorm in which a snow pack of about 10 cm vanished. Profiles of H2O and CO2 mixing ratio, D/H isotope ratio, and several thermodynamic properties were measured from the surface to 300 m every 15 min during four winter days near Boulder, Colorado. Coeval analysis of the D/H ratios and CO2 concentrations find these two variables to be complementary with the former being sensitive to daytime surface fluxes and the latter particularly indicative of nocturnal surface sources. Together they capture evidence for strong vertical mixing during the day, weaker mixing by turbulent bursts and low level jets within the nocturnal stable boundary layer during the night, and frost formation in the morning. The profiles are generally not well described with a gradient mixing line analysis because D/H ratios of the end members (i.e., surface fluxes and the free troposphere) evolve throughout the day which leads to large uncertainties in the estimate of the D/H ratio of surface water flux. A mass balance model is constructed for the snow pack, and constrained with observations to provide an optimal estimate of the partitioning of the surface water flux into contributions from sublimation, evaporation of melt water in the snow and evaporation from ponds. Results show that while vapor measurements are important in constraining surface fluxes, measurements of the source reservoirs (soil water, snow pack and standing liquid) offer stronger constraint on the surface water balance. Measurements of surface water are therefore essential in developing observational programs that seek to use isotopic data for flux attribution.

The Antarctic continent contains the majority of the global ice volume and plays an important role in a changing climate. The nature and causes of Antarctic climate variability are, however, poorly understood beyond interannual time scales due to the paucity of long, reliable meteorological observations. This study analyzes decadal–interdecadal climate variability over Antarctica using a network of annually resolved ice core records and various instrumental and tropical proxy data for the nineteenth and twentieth centuries. During the twentieth century, Antarctic ice core records indicate strong linkages to sea surface temperature (SST) variations in the tropical Pacific and Atlantic on decadal–interdecadal time scales. Antarctic surface temperature anomalies inferred from the ice cores are consistent with the associated changes in atmospheric circulation and thermal advection. A set of atmospheric general circulation model experiments supports the idea that decadal SST variations in the tropics force atmospheric teleconnections that affect Antarctic surface temperatures. When coral and other proxies for tropical climate are used to extend the analysis back to 1799, a similar Antarctic–tropical Pacific linkage is found, although the relationship is weaker during the first half of the nineteenth century. Over the past 50 years, a change in the phase of Pacific and Atlantic interdecadal variability may have contributed to the rapid warming of the Antarctic Peninsula and West Antarctica and related changes in ice sheet dynamics.

The surface of the Greenland ice sheet becomes isolated from the atmosphere during the winter, which acts to conserve ice mass., Despite rapid melting in the coastal regions of the Greenland Ice Sheet, a significant area (~40%) of the ice sheet rarely experiences surface melting. In these regions, the controls on annual accumulation are poorly constrained owing to surface conditions (for example, surface clouds, blowing snow, and surface inversions), which render moisture flux estimates from myriad approaches (that is, eddy covariance, remote sensing, and direct observations) highly uncertain. Accumulation is partially determined by the temperature dependence of saturation vapor pressure, which influences the maximum humidity of air parcels reaching the ice sheet interior. However, independent proxies for surface temperature and accumulation from ice cores show that the response of accumulation to temperature is variable and not generally consistent with a purely thermodynamic control. Using three years of stable water vapor isotope profiles from a high altitude site on the Greenland Ice Sheet, we show that as the boundary layer becomes increasingly stable, a decoupling between the ice sheet and atmosphere occurs. The limited interaction between the ice sheet surface and free tropospheric air reduces the capacity for surface condensation to achieve the rate set by the humidity of the air parcels reaching interior Greenland. The isolation of the surface also acts to recycle sublimated moisture by recondensing it onto fog particles, which returns the moisture back to the surface through gravitational settling. The observations highlight a unique mechanism by which ice sheet mass is conserved, which has implications for understanding both past and future changes in accumulation rate and the isotopic signal in ice cores from Greenland.

