Assessment of the Mechanisms in the Hindcasts of Strong El Niño by the NCEP CFSv2 Model and Comparison to the 2015-2016 Forecasts

Abstract:

Strong El Niño events, like the ones observed in 1982-83 and 1997-98, have been proposed to be dynamically different from other El Niño, involving nonlinear feedbacks associated with deep convection in the eastern Pacific. However, climate models have strong biases in this region, so it is critical to assess whether the key processes are correctly represented in prediction systems. In this study, the mechanisms leading to the prediction of a strong El Niño are analyzed in the hindcasts (1982-2010) from the NCEP CFSv2 model and are compared to the 2015-16 forecasts. We focus on the differences in the evolution of the members that produce the largest and smallest warming in the ensemble runs in both the hindcasts and forecasts, particularly on the differences in the drivers or precursors that lead to the ensemble spread, like the zonal wind stress predictor in boreal fall that has been proposed to be determinant for strong El Niño (Takahashi & Dewitte, 2015). Preliminary results indicate that the ocean heat content plays a larger role for the development of El Niño in CFSv2 than in nature, such that the hindcast of the 2009-2010 central Pacific event was similar to that of the strong El Niño of 1997-98, while the response to the zonal wind stress predictor is weaker than observed. This study provides some insights into the processes or mechanisms in the model that need to be better represented in order to improve the prediction of a strong El Niño event.