Fig. 3: Evolution of North American SAT anomalies linked to extreme stratospheric wave events.
From: Extreme stratospheric wave activity as harbingers of cold events over North America
a, b Composites of North American SAT (NA SAT) anomalies for weak (a) and strong (b) stratospheric wave events in ERA5 reanalysis and CMIP6 models. ERA5 is depicted as black lines, the CMIP6 multi-model ensemble (MME) means in red, and individual models in light gray. c, d As in (a, b), but for the NAO index. e Evolution of the stratospheric wave index and NA SAT anomalies for winter 2020/2021. Solid parts of the lines for ERA5 and CMIP6 MME in (a–d) represent the composites significant at the 95% confidence level based on the Student’s t-test. Gray lines in (e) denote the onset date of the SSW on January 5, 2021, and the lowest NA SAT on February 13, 2021, for the winter. NA SAT anomalies are averaged over the land regions of 40∘–70∘N, 70∘–130∘W. The NAO index is defined as the SLP difference between 20∘–55∘N, 90∘W–60∘E and 55∘–90∘N, 90∘W–60∘E.
