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Warm Arctic Episodes Linked with Increasing Frequency of Extreme Winter Weather in the US
A large scale seesaw pattern of temperature characterised by an unusually warm Arctic and cold continents has been exhibited by recent boreal winters. An active area of research has been whether there has been any physical link between variability in the Arctic and Northern Hemisphere (NH) extreme weather. In this paper Cohen et al. show, by the use a recent index of severe weather, that the occurrence of severe winter weather in the US is related significantly to anomalies in pan-Arctic geopotential heights and temperatures. The frequency of severe winter weather in mid-latitudes increases through the transition as the Arctic transitions from a relatively cold state to a warmer one. In the eastern US this relationship is, however, strongest and mixed to even opposite along the west coast. The study also shows that when the warming trend in the Arctic is at its greatest during mid-winter to late winter of recent decades, the warming trend of the Arctic is greatest and extends into the upper troposphere and lower stratosphere, severe winter weather, which includes both cold spells and heavy snows, becomes more frequent in the eastern US.
A combination of forced and natural variability is responsible for variability in the day-to-day weather. Forced variability is a result of boundary conditions, such as sea surface temperatures (SST), and variability, that is natural or internal, is a result of the chaotic nature of dynamical systems (Madden, 1996; Shukla & Guzzler, 1983). It is argued by recent studies that the arctic is playing an increasingly important role as a boundary-forcing agent as a result of its accelerated warming relative to other regions of the globe (Vihma, 2014; Cohen et al., 2014), though the tropics are usually the main driver of variability that is boundary-forced (Phelps, Kumar & O’Brien, 2004; Cohen, 2016).
Globally, increasing greenhouse gases in the atmosphere are contributing to a general warming of the atmosphere and oceans (IPCC. In Climate Change, 2013). Warming has dominated global temperature trends during 3 of the seasons (Cohen et al., 2012), over recent decades. Cooling trends have been observed across Eurasia and the eastern US (Cohen et al., 2012; Nakamura et al., 2012; Chen & Lou, 2017) in winter, however, along with rapid warming in the Arctic (Vihma, 2014; Cohen et al., 2014; Overland et al., 2015). This seesaw winter temperature pattern is known as the “warm-Arctic/cold-continents pattern” (Overland, Wood & Wang, 2011). There has been a vigorous debate in the climate community regarding whether (Francie, Vavrus & Cohen, 2017; Kintisch, 2014) and/or how much the mid-latitude weather can be influenced by the Arctic and, in particular, whether the likelihood of severe cold spells in the continents (Cohen et al., 2013) in the mid-latitudes is increased by a warmer Arctic increases.
It is widely expected that anthropogenic global warming will influence certain types of weather extremes, including heat waves and droughts that are more intense and frequent as well as heavy precipitation events (Coumou & Rahmstorf, 2012; Diffenbaugh et al., 2017; Lou et al., 2017). Surprisingly over the past 2 or 3 decades, however, the increases in extreme weather has included more, not fewer, outbreaks of severe cold air and heavy snowfalls that have been observed in North America and Eurasia (Cohen et al., 2014; Cohen et al., 2012; Overland et al., 2015; Cohen et al., 2013; Mori et al., 2014; Messori, Caballero & Gaetani et al., 2016; Cohen, Barlow & Saito, 2009; Zhang et al., 2016).
It has been shown qualitatively by recent studies that geopotential heights across the Arctic that are anomalously high are linked with extreme weather events across the mid-latitudes in winter (Cohen et al., 2013; Wang et al., 2017) and even into spring (Cohen et al., 2017). Those studies were limited to, however, just a few months of a particular year. Cohen et al. present in this paper a more extensive, quantitative analysis of the link between Arctic variability and severe winter weather across the mid-latitudes. A robust relationship was found in this study between temperature in the Arctic and extreme winter weather in the US. Cold temperatures and heavy snowfall are more frequent when the Arctic is warm compared to when the Arctic is cold. Also, this study found that during the period of accelerated Arctic warming when the Arctic warming reaches into the upper troposphere and lower stratosphere during mid-winter to late-winter severe winter weather has been increasing.
Arctic variability and mid-latitude weather
The daily change in seasonal AWSSI (or the daily accumulation for that day) is composited for all standardised PCHs at 12 representative cities across the US by computing the mean change in the AWSSI that is associated with the daily PCH values at each isobaric level during winter (DJF) from 1950 to 2016. A strong relationship between a warmer Arctic and increased frequency of severe winter weather appears to be the case for all stations to the east of the Rockies, the strongest association in the eastern ⅓ of the US, where a statistically significant (p<0.01) and nearly linear relationship between height changes in the Arctic throughout the troposphere and AWSSI. Severe winter weather is not likely when the Arctic heights are at their lowest (PCH<~ -1). The likelihood of severe weather increases for larger values of PCH (PCH> +1), and correlations peaking when the PCH is greater than +1.5. Throughout the troposphere this relationship is fairly consistent over the full range of Arctic height anomalies. The correlation generally holds in the stratosphere as well. However the relationship is weak in the Rockies and along the west coast, and some stations even exhibit the opposite relationship, i.e., milder winter weather is favoured by an Arctic that is relatively warm. This result is consistent with the predominance of an anomalous western ridge during the recent period of pronounced Arctic warming.
Cohen et al. also investigated the separate association between the variability of PCH and extreme cold temperatures or heavy snowfall. A stronger and more extensive relationship with temperature than with snowfall is exhibited by the positive values of PCH. In the northeastern US the relationship between PCH and snowfall is the most robust, therefore it is likely that in this region snowfall is most sensitive to variability of the Arctic, with higher geopotential heights and temperatures that are relatively warmer favouring heavier snowfalls (Liu et al., 2012).
