Fig. 4: Risk ratio of extreme cold days and probability density function (PDF) of NA SAT anomalies following strong stratospheric wave events.
From: Extreme stratospheric wave activity as harbingers of cold events over North America
a, b The spatial pattern of the risk ratio of extreme cold days (a) and the PDF of NA SAT anomalies (b) in ERA5 during days 5–25 after the onset of strong stratospheric wave events, compared with the statistics of all winters. The risk ratio in (a) is defined as the probability of cold days (i.e., SAT is at least 1.5 SD below its climatology) in days 5–25 divided by the probability of cold days in any random 21-day period in winter. Stippling indicates where the risk ratio is significant at the 95% confidence interval based on a Student’s t-test. c, d As in (a, b), but for CMIP6 models. The red boxes in (a) and (c) indicate the region where the NA SAT anomalies are calculated. The vertical gray lines in (b) and (d) denote −1, −1.5, and −2 SD of NA SAT anomalies in all the winter days, and the values in red depict the risk ratios of the exceedance frequency due to strong wave events.
