Rapid development of systematic ENSO-related seasonal forecast errors

El Niño–Southern Oscillation (ENSO) is the leading source of global seasonal climate variability, with teleconnections to many regions of the world, so its prediction is crucial to overall extratropical seasonal forecast skill. In this study, we evaluate systematic ENSO-related seasonal forecast errors within several different operational forecast models, based on multi-decade seasonal hindcast datasets. We find that the predictions have a systematic westward SST anomaly bias, whereby the eastern-central tropical Pacific SST anomalies associated with ENSO events extend or are shifted too far to the west for anomalies of either sign. Associated with this SST forecast error is a westward shift of ENSO rainfall anomalies, which in turn affects extratropical seasonal forecast skill through errors in wave propagation from the tropical Pacific. The ENSO-related forecast errors, which are also typical of long free-running climate model simulations, are apparent almost immediately; in fact, they develop so rapidly that they are primarily a function of the seasonal cycle, rather than lead time. That is, the pattern and even amplitude of ENSO-related tropical anomaly errors for a given month are very similar over a range of forecast lead times. Predicted ENSO events also tend to decay too slowly compared to observations, resulting in large systematic forecast errors in the eastern tropical Pacific in late winter/early spring, which are also well-developed at short forecast lead times. The effect of these errors on extratropical seasonal forecast skill will be discussed, as well as possible causes of the ENSO-related biases as seen in the development of the daily forecast error over the first few forecast months.