C52A-01
Significant warming of continental West Antarctica in the last 50 years
We use statistical climate field reconstruction techniques to determine monthly temperature anomalies for the near-surface of the Antarctic ice sheet since 1957. Two independent data sets are used to provide estimates of the spatial covariance patterns of temperature: automatic weather stations and thermal infrared satellite observations. Quality-controlled data from occupied instrumental weather stations are used to determine the amplitude of changes in those covariance patterns through time. We use a modified principal component analyses technique (Steig et al., in review, Nature) to optimize the combination of spatial and temporal information. Verification statistics obtained from subsets of the data demonstrate the resulting reconstructions represent improvements relative to climatological mean values. We find that significant warming has occurred over most of continental West Antarctica. This is an area much larger than previously reported; most studies have concluded that warming is limited to the Antarctic Peninsula. An updated version of the recent temperature reconstruction of Monaghan et al. (2008, JGR) independently confirms our results. Warming in continental West Antarctica in the last 50 years exceeds 0.1 °C/decade, and is strongest in Spring. A possible explanation is an increase in storms in the Amundsen-Bellinghausen sea, resulting in enhanced warm air fluxes to the continent. Increased storminess in this sector is associated with the positive phase of the zonal wave-3 pattern, which independent observations suggest has increased since the 1970s (Raphael, GRL, 2004). The substantial negative sea ice anomalies in the Amundsen-Bellinghausen sea may also play a role. Our results suggest that changes in the wave-3 pattern dominates over (possibly anthropogenic) changes in the Southern Annular Mode in explaining recent Antarctic temperature variability.
C52A-02
Wind-Driven Changes in Sea Ice Advance and Retreat and Ocean Freshening Near Antarctica
The timing of sea ice edge advance during autumn-winter and its retreat during spring-summer is mapped from SMMR-SSM/I satellite data from 1979 to 2008 in order to further investigate when, where and how the largest changes are occurring in the Southern Ocean. Previously it was shown that two Pacific regions show large but opposing sea ice trends. In the western Antarctic Peninsula/southern Bellingshausen Sea, sea ice is retreating earlier (up to 1 month) and advancing later (up to 2 months). In the western Ross Sea, sea ice is retreating later (up to 1 month) and advancing earlier (up to 1 month). These seasonal sea ice trends are largely a response to atmospheric circulation changes associated with ENSO and SAM over spring-autumn. We now explore how these wind driven changes may, or may not, be contributing to ocean freshening near Antarctica. In the eastern Pacific region we infer that the wind-driven delay in ice advance is mechanically thickening the sea ice near Antarctica and incorporating large amounts of snow into the ice matrix during rafting events. The subsequent wind-driven acceleration of sea ice retreat contributes further to increased ice and snow melting over that shelf region, and to observed freshening downstream in the Ross Sea. The sea ice trend in the Ross Sea could increase or decrease that freshening rate.
C52A-03 INVITED
A Synthesis of Arctic Weather and Climate
In the polar regions, its is difficult to place current weather and climate trends in a long-term climatological perspective, primarily because the meteorological records there are limited in time and space in comparison with other regions of the globe. The low spatial density of polar meteorological data makes it challenging to attribute changes to local effects or to hemispheric teleconnections. Reanalyses, which assimilate all available observations into physically-consistent, regularly-spaced and comprehensive datasets, can be especially helpful in these latitudes. The timeliness of such efforts is especially pronounced given the recently-observed dramatic changes in Arctic sea ice, land ice, and permafrost regions, whose causes are being debated. A new physically-consistent synthesis of Arctic observations will be achieved through the high-resolution reanalysis of the northern high latitude region, spanning poleward from the headwaters of the northward flowing rivers. The ASR is a collaboration of the Ohio State University, the National Center Atmospheric Research, the University of Colorado, the University of Illinois, and the University of Alaska-Fairbanks. The production phase of the initial ASR has been funded by the National Science Foundation as an International Polar Year (IPY 2007-2009) project covering 2000-2010. The ASR will provide a high resolution description in space (15 km) and time (1-3 h) of the coupled atmosphere-sea ice-land surface system of the Arctic. Ingested historical data streams from the surface and space, along with measurements of the physical components of the Arctic Observing Network being developed as part of IPY will be assimilated by the ASR. Gridded output fields from the ASR will serve a variety of uses such drivers for coupled ice-ocean, land surface and other models, and will offer a focal point for coordinated model inter-comparison efforts. The ASR will permit detailed reconstructions of the Arctic system's variability and change, thereby complementing efforts of the global reanalyses in illuminating the causes of high northern latitude climate change. The project will also shape the legacy observing network of the IPY by providing a vehicle for observing system sensitivity studies of the Sustained Arctic Observing Network (SAON). ASR will result in improved Arctic atmospheric and land-surface modeling and advances in high latitude mesoscale data assimilation.
C52A-04
Synoptic-scale Atmospheric Forcing of Frozen Ground in the Eurasian High Latitudes
Seasonal freezing and thawing processes of the ground thermal regime in cold regions play an important role in ecosystem diversity, productivity, and the Arctic hydrologic system. Long-term changes in seasonal freeze and thaw depths are useful indicators of climate change. In this analysis we improve on a previous 1956- 1990 assessment of seasonal freeze depths based on only 242 sites by employing a greatly expanded station database of soil temperatures for 423 sites with updated observations for 1930-2000. The addition of 181 sites throughout the Russian Arctic combined with 37 more years of observations allows for a significantly more comprehensive evaluation. The addition of 1991-2000 in particular allows us to quantify changes in frozen ground during a decade when accelerated climate warming has occurred. We find a statistically significant overall change in seasonal freeze depth of -29 cm for 1930-2000, averaged across the Eurasian high latitudes. This time series also indicates some patterns of interdecadal variability: increasing freeze depths until ~1970, followed by a sharp decrease until ~1995. From ~1995 on, freeze depths may actually be increasing again. Exploring potential climatic drivers of these changes, we find that seasonal freeze depth correlates most strongly with the annual freezing index, and also with air temperature. However, when all potential drivers are working together to jointly influence ground freezing, snow depth becomes the most important driver, more so than temperature. The most prominent feature in this time series is the strong 1970-1995 decrease, which coincides with a strong positive phase in the North Atlantic Oscillation (NAO). We find that when the NAO is in a strong positive phase, as it was from the late 1960s to the mid 1990s, it accounts for 80% of the variability in frozen ground conditions. It could thus be argued that the prominent decrease in freeze depths from ~1970- 1995 is due to internal variability in the atmosphere, as represented by the NAO.