A preindustrial control run and an ensemble of twentieth-century integrations of the Community Climate System Model, version 4 (CCSM4), are evaluated for Antarctic sea ice climatology, modes of variability, trends, and covariance with related physical variables such as surface temperature and sea level pressure. Compared to observations, the mean ice cover is too extensive in all months. This is in part related to excessively strong westerly winds over ~50°–60°S, which drive a large equatorward meridional ice transport and enhanced ice growth near the continent and also connected with a cold bias in the Southern Ocean. In spite of these biases in the climatology, the model’s sea ice variability compares well to observations. The leading mode of austral winter sea ice concentration exhibits a dipole structure with anomalies of opposite sign in the Atlantic and Pacific sectors. Both the El Niño–Southern Oscillation and the southern annular mode (SAM) project onto this mode. In twentieth-century integrations, Antarctic sea ice area exhibits significant decreasing annual trends in all six ensemble members from 1950 to 2005, in apparent contrast to observations that suggest a modest ice area increase since 1979. Two ensemble members show insignificant changes when restricted to 1979–2005. The ensemble mean shows a significant increase in the austral summer SAM index over 1960–2005 and 1979–2005 that compares well with the observed SAM trend. However, Antarctic warming and sea ice loss in the model are closely connected to each other and not to the trend in the SAM.

This study reviews the mechanisms associated with Antarctic–tropical climate linkages and presents new analyses of the seasonality and spatial patterns of tropical climate signals in the Antarctic for the late 1950s to the present. Tropical climate signals are primarily communicated to the Antarctic via the Pacific–South America (PSA) pattern and the southern annular mode (SAM). The impacts of these circulation patterns and their tropical linkages are evident in regressions of seasonally stratified Antarctic station temperature data and annually resolved ice core records on global fields of sea surface temperature, sea level pressure, and precipitation. Temperature and ice core anomalies in the Antarctic Peninsula region and adjoining areas of West Antarctica are significantly impacted by the PSA, interpreted as a Rossby wave train driven by anomalous tropical deep convection during ENSO events. This pattern is most evident in the austral spring, consistent with recent studies, suggesting that atmospheric conditions for Rossby wave propagation are most favorable during this season. During austral summer at the peak of the ENSO cycle, temperature anomalies at East Antarctic coastal stations exhibit significant correlations with tropical Pacific anomalies. This linkage reflects the influence of anomalous tropical heating on the position and strength of the subtropical jets and is consistent with changes in eddy momentum fluxes that alter the mean meridional circulation associated with the SAM. Of the ice cores that exhibit tropical linkages, most tend to be associated with the PSA teleconnection. The implications of the study’s findings for understanding Antarctic climate variability and climate change from seasonal to decadal time scales are also discussed.

Purpose To assess the efficacy of high-frequency ultrasound biomicroscopy (UBM) in improving the accuracy of phakic intraocular lens (pIOL) sizing by increasing the incidence of acceptable postoperative vault. Setting Multiple private practices, United States. Design Evaluation of diagnostic test or technology. Methods This prospective multicenter clinical study evaluated eyes having pIOL (Visian Implantable Collamer Lens) implantation. A retrospective data analysis was performed using UBM measurements (VuMax-II) of preoperative sulcus-to-sulcus (STS) distance and postoperative vault. The regression data and clinical input from investigators were then used to develop a pIOL sizing nomogram. The nomogram used only STS and pIOL power as variables to determine length. Inadequate vault ( 1000 μm) were defined based on peer-reviewed literature. Sizing recommendations using the nomogram were studied prospectively and compared with 2 sizing methods used in the United States that are based on white-to-white (WTW) measurements. Results One eye was excluded from the analysis because the wrong length pIOL was placed (12.6 mm instead of nomogram-recommended 13.2 mm), resulting in 51 μm of vault. The mean postoperative vault in the remaining 72 cases was 340 μm ± 174 (SD) (range 90 to 952 μm); there were no cases of inadequate or excessive vault with the newly developed UBM nomogram. Sizing methods using WTW measurements would have resulted in different-sized pIOLs in 36% to 69% of cases compared with the STS method. Conclusion There were no cases of inadequate or excessive vault when the UBM nomogram for pIOL sizing was used. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.