As discussed earlier, the PCH combines influences of thermodynamics and dynamics. The prior analysis was repeated substituting PCT for PCH, in order to determine whether there is a direct relationship between Arctic temperatures alone and weather at mid-latitudes, demonstrating that a significant fraction of the relationship between Arctic variability and severe winter weather across the mid-latitudes is related to variability in Arctic tropospheric temperature.
Cohen et al. analysed 2 metrics of Arctic variability in order to demonstrate that an Arctic that is relatively warm is associated with an increase in winter weather that is severe across the continents of the Northern Hemisphere, and in particular, the eastern US. In the troposphere, PCH+ and PCT+, as well as PCH+ in the lower stratosphere, are correlated with high values of the AWSSI, which is a severe weather index that includes cold spells and heavy snowfalls. Cohen et al. found, based on their analysis of that the in the lower stratosphere to mid-troposphere (70 to 500 hPa), that PCH+ of 2 standard deviations or greater is associated with a 2-fold to 4-fold increase in the likelihood of winter weather extremes. These extremes were typically on the order of 2 – 6 standard deviations based on the AWSSI. In the northeast and upper mid-western US this relationship is most apparent.
Their studies are consistent with earlier studies linking a warming Arctic with extreme weather in winter in mid-latitudes of the Northern Hemisphere, though they did not offer mechanistic relationships for these relationships. According to Cohen et al. most theories begin with melting sea ice (Chen & Luo, 2017; Kim et al., 2014; Honda, Inue & Yamane, 2009; Orsolini et al., 2012), as the lowest minimum extents of sea ice have been exhibited over the past 10 years since the beginning of satellite observations, and sea ice is a variable that is relatively easy to manipulate in models. The increase in autumn snow cover on high latitude continents is another possible contributor to extreme weather in winter, especially across Eurasia (Cohen et al., 2014). Less extensive Arctic sea ice cover, as well as more extensive fall snow cover, is related to a warmer Arctic and colder East Asia (Cohen et al., 2013).
Most of the mechanisms that have been proposed linking sea ice and/or increased Eurasian snow cover to extreme winter weather across mid-latitudes of the Northern Hemisphere continents involve a pathway through SPV (Cohen et al., 2014; Cohen, Barlow & Saito, 2009; Zhang et al., 2016; Kim et al., 2014; Kretschmer et al., 2016; Jaiser et al., 2012; Jaiser et al., 2016; Handorf et al., 2015). Boundary forcing that results from the loss of sea ice and a more extensive cover of snow can interact constructively with climatological large-scale waves to enhance the activity of the waves and increase the transfer of energy from the troposphere to the stratosphere, which can then trigger a SSW and weaken the SPV. Polar air masses then spill southwards into mid-latitudes, first in the stratosphere and also later into the troposphere (Cohen et al., 2014; Overland et al., 2015). Figures 2 and 6 in this paper are consistent with the stratigraphic pathway and weakened polar vortex as 1 possible dynamical pathway between variability of the Arctic and weather in mid-latitudes. Other mechanisms that have been proposed confine the influence of the Arctic on changes of large-scale circulation to the troposphere, in which a wavier flow is favoured by a warmer Arctic and atmospheric blocking that is more consistent, which often spawns extreme weather events (Francis & Vavrus, 2012; Yao, Luo, Dai & Simmonds, 2017).
How is the debate informed as to whether AA in general and sea ice loss in particular are contributing to winter weather that is more extreme? The focus of Cohen et al. on 2 Arctic-only indicators and an index of extreme winter weather offer new evidence and clues about the effects of a melting Arctic that is warming rapidly in the remainder of the world. Cohen et al. found that a warmer Arctic atmosphere contributes to dilated geopotential heights locally accompanied by lower heights across mid-latitudes and a jet stream that has been shifted towards the equator. Airmasses of the Arctic are allowed by this to expand further south while increasing the likelihood of heavy snowfalls. They found a distinction between early winter, when only the lower troposphere is affected by Arctic warming trends, and mid-winter to late-winter when PCH+ is evident throughout the troposphere and lower stratosphere. The probability of severe winter weather in mid-latitudes increases, when the entire Arctic atmospheric column is affected, as observed in the era of AA in late winter. The opposite effect is elicited by the opposite response. It is suggested by these findings that the continuation of rapid Arctic warming and melting contribute to episodes of severe winter weather that are more frequent across the continents of the Northern Hemisphere.
In terms of indirect linkages, uncertainties remain, though much has been learned in recent years concerning direct connections between climate change and weather patterns. It has been revealed by recent work that there is a variety of possible mechanisms, yet it has been concluded by some recent studies that a warming Arctic does not force robust cooling over mid-latitudes continents, and that internal variability can explain recent trends (McCusker, Fyfe & Simmond, 2016; Sun, Perlwitz & Hoerling, 2016; Sigmond & Fyfe, 2016; Collow, Wang & Kumar, 2017). Also, climate model simulations that simulate realistically AA indicate that cold extremes and heavy snowfall will decrease as the Arctic continues to warm (Screen, Deser & Sun, 2015; Schneider, Bischoff & Ayarzaguena & Screen, 2016). The discrepancies between observational and modelling studies, and also among modelling studies, are recognised (Cohen et al., 2014; Overland et al., 2016; Screen, Deser & Sun, 2015; Peings & Magnusdottir, 2014) though not well understood (Francis, Vavrus & Cohen, 2017; Francis, 2017). According to Cohen et al. research should continue rapidly to elucidate the sources of uncertainty in these linkages, because of the important and costly ramifications of changing weather patterns – particularly extreme weather – on society.
Cohen, J., et al. (2018). "Warm Arctic episodes linked with increased frequency of extreme winter weather in the United States." Nature Communications 9(1): 869.
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