The equatorial Pacific cold tongue bias in CESM1 and its influence on ENSO forecasts

The equatorial Pacific cold tongue in the forecasts is too strong and extends excessively westward due to a combination of the model's inherent climatological bias, initialization imbalance, and errors in initial ocean data.

The forecasts show a stronger cold tongue bias in the first year than that inherent to the model due to the imbalance between initial subsurface oceanic states and model dynamics. The cold tongue bias affects not only the pattern and amplitude but also the duration of ENSO in the forecasts by altering ocean-atmosphere feedbacks. The predicted sea surface temperature anomalies related to ENSO extend to the far western equatorial Pacific during boreal summer when the cold tongue bias is strong, and the predicted ENSO anomalies are too weak in the central-eastern equatorial Pacific.