Northern Hemisphere summer cooling through the Holocene is largely driven by the steady decrease in summer insolation tied to the precession of the equinoxes. However, centennial-scale climate departures, such as the Little Ice Age, must be caused by other forcings, most likely explosive volcanism and changes in solar irradiance. Stratospheric volcanic aerosols have the stronger forcing, but their short residence time likely precludes a lasting climate impact from a single eruption. Decadally paced explosive volcanism may produce a greater climate impact because the long response time of ocean surface waters allows for a cumulative decrease in sea-surface tempera- tures that exceeds that of any single eruption. Here we use a global climate model to evaluate the potential long-term climate impacts from four decadally paced large tropical eruptions. Direct forcing results in a rapid expansion of Arctic Ocean sea ice that persists throughout the eruption period. The expanded sea ice increases the flux of sea ice exported to the northern North Atlantic long enough that it reduces the convective warming of surface waters in the subpolar North Atlantic. In two of our four simulations the cooler surface waters being advected into the Arctic Ocean reduced the rate of basal sea-ice melt in the Atlantic sector of the Arctic Ocean, allowing sea ice to remain in an expanded state for ( 100 model years after volcanic aerosols were removed from the stratosphere. In these simulations the coupled sea ice-ocean mechanism main- tains the strong positive feedbacks of an expanded Arctic Ocean sea ice cover, allowing the initial cooling related to the direct effect of volcanic aerosols to be perpetuated, potentially resulting in a centennial-scale or longer change of state in Arctic climate. The fact that the sea ice-ocean mechanism was not established in two of our four simu- lations suggests that a long-term sea ice response to vol- canic forcing is sensitive to the stability of the seawater column, wind, and ocean currents in the North Atlantic during the eruptions.

Ice-core records are a key resource for reconstructing Antarctic climate. However, a number of physical processes preclude the simple interpretation of ice-core properties such as oxygen isotopic ratios in terms of climate variables like temperature or sea-level pressure. We show that well-dated, sub-annually resolved stable-isotopic records from the United States International Trans-Antarctic Scientific Expedition (US-ITASE) traverses and other sources have a high correlation with local seasonal temperature variation. However, this temporal relationship cannot be simply extended to quantitative interannual resolution reconstructions of site temperature. We suggest that a consistent and important target for ice-core calibrations is a composite of annual mean temperature records from Antarctic weather stations, which covaries strongly with the dominant mode (from principal component analysis) of temperature variability in the Antarctic. Significant correlations with this temperature index are found with individual ice-core records, with a composite of the ice cores, and through a multiple linear regression model with the ice cores as predictors. These results imply that isotopic signals, like the instrumental temperature mode itself, have a strong response to large-scale atmospheric circulation variability, which in the Antarctic region is dominated by the Southern Annular Mode.

Shallow ice cores were obtained from widely distributed sites across the West Antarctic ice sheet, as part of the United States portion of the International Trans-Antarctic Scientific Expedition (US ITASE) program. The US ITASE cores have been dated by annual-layer counting, primarily through the identification of summer peaks in non-sea-salt sulfate (nssSO42–) concentration. Absolute dating accuracy of better than 2 years and relative dating accuracy better than 1 year is demonstrated by the identification of multiple volcanic marker horizons in each of the cores, Tambora, Indonesia (1815), being the most prominent. Independent validation is provided by the tracing of isochronal layers from site to site using high-frequency ice-penetrating radar observations, and by the timing of mid-winter warming events in stable-isotope ratios, which demonstrate significantly better than 1 year accuracy in the last 20 years. Dating precision to ±1 month is demonstrated by the occurrence of summer nitrate peaks and stable-isotope ratios in phase with nssSO42–, and winter-time sea-salt peaks out of phase, with phase variation of

Recent Antarctic climate variability on month-to-month to interannual time scales is assessed through joint analysis of surface temperatures from satellite thermal infrared observations (T(sub IR)) and passive microwave brightness temperatures (T(sub B)). Although Tw data are limited to clear-sky conditions and T(sub B) data are a product of the temperature and emissivity of the upper approx. 1m of snow, the two data sets share significant covariance. This covariance is largely explained by three empirical modes, which illustrate the spatial and temporal variability of Antarctic surface temperatures. T(sub B) variations are damped compared to TIR variations, as determined by the period of the temperature forcing and the microwave emission depth; however, microwave emissivity does not vary significantly in time. Comparison of the temperature modes with Southern Hemisphere (SH) 500-hPa geopotential height anomalies demonstrates that Antarctic temperature anomalies are predominantly controlled by the principal patterns of SH atmospheric circulation. The leading surface temperature mode strongly correlates with the Southern Annular Mode (SAM) in geopotential height. The second temperature mode reflects the combined influences of the zonal wavenumber-3 and Pacific South American (PSA) patterns in 500-hPa height on month-to-month timescales. ENSO variability projects onto this mode on interannual timescales, but is not by itself a good predictor of Antarctic temperature anomalies. The third temperature mode explains winter warming trends, which may be caused by blocking events, over a large region of the East Antarctic plateau. These results help to place recent climate changes in the context of Antarctica's background climate variability and will aid in the interpretation of ice core paleoclimate records.

