Fig. 1: Spatial distribution of Euro-Atlantic winter blocking frequency in the reanalysis and Energy Exascale Earth System Model (E3SM) experiments.

a Wintertime (December–February) blocking frequency climatology in the MERRA-2 (shading) and the control (CTL) simulation (contour lines, with intervals of 2 starting at 2). The unit is percentage of blocked days in the season (i.e., with 2% corresponding approximately to two blocked days per winter). b Difference between the cloud-locking (CLOCK) experiment and CTL (shading) and the climatology in the CTL run (contour lines). c Similar to (b) but for the difference between the longwave cloud radiative effect–off (LWOFF) experiment and CTL (shading). Dots in (b) and (c) indicate differences exceeding the 95% confidence interval based on the two-tailed Student’s t-test. d Changes in blocked days averaged over the Euro-Atlantic sector (43^∘–65^∘N, 40^∘W–20^∘E; orange box, and West/Central Europe (45^∘–60^∘N, 10^∘W–15^∘E; red box in (c)) in half-violin-style box-whisker plot. The vertical widths of boxes represent the interquartile range, whiskers extend from 5% to 95%, and horizontal lines and x symbols indicate median and mean values, respectively. The dots represent individual data points. The p-values from the t-test of CLOCK (LWOFF) versus CTL differences are shown in each panel.