Evaluating ENSO's Spatiotemporal Complexity in the CWB CFS 1-Tiered Model Hindcasts

Using hindcasts produced by a coupled climate model, this study evaluates whether the model can forecast the observed spatiotemporal complexity in the El Nino-Southern Oscillation (ENSO) during the period 1982-2011: the eastern Pacific (EP), central Pacific-I (CP-I) and -II (CP-II) types of El Nino, and the multi-year evolution events of El Nino occurred in 1986-1988 (i.e., 1986/87/88 El Nino) and La Nina occurred in 1998-2000 (i.e., 1998/99/00 La Nina). With regard to the spatial complexity, it is found that the CP-I type of El Nino is the easiest to hindcast, the CP-II is second, and the EP is most difficult to hindcast as its amplitude is significantly underestimated in the model used here. The model deficiency in hindcasting the EP El Nino is related to a warm bias in climatological sea surface temperatures (SSTs) in the tropical eastern Pacific. This warm bias is related to model biases in the strengths of the Pacific Walker circulation and South Pacific high, both of which are notably weaker than observed. As for the temporal complexity, the model successfully hindcasts the multi-year evolution of the 1998/99/00 La Nina but fails to accurately hindcast the 1986/87/88 El Nino. This contrasting model performance in hindcasting multi-year events is found to be related to a cold bias in climatological SSTs in the tropical central Pacific. This cold bias result enables the model La Nina, but not El Nino, to activate intrabasin tropical-subtropical interactions associated with the Pacific Meridional Mode that produce the multi-year evolution pattern.

Automated summary

Using hindcasts produced by a coupled climate model, this study evaluates the model's ability to forecast the observed spatiotemporal complexity in ENSO. It is found that the model underestimates the amplitude of the eastern Pacific type of El Nino and fails to accurately hindcast the multi-year evolution of the 1986/87/88 El Nino. This is related to model biases in climatological SSTs in the tropical eastern and central Pacific.