Elizabeth N. Cassano

Bio: Elizabeth N. Cassano is an academic researcher from Cooperative Institute for Research in Environmental Sciences. The author has contributed to research in topics: Sea ice & Arctic ice pack. The author has an hindex of 13, co-authored 19 publications receiving 616 citations.

Predicted changes in synoptic forcing of net precipitation in large Arctic river basins during the 21st century

Abstract: [1] Daily output from 15 global climate system models and 2 global reanalyses were analyzed to create a synoptic climatology of Arctic sea level pressure and to assess predicted changes in net precipitation over the Arctic. The method of self-organizing maps was used to create the synoptic climatology from 3 decades of model output: 1991–2000, 2046–2055, and 2091–2100. The model-derived synoptic climatology was compared to that from two reanalyses, for the period 1991–2000, and this comparison was used to select a subset of models that best reproduced the currently observed synoptic climate of the Arctic. Of the 15 models evaluated in this way, only 4 models were able to reproduce the key features of the Arctic synoptic climate as depicted by the two global reanalyses. The synoptic climatology created using the self-organizing map technique lends itself to the study of extreme events and the projections from this subset of 4 models indicate an increase in cyclonically dominated weather patterns over the 21st century. The models also projected an increase in net precipitation over the Arctic cap and the large Arctic river watersheds during the 21st century. Using the synoptic climatology, a method to assess thermodynamic- and circulation-related changes in net precipitation was derived. The results of this assessment indicate that thermodynamic changes are responsible for more than 75% of the predicted change in Arctic net precipitation during the 21st century.

Classification of synoptic patterns in the western Arctic associated with extreme events at Barrow, Alaska, USA

Abstract: The coastal geography of Barrow, Alaska, makes the city vulnerable to weather events that cause flooding and erosion. This study uses the self-organizing map (SOM) algorithm, an unsu- pervised learning process that codifies large, multivariate datasets onto a 2-dimensional array, or map, to study large-scale circulation patterns associated with temperature and high wind extremes at Barrow. The analysis first uses the SOM algorithm to produce an automated 55 yr synoptic climatology of daily sea level pressure patterns for the western Arctic for August to November, when the area is potentially ice free. The results are in agreement with previous Arctic climatologies, showing the Aleutian Low to be dominant in southern Alaska, and high pressure prevalent over the Beaufort and Chukchi Seas. The SOM algorithm is then used to study circulation patterns associated with temperature and high wind extremes at Barrow. These results show that high winds are associ- ated with patterns containing a strong pressure gradient between the Aleutian Low and the Beaufort and Chukchi Seas and also with patterns that contain a low pressure system to the north of Barrow. High (low) air temperature extreme anomalies are associated with patterns that produce strong, southerly (northerly) air flow at Barrow. This study demonstrates the utility of using SOMs to investi- gate the relationship between local weather conditions and large-scale patterns. This approach can be applied to future global climate model (GCM) simulations to investigate the impact of changes in large-scale circulation patterns to local extreme events.

Self-organizing map analysis of widespread temperature extremes in Alaska and Canada

Abstract: This paper demonstrates how self-organizing maps (SOMs) can be used to evaluate the large-scale environment, in particular the synoptic circulation associated with widespread temperature extremes. The paper provides details on how SOMs are created, how they can be used to understand extreme events, and lessons learned in applying this methodology for extremes analysis. Using a SOM can be helpful in understanding the underlying physical processes that control extreme events, and how the extremes and the processes that control them may change in time or differ across space. Examples of widespread daily temperature extremes in 4 regions: 2 each in Alaska and in northern Canada during winter (December, January, and February) for 1989−2007 are presented to illustrate the application of the methodology. For the regions studied, the size of the domain over which the synoptic circulation was defined—in particular using a smaller domain focused on particular regions—and a greater number of classes to represent the archetypical synoptic patterns for the regions, give the best relationship between synoptic circulation and extremes. The results are most robust for the Alaskan domains and less so for the Canadian domains, leading to the conclusion that further study is warranted to better understand extremes in the Canadian regions.

