Fig. 4: Mean state changes facilitate northward-broadened anomalies conducive to multi-year La Niña.

a, Multi-model mean changes of grid-point SST (colouring), SLP (contour; positive in solid lines and negative in dashed lines, with an interval of 5 Pa °C⁻¹) and 10-m wind (vectors) between 1900–1999 and 2000–2099, all scaled by the increase in global mean SST in each of the selected models. b, Inter-model correlation between changes (2000–2099 minus 1900–1999) in grid-point mean SST with changes in the strength of extratropical response to tropical forcing, both scaled by the increase in global mean SST of each model. Stippling indicates statistical significance above the 90% level based on a two-tailed Student’s t-test. c, Inter-model relationship between mean SST warming (°C) and WESp change (W s m⁻³, averaged from February to July) in the subtropical northeastern Pacific (155° W–115° W, 15° N–35° N; blue box in a). A linear fit (solid black line) is shown, together with correlation coefficient r and corresponding P value. Note that, even after excluding a seeming outlier model (NorESM2-MM), the inter-model correlation is still significant (r = 0.67, P < 0.01). d, Relationship between normalized NPMM-like SST anomalies in MAM(1) and normalized Niño3 SST anomalies (5° S–5° N, 150° W–90° W) in D(0)JF(1) for 1900–1999 (blue dots) and 2000–2099 (orange dots). The NPMM-like anomalies (normalized SST anomalies in 15° N–25° N, 150° W–120° W; green box in a) are binned in 0.1 s.d. intervals of Niño3 SST anomalies to obtain median values (circles) in each bin first. Only positive Niño3 SST anomalies are considered here. Also shown are respective slopes with 95% confidence intervals for the two periods. Warming maximum in the subtropical northeastern Pacific enhances the local thermodynamic response to convective anomalies from El Niño, generating anomalous easterlies at further northern latitudes than in 1900–1999, and warming maximum in the eastern equatorial Pacific enhances the convective response, both contributing to the increased frequency of multi-year La Niña.