Satellite observations of microwave emission are a key resource for estimating surface temperatures in Antarctica. Use of these data to examine climate variability, however, relies on the assumption of constancy through time in the relationship between surface temperatures and the proxy brightness temperatures. Thus we are motivated to study the physical relationship between surface and brightness temperature time series, and to seek indicators of possible temporal variability in that relationship. Here we report an initial study using near-surface temperatures from the Byrd Station automated weather station in West Antarctica and 37 GHz, vertically polarized brightness temperatures from the Scanning Multichannel Microwave Radiometer. We begin with the simplest model of the relevant thermal and microwave physics and derive a convolution expression that relates surface and brightness temperatures. The convolution kernel depends on firn thermal diffusivity and the microwave extinction coefficient in a particularly simple way: solely through a single characteristic time-scale. For the Byrd data, we find that the (fractional variation in) observed brightness temperatures can be reproduced by our model in considerable detail, on scales from interannual down to a few days. The time-scale is tightly constrained by minimization of the discrepancy between observed and simulated time series, and the optimized value agrees closely with that derived from independent estimates of firn thermal and microwave parameters. We find no evidence thus far of temporal variability in the relation between surface and brightness temperatures, though investigation across a wider domain in space and time is needed before such variability can be ruled out.

Dual-energy computed tomography (DECT) offers various fields of application, especially in angiography using virtual monoenergetic imaging. The aim of this study was to evaluate objective image quality indices of calculated low-kiloelectron volt monoenergetic DECT angiographic cervical and cerebral data sets compared to virtual 120-kV polyenergetic images.Forty-one patients (21 men, mean age 58 ± 14) who underwent DECT angiography of the cervical (n = 7) or cerebral vessels (n = 34) were retrospectively included in this study. Data acquired with the 80 and 140 kVp tube using dual-source CT technology were subsequently used to calculate low-kiloelectron volt monoenergetic image data sets ranging from 120 to 40 keV (at 10-keV intervals per patient). Vessel and soft tissue attenuation and image noise were measured in various regions of interest, and contrast-to-noise ratio (CNR) was subsequently calculated. Differences in image attenuation and CNR were compared between the different monoenergetic data sets and virtual 120-kV polyenergetic images.For cervical angiography, 60-keV monoenergetic data sets resulted in the greatest improvements in vessel attenuation and CNR compared to virtual 120-kV polyenergetic data sets (+40%, +16%; all P.01). Also for cerebral vessel assessment, 60-keV monoenergetic data sets provided the greatest improvement in vessel attenuation and CNR (+40%, +9%; all P.01) compared to virtual 120-kV polyenergetic data sets.60-keV monoenergetic image data significantly improve vessel attenuation and CNR of cervical and cerebral DECT angiographic studies. Future studies have to evaluate whether the technique can lead to an increased diagnostic accuracy or should be used for dose reduction of iodinated contrast material.

[1] A bipolar seesaw of Arctic and Antarctic temperature anomalies has been reported to be evident in instrumental data on decadal timescales during the last century. This finding hinges upon a global temperature data set that for the area poleward of ∼60°S is derived from only one sub-Antarctic station prior to the mid-1940s, and does not include a substantial number of Antarctic stations until the late 1950s. The timeseries of the single-station record for the early period spliced to the data based on broader coverage for the latter period is an artificial estimate of the Antarctic climate trend and its variability. We estimate the real variability using the original timeseries from the sub-Antarctic station, a reconstruction of the Southern Annular Mode index, and an ice-core based reconstruction of Antarctic temperature. None of these Antarctic timeseries are significantly correlated with Arctic or North Atlantic climate records, nor with the index of the Atlantic Multidecadal Oscillation, which was proposed as the driving mechanism of the seesaw. Instead, each of these records is consistently correlated with tropical Pacific sea surface temperatures. However, neither the seesaw nor the tropics alone can fully capture the complexity of Antarctic climate variability and climate change.

a b s t r a c t The purpose of this study was to evaluate the discriminant and concurrent (criterion-related) validity of a recently introduced floor-based photocell system (Optogait, Microgate, Bolzano, Italy) with a validated electronic walkway for the assessment of spatiotemporal gait parameters. Fifteen patients (mean age ± standard deviation: 65 ± 7 years) with total knee arthroplasty and 15 healthy matched control subjects were asked to walk at different velocities while gait variables were recorded simultaneously by the two instruments. The Optogait and the criterion instrument detected the same differences in walking parameters between patients and controls. Intraclass correlation coefficients ranged between 0.933 (swing time) and 0.999 (cycle time, cadence and walking speed). Cycle time and stance time were significantly longer, while swing time, step length, cadence and walking speed were significantly lower for Optogait (p < 0.001) compared to the criterion instrument. The Optogait system demonstrated high discriminant and concurrent validity with an electronic walkway for the assessment of spatiotemporal gait parameters in orthopedic patients and healthy controls. However, the two measuring instruments cannot be used interchangeably for quantitative gait analysis, and further validation of floor-based photocell technology is warranted.

[1] Explosive volcanism is known to be a leading natural cause of climate change. The second half of the 13th century was likely the most volcanically perturbed half-century of the last 2000 years, although none of the major 13th century eruptions have been clearly attributed to specific volcanoes. This period was in general a time of transition from the relatively warm Medieval period to the colder Little Ice Age, but available proxy records are insufficient on their own to clearly assess whether this transition is associated with volcanism. This context motivates our investigation of the climate system sensitivity to high- and low-latitude volcanism using the fully coupled NCAR Community Climate System Model (CCSM3). We evaluate two sets of ensemble simulations, each containing four volcanic pulses, with the first set representing them as a sequence of tropical eruptions and the second representing eruptions occurring in the mid-high latitudes of both the Northern and Southern hemispheres. The short-term, direct radiative impacts of tropical and high-latitude eruptions include significant cooling over the continents in summer and cooling over regions of increased sea-ice concentration in Northern Hemisphere (NH) winter. A main dynamical impact of moderate tropical eruptions is a winter warming pattern across northern Eurasia. Furthermore, both ensembles show significant reductions in global precipitation, especially in the summer monsoon regions. The most important long-term impact is the cooling of the high-latitude NH produced by multiple tropical eruptions, suggesting that positive feedbacks associated with ice and snow cover could lead to long-term climate cooling in the Arctic.

Although the 20th Century warming of global climate is well known, climate change in the high-latitude Southern Hemisphere (SH), especially in the first half of the century, remains poorly documented. We present a composite of water stable isotope data from high-resolution ice cores from the West Antarctic Ice Sheet. This record, representative of West Antarctic surface temperature, shows extreme positive anomalies in the 1936–45 decade that are significant in the context of the background 20th Century warming trend. We interpret these anomalies—previously undocumented in the high-latitude SH—as indicative of strong teleconnections in part driven by the major 1939–42 El Niño. These anomalies are coherent with tropical sea-surface temperature, mean SH air temperature, and North Pacific sea-level pressure, underscoring the sensitivity of West Antarctica's climate, and potentially its ice sheet, to large-scale changes in the global climate.

[1] We compare new observationally-based data sets of Antarctic near-surface air temperature and snowfall accumulation with 20th century simulations from global climate models (GCMs) that support the Intergovernmental Panel on Climate Change Fourth Assessment Report. Annual Antarctic snowfall accumulation trends in the GCMs agree with observations during 1960–1999, and the sensitivity of snowfall accumulation to near-surface air temperature fluctuations is approximately the same as observed, about 5% K−1. Thus if Antarctic temperatures rise as projected, snowfall increases may partially offset ice sheet mass loss by mitigating an additional 1 mm y−1 of global sea level rise by 2100. However, 20th century (1880–1999) annual Antarctic near-surface air temperature trends in the GCMs are about 2.5-to-5 times larger-than-observed, possibly due to the radiative impact of unrealistic increases in water vapor. Resolving the relative contributions of dynamic and radiative forcing on Antarctic temperature variability in GCMs will lead to more robust 21st century projections.