Modeled precipitation variability over the Greenland Ice Sheet

Abstract: On the basis of the evaluation of recent Greenland precipitation studies, some of the deficiencies in the modeled precipitation are probably related to the topographic data employed in modeling. In this paper the modern digital elevation data of Ekholm [1996] is used. If the horizontal pressure gradient force in σ coordinates is separated into its irrotational and rotational parts, which are expressed by the equivalent geopotential and geo-stream-function, respectively, the topographic effect on the precipitation can be accurately modeled. The equivalent geopotential and geo-stream-function are implemented in a fully consistent manner in the generalized ω-equation in this paper. A simplified large-scale condensation without evaporation of condensate is also used. These improvements are combined to yield an improved dynamic method. Two aspects of the precipitation distribution are refined by the improved dynamic method. One is the 10 cm yr−1 contour near Summit, Greenland, and the other is a relative large precipitation area centered near the point (70°N, 49°W). Extensive comparisons are made between the retrieved precipitation and the observed annual accumulation time series from 11 ice core sites on the ice sheet. The modeled precipitation from the original method must use sealers to have a high degree of interannual correspondence between the measured accumulation and the retrieved precipitation, but the retrieved precipitation from the improved method increases at all ice core sites and a good correspondence is obtained without any sealer being required. The spatial average of multiyear mean error ( e¯j) is 11.5 cm yr−1 for the modeled precipitation from the improved method, while that for P from ERA-15 is 14.5 cm yr−1. The total mean error (eM) is 3.0 cm yr−1 for the improved method, while eM for the P from ERA-15 is 4.0 cm yr−1. These two errors show that the precipitation modeled by the improved method is better than the P from ERA-15. Thus the distribution of precipitation over the 11 sites retrieved by the improved dynamic method is considerably refined. Large downward trends in annual amounts are present in the precipitation retrieved by the improved dynamic method for all of Greenland and its southern and central west coastal regions. The modeled precipitation from the improved dynamic method and observed accumulation from ice cores are all in agreement with the Thomas et al. [1999] result that the southern Greenland ice sheet above 2000 m is approximately in balance. It also shows that local thickening and thinning areas of the ice sheet derived by airborne laser altimetry from 1993 to 1999 over the entire Greenland above 2000 m [Krabill et al., 2000] are approximately consistent with precipitation change retrieved by the improved dynamic method.

Atmospheric impacts of an Arctic sea ice minimum as seen in the Community Atmosphere Model

Abstract: There is growing recognition that reductions in Arctic sea ice extent will influence patterns of atmospheric circulation both within and beyond the Arctic. We explore the impact of 2007 ice conditions (the second lowest Arctic sea ice extent in the satellite era) on atmospheric circulation and surface temperatures and fluxes through a series of model experiments with the NCAR Community Atmospheric Model version 3 (CAM3). Two 30-year simulations were performed; one using climatological sea ice extent for the end of the 20th century and other using observed sea ice extent from 2007. Circulation differences over the Northern Hemisphere were most prominent during autumn and winter with lower sea level pressure (SLP) and tropospheric pressure simulated over much of the Arctic for the 2007 sea ice experiment. The atmospheric response to 2007 ice conditions was much weaker during summer, with negative SLP anomalies simulated from Alaska across the Arctic to Greenland. Higher temperatures and larger surface fluxes to the atmosphere in areas of anomalous open water were also simulated. CAM3 experiment results were compared to observed SLP anomalies from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data. The observed SLP anomalies during spring are nearly opposite to those simulated. In summer, large differences were shown between the observed and simulated SLP also, suggesting that the sea ice conditions in the months preceding and during the summer of 2007 were not responsible for creating an atmospheric circulation pattern which favoured the large observed sea ice loss. The simulated and observed atmospheric circulation anomalies during autumn and winter were more similar than spring and summer, with the exception of a strong high pressure system in the Beaufort Sea which was not simulated, suggesting that the forced atmospheric response to reduced sea ice was in part responsible for the observed atmospheric circulation anomalies during autumn and winter.

Evidence and implications of recent climate change in northern Alaska and other arctic regions.

Abstract: The Arctic climate is changing. Permafrost is warming, hydrological processes are chang- ing and biological and social systems are also evolving in response to these changing conditions. Knowing how the structure and function of arctic terrestrial ecosystems are responding to recent and persistent climate change is paramount to understanding the future state of the Earth system and how humans will need to adapt. Our holistic review presents a broad array of evidence that illustrates con- vincingly; the Arctic is undergoing a system-wide response to an altered climatic state. New extreme and seasonal surface climatic conditions are being experienced, a range of biophysical states and pro- cesses influenced by the threshold and phase change of freezing point are being altered, hydrological and biogeochemical cycles are shifting, and more regularly human sub-systems are being affected. Importantly, the patterns, magnitude and mechanisms of change have sometimes been unpredictable or difficult to isolate due to compounding factors. In almost every discipline represented, we show

More Evidence Linking Arctic Amplification to Extreme Weather in Mid-Latitudes

Abstract: [1] Arctic amplification (AA) – the observed enhanced warming in high northern latitudes relative to the northern hemisphere – is evident in lower-tropospheric temperatures and in 1000-to-500 hPa thicknesses. Daily fields of 500 hPa heights from the National Centers for Environmental Prediction Reanalysis are analyzed over N. America and the N. Atlantic to assess changes in north-south (Rossby) wave characteristics associated with AA and the relaxation of poleward thickness gradients. Two effects are identified that each contribute to a slower eastward progression of Rossby waves in the upper-level flow: 1) weakened zonal winds, and 2) increased wave amplitude. These effects are particularly evident in autumn and winter consistent with sea-ice loss, but are also apparent in summer, possibly related to earlier snow melt on high-latitude land. Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves.

Effects of Arctic Sea Ice Decline on Weather and Climate: A Review

Abstract: The areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air tempera- ture, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia.