Excessive Momentum and False Alarms in Late-Spring ENSO Forecasts

The unanticipated stalled El Nino-Southern Oscillation (ENSO) evolution of 2014 raises questions about the reliability of the coupled models that were used for forecast guidance. Here we have analyzed the skill and reliability of forecasts of the Nino 3.4 tendency (3-month change) in the North American multimodel ensemble (1982-2018). We found that forecasts initialized April-June (AMJ) have excessive momentum in the sense that the forecast Nino 3.4 tendency is more likely to be a continuation of the prior observed conditions than it should be. Models tend to predict warming when initialized after observed warming conditions and cooling when initialized after observed cooling conditions. Excessive momentum appears in AMJ forecast busts and false alarms including the 2014 one. In some models, excessive momentum appears to be related to model formulation rather than initialization. A concerning trend is that four of the nine years with AMJ forecast busts occurred in the last decade.