polar-vortex-final-warming-20240404.png

In years with sudden stratospheric warmings (red dots), the final warming of the polar vortex tends to occur later than the historical median date of April 12 (solid black line). Final stratospheric warming dates are calculated as the first date that the daily-mean west-to-east winds at 60N and 10-hPa altitude fall below 0 m/s (i.e., become east-to-west) and do not return to a west-to-east state for more than 10 consecutive days. Red dots indicate years in which a sudden warming occurred during the preceding Jan-Mar. Dates are calculated for JRA55 from 1958-2019 and for MERRA2 reanalysis for 2020-2024. NOAA Climate.gov image, adapted from original by Amy Butler.