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Many publications document an increase in air temperature at many weather stations in Alaska and for many different periods of time starting in the early twentieth century (Stafford et al. 2006 Kyselý 2010).Īlaska possesses a highly diverse climate due to its large geographic area (1,525,953 km 2), meridional extent, long coastline on the one side, and the large landmass of the North American continent on the other side, as well as variable relief (Shulski and Wendler 2007 Bieniek et al. 2016), as well as in other regions of the Earth (Meehl and Tebaldi 2004 Alexander et al. Research results based on climate scenarios show a continuation of currently noted tendencies in the future, both in North America and in Alaska alone (Lau and Nath 2012 Bennett and Walsh 2015 Jeong et al. ( 2015) were able to document a clear and statistically significant decreasing trend in the frequency of cold spells in the Arctic in the winter and summer as well as less pronounced, seasonal, and regional tendencies (either positive or negative) in the frequency of the occurrence of warm spells (1979–2013). In Canada, in the period 1911–2010, warm and cold extremes became increasingly warmer, especially in the winter and in the Arctic regions of the country (Wang et al. Tendencies noted in the USA vary by region however, most areas of the country including Alaska have experienced warm spells and heatwaves more often since the 1950s, while cold spells have become less frequent (Barnett et al. ( 2007) found that in the 1880–2003, the length of heatwaves doubled in Western Europe, while the occurrence of hot days nearly tripled. 2012) as well as in particular regions of the Earth, especially at high latitudes of the northern hemisphere (AMAP 2011 IPCC 2013). Increases in the frequency of occurrence, duration, and severity of warm extremes along with a decrease in the case of cold extremes are observed on a global scale (Alexander et al.

Humans, animals, and plants are sensitive to the severity, frequency, and persistence of extremes (Wigley 1985), which explains why these parameters are most often examined by climatologists. The change in the distribution of mean annual air temperatures is also linked with a change in the distribution of air temperature extremes (IPCC 2013), which leads to a change in the frequency of extreme temperature events-hereafter temperature extremes-or days with exceptionally high or low air temperatures as well as warm and cold spells. This trend is particularly strong in the Arctic, where the warming occurs at roughly twice the rate of that at middle latitudes in the northern hemisphere (AMAP 2011 Walsh et al. The occurrence of temperature extremes is generally favored by anticyclonic weather with advection direction indicating air mass flows from the interior of the North American continent as well as the south (warm extremes in summer) and north (cold extremes in winter).Īir temperatures have been observed to increase across the planet Earth since the late nineteenth century (IPCC 2013). This is reflected in changes in the frequency of occurrence and intensity of temperature extremes which are much more pronounced in the case of winter cold extremes (decreasing tendencies) than summer warm extremes (increasing tendencies). The results show that the general trend is towards the warmer temperatures, and the warming is greater in the winter than summer and for T MAX as opposed to T MIN. In order to characterize synoptic conditions during temperature extremes, the objective classification scheme of advection types considering jointly the direction of the air influx and type of pressure system is employed. Frequency of the occurrence and severity of warm and cold extremes as well as duration of WSs and CSs is analyzed.

WD (CD) is defined as a day with T MAX above the 95th ( T MIN below the 5th) percentile of a probability density function calculated from observations, and WS (CS) equals at least three consecutive WDs (CDs). The analysis is performed using daily temperature maxima ( T MAX) and minima ( T MIN) measured at 10 weather stations in Alaska as well as mean daily values of sea level pressure and wind direction at the 850 hPa isobaric level.

The aims of this study are to characterize the spatial and temporal variability of extremely warm days (WDs) and warm spells (WSs) in summer as well as extremely cold days (CDs) and cold spells (CSs) in winter in Alaska in the years 1951–2015 and to determine the role of atmospheric circulation in their occurrence.