全球变化知识资源中心(globalchange): Arctic amplification decreases temperature variance in northern mid- to high-latitudes

Changes in climate variability are arguably more important for society and ecosystems than changes in mean climate, especially if they translate into altered extremes^{1, 2, 3}. There is a common perception and growing concern that human-induced climate change will lead to more volatile and extreme weather⁴. Certain types of extreme weather have increased in frequency and/or severity^{5, 6, 7}, in part because of a shift in mean climate but also because of changing variability^{1, 2, 3, 8, 9, 10}. In spite of mean climate warming, an ostensibly large number of high-impact cold extremes have occurred in the Northern Hemisphere mid-latitudes over the past decade¹¹. One explanation is that Arctic amplification—the greater warming of the Arctic compared with lower latitudes¹² associated with diminishing sea ice and snow cover—is altering the polar jet stream and increasing temperature variability^{13, 14, 15, 16}. This study shows, however, that subseasonal cold-season temperature variability has significantly decreased over the mid- to high-latitude Northern Hemisphere in recent decades. This is partly because northerly winds and associated cold days are warming more rapidly than southerly winds and warm days, and so Arctic amplification acts to reduce subseasonal temperature variance. Previous hypotheses linking Arctic amplification to increased weather extremes invoke dynamical changes in atmospheric circulation^{11, 13, 14, 15, 16}, which are hard to detect in present observations^{17, 18} and highly uncertain in the future^{19, 20}. In contrast, decreases in subseasonal cold-season temperature variability, in accordance with the mechanism proposed here, are detectable in the observational record and are highly robust in twenty-first-century climate model simulations.

Arctic amplification is clearly identified in autumn zonal-mean land near-surface temperature anomalies since the year 1979 in a contemporary reanalysis (Fig. 1a) and gridded station observations (Supplementary Fig. 1). In the last decade, positive zonal-mean temperature anomalies are particularly evident across the entire mid- to high-latitude Northern Hemisphere, but notably becoming larger in magnitude with increasing latitude. The linear trend for the period 1979–2013 is 0.86 °C per decade at latitudes 70°–80° N compared with only 0.30 °C per decade at 30°–40° N (Fig. 1e; green line). Arctic amplification is observed in all seasons except summer¹², but because it is largest in autumn, the focus of the main material is on this season with results from the other seasons provided in the Supplementary Information. Coincident with Arctic amplification, the zonal-mean variance of autumn daily temperature anomalies has decreased in both the reanalysis (Fig. 1b) and observations (Supplementary Fig. 1). Here and in what follows, the variance is calculated at each grid point before area averaging (Methods). Negative variance anomalies emerge in the last decade for latitudes 40°–80° N. The negative linear trend in zonal-mean autumn variance is statistically significant for latitudes 60°–80° N (Fig. 1e; black line). Decreases in grid-point variance are observed over large parts of the extratropical Northern Hemisphere, with the largest declines found over Canada and northern Siberia (Supplementary Figs 2 and 3). Zonal-mean temperature anomalies for the 5% coldest (that is, most negative daily anomalies) and 5% warmest (that is, most positive daily anomalies) days per autumn, reveal asymmetric warming tendencies. Cold autumn days have warmed substantially with the largest changes in high latitudes (Fig. 1c and Supplementary Fig. 1). Warm autumn days have also warmed (Fig. 1d and Supplementary Fig. 1), but at a slower rate, especially at higher latitudes (Fig. 1e; blue and red lines). The geographical regions with decreased variance well match those where cold autumn days have warmed faster than warm autumn days (Supplementary Figs 2 and 3).