Amplified mid-latitude planetary waves favour particular regional weather extremes

There has been an ostensibly large number of extreme weather events in the Northern Hemisphere mid-latitudes during the past decade(1). An open question that is critically important for scientists and policy makers is whether any such increase in weather extremes is natural or anthropogenic in origin(2-13). One mechanism proposed to explain the increased frequency of extreme weather events is the amplification of mid-latitude atmospheric planetary waves(14-17). Disproportionately large warming in the northern polar regions compared with mid-latitudes-and associated weakening of the north-south temperature gradient-may favour larger amplitude planetary waves(14-17), although observational evidence for this remains inconclusive(18-21). A better understanding of the role of planetary waves in causing mid-latitude weather extremes is essential for assessing the potential environmental and socioeconomic impacts of future planetary wave changes. Here we show that months of extreme weather over mid-latitudes are commonly accompanied by significantly amplified quasi-stationary mid-tropospheric planetary waves. Conversely, months of near-average weather over mid-latitudes are often accompanied by significantly attenuated waves. Depending on geographical region, certain types of extreme weather (for example, hot, cold, wet, dry) are more strongly related to wave amplitude changes than others. The findings suggest that amplification of quasi-stationary waves preferentially increases the probabilities of heat waves in western North America and central Asia, cold outbreaks in eastern North America, droughts in central North America, Europe and central Asia, and wet spells in western Asia.