Fig. 2: Summary statistics from the Clouds On/Off Klimate Intercomparison Experiment, phase 2 (COOKIE-2).

a Differences in blocking frequency between the Atmospheric Model Intercomparison Project (AMIP) and AMIP longwave cloud radiative effect–off (AMIP–LWOFF) simulations from the multi-model mean (color shading). Contour lines show the ensemble mean of AMIP simulations, drawn with intervals of 2 starting at 2. Black dots mark regions where at least 75% of models agree on the sign of change. b Domain-averaged blocking frequency over the Euro-Atlantic sector (43^∘–65^∘N, 40^∘W–20^∘E; orange box in (a)) for each model. Lines connect paired AMIP and AMIP–LWOFF runs, with filled (open) circles indicating statistically significant (not significant) differences at the 95% level. For comparison, the Energy Exascale Earth System Model (E3SM) control (CTL) and longwave cloud radiative effect–off (LWOFF) simulations are also shown, following a similar protocol but with prescribed sea surface temperatures (SSTs) in year-2000s conditions. The vertical widths of boxes represent the interquartile range, whiskers extend from 5% to 95%, and horizontal lines and * symbols indicate median and mean values, respectively. c As in (b) but averaged over West/Central Europe (45^∘–60^∘N, 10^∘W–15^∘E; red box in (a)). Colors denote models, with filled (raster) boxes indicating AMIP (AMIP–LWOFF).