Supplementary material to "Cold spells induced by slow and amplified atmospheric waves"

Cold spell location selectionTo select cold spell regions, we first identified historical extreme cold days on the hemispheric scale.To do so, a non-standardized version of the midlatitude extreme (MEX; Riboldi et al., 2020) index is applied for land in four latitude bands (35-45N, 45-55N, 55-65N, and entire mid-latitudes: 35-65N).Dividing the mid-latitudes into three sub-bands enables an identification of extreme cold days in the lower, middle, and upper sections of the mid-latitudes.The MEX provides an overall measure of mid-latitude temperature daily variance by averaging over each grid point in the mid-