Transition of El Niño to La Niña can be driven by regional perturbations a year ahead

Abstract Interannual forecasts provide skilful predictions of El Niño-Southern oscillation (ENSO) up to a year in advance, however our understanding of what drives the ensemble skill and diversity of outcomes across members is limited. Using a fully coupled ocean–atmosphere ensemble forecasting system, we investigate the causality of regional perturbations on the evolution of ENSO at interannual timescales. Using forecasts initialised on 1 November 2009, transplanting more realistic cooler conditions in the South Pacific across ensemble members on 1 January 2010 significantly cools the resulting 2010/2011 winter ENSO one year later. The imposed perturbations migrate equatorward via wind–evaporation–sea surface temperature feedback and significantly alter tropical zonal gradients during late spring and summer. This drives the ensemble towards La Niña conditions, in line with observations. Repeating the experiment with warmer South Pacific conditions, results in the reverse signal and warms ENSO one year later. Across the experiments we find an almost four-fold increase in probability of La Niña and a three-fold decrease in probability of El Niño, demonstrating that long lead regional perturbations can systematically tip the climate system between ENSO states. Predicted surface conditions are significantly impacted across many parts of the world and the forecast global annual mean surface temperature for 2010 is significantly cooled, resulting in better agreement with observations. Our results demonstrate sensitivity of ENSO evolution and the global climate system to specific regional perturbations and provide new insights for interannual climate prediction.