Synoptic conditions during wintertime temperature extremes in Alaska

Abstract

The large-scale atmospheric state associated with widespread wintertime warm and cold extremes in southern Alaska was identified using 1989 to 2007 European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-I) data. Extremes were defined as days with the coldest and warmest 1% of daily temperatures. Widespread extreme events were identified for days when at least 25 50 km grid cells in the study domain met the extreme temperature criteria. A total of 55 cold and 74 warm extreme days were identified in 19 winters. Composites of the atmospheric state from 5 days before through the day of the extreme events were analyzed to assess the large-scale atmospheric state associated with the extremes. The method of self-organizing maps (SOMs) was used to identify the range of sea level pressure (SLP) patterns present in the ERA-I December-February data, and these SLP patterns were then used to stratify the extreme days by their large-scale atmospheric circulation. Composites for all warm or cold extreme days showed less intense features than those for specific SLP patterns. In all of the composites temperature advection, strongest at 700 hPa, and anomalous longwave radiation were the primary factors that led to the extreme events. The anomalous downwelling longwave radiation was due to either reduced cloud cover, during cold extremes, or to increased cloud cover, during warm extremes. The SOM composites provided additional insight into the temporal evolution of the extreme days and highlighted different portions of southern Alaska most likely to experience temperature extremes for a given SOM SLP pattern.

Evaluation of synoptic‐scale patterns during extreme temperature and precipitation events in Alaska

“…This circulation pattern is consistent with results from Cassano et al . () who found temperature advection to be a principle driver of cold extremes during the winter for a region of Alaska containing Nome, Fairbanks, and Anchorage.…”

“…Cassano et al . () identified 700 hPa as a critical height of observing cold air advection. Our analysis compliments their results through consistent, strongly correlated patterns of a northerly geostrophic flow at 700 hPa for cold events in the locations of their analysis (Nome, Fairbanks, and Anchorage).…”

“…Cassano et al . () identified the 700‐hPa geopotential height layer as strongly associated with temperature advection producing extreme temperatures during the winter months. Data for mean sea level pressure (MSLP), 700 and 500‐hPa geopotential heights were obtained for the CFSR through NOAA's National Operational Model Archive and Distribution System (NOMADS; Rutledge et al ., ).…”

On the use of self‐organizing maps for studying climate extremes

“…Horton et al [] provided evidence that changes in the frequency of geopotential height patterns since the 1980s have already added to changes in extreme temperature trends in some Northen Hemisphere midlatitudinal regions. Cassano et al [] and Cassano et al [] examined surface circulation patterns conducive to Arctic winter temperature extremes, while Loikith and Broccoli [] compared circulation links to North American temperature extremes between observations and climate models. Focusing on rainfall, Alexander et al [] found that changes in circulation patterns across parts of southern Australia for the 20th century have likely contributed to decreasing trends in rainfall and rainfall extremes and decreases in “storminess.” Similarly, but focusing on South African rainfall, Lennard and Hegerl [] suggest that some seasonal rainfall trends and seasonal timing of rainfall extremes can be linked to trends in regional circulation patterns.…”

Synoptic conditions during summertime temperature extremes in Alaska

“…Although quite a few studies have focused on the relationship between circulation and temperature extremes (e.g. Shabbar and Bonsal, ; Shulski et al , ; Cassano et al , ; Lebedeva et al , ; Grotjahn et al , and references therein), there is a comparative dearth of these types of studies focusing on these relationships for Alaska summertime temperature extremes. Horton et al () found increasing trends in anticyclonic circulation related to summer and autumn hot extremes over portions of Eurasia and North America.…”