The Antarctic Peninsula is one of the most rapidly warming locations on Earth, but it has been difficult to establish whether continent-wide changes are comparable to the clear upward trend in global average temperature. This is because most of the continuous records from ice-sheet weather stations are coastal, providing little information on the continental interior. This problem is by-passed in a new reconstruction of Antarctic surface temperature trends for 1957–2006, based on satellite data (with good spatial coverage for a short period) and air temperatures from weather stations (for a long timescale), blended via an algorithm that estimates missing data points in climate fields. The resulting record, more reliable than previous gap-filling exercises, suggests that overall the continent is warming by about 0.1 °C per decade, with stronger warming in winter and spring and over West Antarctica. The Antarctic Peninsula is known to be warming rapidly, but the overall pattern of climate change for the full Antarctic continent has been uncertain. This work finds that the entire continent is warming at a rate of 0.12 ± 0.07 °C per decade, with stronger warming in winter and spring and over West Antarctica. Assessments of Antarctic temperature change have emphasized the contrast between strong warming of the Antarctic Peninsula and slight cooling of the Antarctic continental interior in recent decades1. This pattern of temperature change has been attributed to the increased strength of the circumpolar westerlies, largely in response to changes in stratospheric ozone2. This picture, however, is substantially incomplete owing to the sparseness and short duration of the observations. Here we show that significant warming extends well beyond the Antarctic Peninsula to cover most of West Antarctica, an area of warming much larger than previously reported. West Antarctic warming exceeds 0.1 °C per decade over the past 50 years, and is strongest in winter and spring. Although this is partly offset by autumn cooling in East Antarctica, the continent-wide average near-surface temperature trend is positive. Simulations using a general circulation model reproduce the essential features of the spatial pattern and the long-term trend, and we suggest that neither can be attributed directly to increases in the strength of the westerlies. Instead, regional changes in atmospheric circulation and associated changes in sea surface temperature and sea ice are required to explain the enhanced warming in West Antarctica.

[1] Estimating the spatial extent of past climate changes has been an ongoing challenge for paleoclimatology. For such estimates to be made with confidence, it is important to establish an understanding of the spatial coherence of proxy records during an interval of known climate change. We use water stable isotopes from high-resolution ice cores and twentieth-century observations of sea level pressures and sea surface temperatures to assess the covariance among isotopic records and its link to organized patterns of climate variability. Covarying signals in the cores are identified using empirical orthogonal function analysis. Results from regression analysis show that the leading signals are consistent with key climate patterns including the Northern Atlantic Oscillation and Southern Annular Mode and variability in tropical Pacific sea surface temperatures associated with the El Nino–Southern Oscillation. Patterns that have recently been identified in instrumental data, such as positive tropical Pacific SST anomalies associated with the negative phase of the SAM, are evident in the ice cores. These explanations for the variance of stable isotopes are consistent with recent studies using isotope-enabled general circulation models and provide a physical basis for interpreting the observed isotopic signals. While there is also a global change signal that is evident when analyzing the records collectively, there are some limitations in reconstructing global temperatures due to the geographic coverage of the available records and the current lack of modeling studies to explain the observed global-scale changes. Still, water stable isotope ratios preserved in ice cores provide a sufficiently rich sampling of large-scale climate variability that they can be more widely used in physically based paleoclimate reconstructions covering the last millennium and other periods.

Highly accurate and stable observations—beyond those provided by routine weather monitoring—are essential for understanding the behavior of the climate system, developing and validating Earth system models, and attributing extreme weather events and long-term trends to causes [National Research Council, 2012; Trenberth et al., 2013]. In parallel with an exploding volume of climate data, ready access to data in user-friendly formats is important to an expanding number and increasing diversity of individuals worldwide across public, private, and academic sectors [Overpeck et al., 2011].

Purpose To assess the efficacy of the Toric Implantable Collamer Lens (ICL) to treat moderate to high myopic astigmatism. Design Prospective nonrandomized clinical trial. Participants Two hundred ten eyes of 124 patients with between 2.38 and 19.5 diopters (D) of myopia (spherical equivalent [SE]) and 1 to 4 D of astigmatism participating in the United States Food and Drug Administration clinical trial of the Toric ICL. Intervention Implantation of the Toric ICL. Main Outcome Measures Uncorrected visual acuity (UCVA), refraction, best spectacle-corrected visual acuity (BSCVA), adverse events, and postoperative complications. Results At 12 months postoperatively, the proportion of eyes with 20/20 or better UCVA (83.1%) was identical to the proportion of eyes with preoperative 20/20 or better BSCVA (83.1%); 76.5% had postoperative UCVA better than or equal to preoperative BSCVA. The mean manifest refractive cylinder dropped from 1.93 D (±0.84) at baseline to 0.51 D (±0.48) postoperatively, a 73.6% decrease in astigmatism. Although only 21.0% of eyes had 1-D refractive cylinder preoperatively (none less), 91.4% of cases had ≤1 D of cylinder postoperatively. Furthermore, 65.6% had ≤0.5 D and 40.9% had ≤0.25 D of refractive cylinder postoperatively. Mean manifest refraction SE (MRSE) improved from −9.36 D (±2.66) preoperatively to 0.05 D (±0.46) postoperatively. A total of 76.9% of eyes were predicted accurately to within ±0.5 D, 97.3% to within ±1.0 D, and 100% to within ±2.0 D of predicted MRSE. Postoperatively, 37.6% of eyes had a BSCVA of 20/12.5 or better, compared with a preoperative level of 4.8%. Furthermore, BSCVA of 20/20 or better occurred in 96.8% postoperatively, compared with 83.1% preoperatively. Mean improvement in BSCVA was 0.88 lines; there were 3 cases (1.6%) that lost ≥2 lines of BSCVA after 12 months postoperatively, whereas 18.9% of cases improved by ≥2 lines. A total of 76.4% of cases gained ≥1 lines of BSCVA, whereas only 7.5% of cases lost the equivalent amount. Three ICL removals were performed without significant loss of BSCVA, and 1 clinically significant lens opacity was observed. Conclusion The results support the efficacy and predictability of Toric ICL implantation to treat moderate to high myopic astigmatism. Important safety concerns were not identified.

[1] Annual and interannual variability of 21 years of passive microwave brightness temperatures on the Antarctic ice sheet is documented through principal component analysis. The leading modes show that brightness temperatures are dominantly forced by the annual temperature cycle, with surface melting signals explaining about 5% of the total variance of the data. Analysis of the data with the annual cycle and melting signals removed results in two significant interannual modes. The first is consistent with the Antarctic Oscillation and its influence on Antarctic surface temperatures, while the second reflects regional scale variability that appears to be linked to sea-ice anomalies and the El Nino-Southern Oscillation. The expression of these characteristic patterns of Southern Hemisphere climate variability on the Antarctic continent provides a framework for interpreting modern climate records, as well as a basis for reconstructing past climate variability with ice cores.

[1] We present glaciological and climatological characteristics of Hercules Dome, Antarctica (86°S, 105°W), which demonstrate its potential as a deep ice core site. Annual layering in δD ratios from a 72 m ice core collected by the US-ITASE 2002 traverse indicate accumulation rates of 0.16–0.20 m/yr ice equivalent over the last 300 years. Age control from stratigraphy seen in the radio-echo sounding data collected during the same traverse suggests a rate of 0.09–0.11 m/yr averaged over the past 18,000 years. Ice stratigraphy also indicates that the ice divide position has been stable through at least this period. Comparison of satellite-derived temperature anomalies with atmospheric reanalysis data show that the site is sensitive to the two dominant patterns of climate variability in the high-latitude Southern Hemisphere. Climate proxy data from a deep ice core at Hercules Dome would be indicative of changes in Pacific Southern Hemisphere climate variability and may provide new information on rapid climate change events in Antarctica. The sensitivity of the site and the combination of relatively high accumulation rates, low temperatures (mean annual −35°C to −40°C), and simple ice flow suggest that Hercules Dome is an ideal site for a future deep ice core.

[1] We present a reconstruction of Antarctic mean surface temperatures over the past two centuries based on water stable isotope records from high-resolution, precisely dated ice cores. Both instrumental and reconstructed temperatures indicate large interannual to decadal scale variability, with the dominant pattern being anti-phase anomalies between the main Antarctic continent and the Antarctic Peninsula region. Comparative analysis of the instrumental Southern Hemisphere (SH) mean temperature record and the reconstruction suggests that at longer timescales, temperatures over the Antarctic continent vary in phase with the SH mean. Our reconstruction suggests that Antarctic temperatures have increased by about 0.2°C since the late nineteenth century. The variability and the long-term trends are strongly modulated by the SH Annular Mode in the atmospheric circulation.