C52A-05 INVITED
Investigating the ability of General Circulation Models to capture the effects of Eurasian snow cover on winter climate
Observations show that October snow fall over Eurasia is strongly correlated with Northern Hemisphere wave activity flux from the troposphere to the stratosphere in December and with tropospheric dynamical fields in January. These correlations are not found in simulations from any of the CMIP-3 General Circulation Models (GCMs). Thus these GCMs do not capture what may be an important influence on Northern Hemisphere winter climate. The mechanism of Cohen et al. for the strong observed correlations is considered, and possible reasons for the absence of this mechanism in GCMs are investigated. Implications for climate simulations and seasonal forecasting are discussed.
C52A-06
North American Snow - Climate Interactions: State and Dynamical Responses to Snow Anomalies
Continental scale snow cover anomalies have the potential to modulate local and remote climate over monthly to seasonal time scales. In this study, physical and dynamical links between anomalous snow conditions during the North American (NA) snow season and seasonal climate parameters throughout the Northern Hemisphere (NH) are examined. Two GCM experiments with high/low NA snow forcing are run as forty one-year integrations and then differenced; creating an ensemble that reflects the climatic response to a sustained year-long NA snow forcing. Local responses in seasonal mean sea level pressure and surface temperature are consistent with previous research and current knowledge of climate dynamics. Additionally, a remote positive temperature response is observed over Northern Eurasia during the spring months. Consistent with the mean surface responses, a variety of high-pass filtered transient eddy statistics exhibit robust responses to the snow forcing. North Atlantic storm tracks diagnosed using geopotential height variances exhibit an intensification that begins in fall and continues through late winter, finally dissipating in spring. Similarly, 500mb eddy kinetic energy, 750 mb poleward and upward heat transport, and 250mb poleward momentum flux all intensify over the storm track regions throughout the NH snow season. The time evolution and spatial extent of the storm track response appears to indicate downstream development as disturbances transfer some of their energy to neighboring eddies, extending the storm tracks from their traditional NA source regions, well into Eurasia. Furthermore, the stationary wave streamfunction response indicates a general intensification of the mean flow, especially during the late winter-early spring transition season. These results build upon previously published GCM experiments with more restricted snow forcings, and shed light on the climatic consequences of NA snow cover anomalies.
C52A-07
The impacts of AO and ENSO on the Great Lakes ice cover
The impacts of ENSO (El Nino and South Oscillation) and Arctic Oscillation (AO) on the Great Lakes ice cover were investigated using ice observations for winters 1963-2008, and NCEP reanalysis data. Signatures of ENSO and AO were found in the Great Lakes ice cover. However, the impacts are nonlinear and asymmetric. Strong El Nino events are often associated with least ice cover on the Great Lakes, while the impacts of weak El Nino and La Nina events on the Great Lakes are marginally significant. Negative AO events are often associated with severe ice cover, while positive AO event has smaller impact. The strong El Nino and negative AO events explained about 50% severe and least ice cover winters on the Great Lakes, respectively. The interference of the effects of ENSO and AO over the Great Lakes makes the relationships complicated. This may be an important cause of nonlinear and asymmetric responses of the regional climate including lake ice in the Great Lakes to ENSO and AO.
C52A-08
Mass balance changes of 30 Swiss glaciers during the 20th century
Climate change during the 20th century has caused major mass losses of Alpine glaciers. This study focuses on rates and trends in glacier surface mass balance on a mountain range scale. We compute spatially distributed surface mass balances of 30 glaciers in the Swiss Alps for the period 1900-2007 in daily resolution. The investigated glaciers range from the largest ice mass of the Alps (Grosser Aletschgletscher) to small ice fields and cover different climatic regions of Switzerland. Thus, the 30-glacier average is assumed to provide a representative response of glaciers in the European Alps to the climatic change. A mass balance model is forced by air temperature and precipitation data, which is compiled from various long-term data series. The model is closely constrained with field measurements: ice volume changes derived from high-resolution digital elevation models are available for all glaciers in decadal to semicentennial intervals. For some study sites in-situ measurements of seasonal mass balance at stakes, partly with records since the 1920s, and discharge data are available. Temporal trends in glacier-specific variables are analyzed for a 100-year period (1907-2007) and a 50-year period (1957-2007). We find significant negative trends in net balance for both periods. The slope of the trend is substantially steeper since 1957. No long-term changes in winter accumulation are observed. Our results indicate a significant prolongation of the melting season, which explains the higher rates of mass loss to some extent. During the 20th century, Swiss glaciers showed two distinct phases of mass gain (1910s and 1970s) and two decades of accelerated glacier wastage (1940s and 1985 to present). The fluctuations of climatic forcing on glaciers (energy consumed for snow and ice melt) was similar for all Swiss glaciers. We show that the climatic forcing at elevations of 3000 m a.s.l.~was higher in the 1940s than during the past two decades.