Changes in North American Atmospheric Circulation and Extreme Weather: Influence of Arctic Amplification and Northern Hemisphere Snow Cover

Abstract

This study tests the hypothesis that Arctic amplification (AA) of global warming remotely affects midlatitudes by promoting a weaker, wavier atmospheric circulation conducive to extreme weather. The investigation is based on the late twenty-first century over greater North America (20°–90°N, 50°–160°W) using 40 simulations from the Community Earth System Model Large Ensemble, spanning 1920–2100. AA is found to promote regionally varying ridging aloft (500 hPa) with strong seasonal differences reflecting the location of the strongest surface thermal forcing. During winter, maximum increases in future geopotential heights are centered over the Arctic Ocean, in conjunction with sea ice loss, but minimum height increases (troughing) occur to the south, over the continental United States. During summer the location of maximum height inflation shifts equatorward, forming an annular band across mid-to-high latitudes of the entire Northern Hemisphere. This band spans the continents, whose enhanced surface heating is aided by antecedent snow-cover loss and reduced terrestrial heat capacity. Through the thermal wind relationship, midtropospheric winds weaken on the equatorward flank of both seasonal ridging anomalies—mainly over Canada during winter and even more over the continental United States during summer—but strengthen elsewhere to form a dipole anomaly pattern in each season. Changes in circulation waviness, expressed as sinuosity, are inversely correlated with changes in zonal wind speed at nearly all latitudes, both in the projections and as observed during recent decades. Over the central United States during summer, the weaker and wavier flow promotes drying and enhanced heating, thus favoring more intense summer weather.

Late Twenty-First-Century Changes in the Midlatitude Atmospheric Circulation in the CESM Large Ensemble

“…However, at 508N this does not result in changes in the midlatitude atmospheric dynamics as suggested by the FV12 mechanism. In winter, only the North American sector exhibits a slowdown of the zonal flow and an increase in sinuosity/blocking, as described in Vavrus et al (2017). Other sectors either exhibit neutral or opposite anomalies, in particular the North Atlantic and North Pacific sectors.…”

“…3c), as expected from the Arctic amplification signal largely forced by sea ice loss. The AA signal is less pronounced in spring and summer, when instead a large warming of the entire NH continents occurs that is promoted by decreased snow cover extent and reduced terrestrial heat capacity (Vavrus et al 2017).…”

“…Based on these results, changes in the midlatitude flow as expected from the FV12 mechanism (ZON2/SIN1/BLO1) only occur over North America in CESM-LENS. This finding is detailed in Vavrus et al (2017), who examine changes in zonal wind and sinuosity over the North American sector. In the other sectors, the response is opposite, with either a reinforced or unchanged westerly flow associated with decreased or unchanged waviness/blockings.…”

“…Recall that our calculation of SIN emphasizes changes centered over the 508N latitude, which can mask latitudinal asymmetry. By calculating SIN at each latitude band over North America, Vavrus et al (2017) report large latitudinal variation in the changes in summertime SIN, which projects onto the dipole in zonal wind anomalies over this region (increased sinuosity with weaker westerlies, and conversely).…”

Resolving Future Arctic/Midlatitude Weather Connections

“…Although global warming should overtake current climate conditions in increasing seasonal temperatures during the future (Wallace et al, 2014), there is interest in the paradox that Arctic warming and other changes might impact cold events on the eastern side of both North American and Asian hemispheres, through reinforcing large north/south meanders in the jet stream (Chen et al, 2016;Cohen, Pfeiffer, & Francis, 2018;Sung et al, 2016;Vavrus et al, 2017). The historical distribution of cold months for eastern US Decembers is shown in Figure 1; the previous decade has A way forward is to assess what atmospheric wind patterns are associated with causing these east coast cold events.…”