Tropical Pacific Trends in the ECMWF Seasonal System and Implications for Predictions of the 2020–22 Triple-Dip La Niña

Abstract This study evaluates trends in the tropical Pacific as represented by hindcasts for 1993–2019 of the coupled fifth generation seasonal forecasting system (SEAS5) by the European Centre for Medium-Range Weather Forecasts (ECMWF). Results show that SEAS5 hindcasts strongly underestimate the observed strengthening of equatorial easterly and cross-equatorial southerly winds. Associated with this, hindcasts exhibit spurious trends toward anomalously warm predicted sea surface temperatures (SSTs) in the equatorial and southeastern tropical Pacific, consistent with a tendency toward overprediction of probabilities for El Niño toward the end of the hindcast period. This tendency persisted during the 2020–22 triple-dip La Niña, with forecast errors reminiscent of the long-term trend errors. A simple trend-aware calibration of the forecasts can improve forecast performance in recent years. Uncoupled hindcasts with prescribed SSTs exhibit similar wind trend errors as SEAS5, which suggests an underestimation of the wind–evaporation–SST feedback by the atmospheric model. Spurious cloud and shortwave radiation trends contribute to SST trend errors as well. Results also indicate that a persistent positive precipitation bias in the western Pacific warm pool keeps the model in a regime where equatorial Pacific winds are too insensitive to changes in zonal gradients. In the coupled model, trend errors of the atmospheric model amplify through the Bjerknes feedback. The excessive warming of the equatorial ocean subsurface in the employed initial conditions also plays a role. Further reduction of model biases as well as temporally homogeneous and balanced initial conditions is thus needed to tap the source of predictive skill represented by climate trends.