Role of high- and low-frequency winds and wave reflection in the onset, growth and termination of the 1997-1998 El Nino

The present study aims at studying the role played by high-frequency wind variability, wave reflection and easterly wind anomalies in the western Pacific in the onset, growth and termination phases of the 1997-1998 El Nino using the Trident intermediate coupled model and observations. While the anomalous strength of the trade winds in 1996 favored the initiation of a warm event in 1997 (via western Pacific boundary Rossby wave reflection), the actual timing of the onset and the amplitude of the event resulted from the large March 1997 wind event. Once initiated, high-frequency westerly winds strongly contributed to the rapid growth of the warm event and to the displacement of the eastern edge of the warm-pool. Moreover, both easterly and westerly high-frequency wind variability in 1997-1998 contributed to the amplitude of the event, set the evolution of the warm event and potentially influenced the equatorial Pacific conditions at least one year after the El Nino event. In addition, eastern boundary reflection also significantly contributed to the amplitude and duration of the warm event, whereas its termination was a combination of various factors: reflection of upwelling Rossby waves at the western boundary and large easterly wind anomalies observed in the western Pacific from November 1997 to early 1998. These factors were sufficient to terminate the event and to switch temperature anomalies from warm to cold. To conclude, understanding the coupling between the high- and low-frequency wind variability, i.e., studying ENSO as a multi-scale phenomenon, will certainly lead to a better comprehension of the diversity of its behavior and potentially to an improvement of its predictability.