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There has been an ostensibly large number of extreme weather events in the Northern Hemisphere mid-latitudes during the past decade. 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^,,,,,,,,,,,. One mechanism proposed to explain the increased frequency of extreme weather events is the amplification of mid-latitude atmospheric planetary waves^,,,. 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^,,,, although observational evidence for this remains inconclusive^,,,. A better understanding of the role of planetary waves in causing mid-latitude weather extremes is essential for assessing the potential environmental and socio-economic impacts of …