J.-P. Boulanger C. Menkes M. Lengaigne Role of high- and low-frequency winds and wave reflection in the onset, growth and termination of the 1997–1998 El Nin ˜o Received: 12 February 2003 / Accepted: 4 November 2003 / Published online: 3 February 2004 Ó Springer-Verlag 2004 Abstract The present study aims at studying the role played by high-frequency wind variability, wave reflec- tion and easterly wind anomalies in the western Pacific in the onset, growth and termination phases of the 1997– 1998 El Nin˜o 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 evo- lution of the warm event and potentially influenced the equatorial Pacific conditions at least one year after the El Nin˜o 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 east- erly wind anomalies observed in the western Pacific from November 1997 to early 1998. These factors were suffi- cient to terminate the event and to switch temperature anomalies from warm to cold. To conclude, under- standing 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. 1 Introduction A few studies (Slingo and Delecluse 1999; Fedorov 2002) seem to suggest a connection between the occurrence and intensity of westerly wind events with the onset of El Nin˜o, connecting atmospheric convection activity over the Indo-Pacific warm-pool region to oceanic variability through oceanic Kelvin waves (e.g., Tang and Weisberg 1984; Giese and Harrison 1991; Kindle and Phoebus 1995; Kessler et al. 1995; Kessler and Kleeman, 2000; Pe´rigaud and Cassou 2000; Lengaigne et al. 2002) or through the eastward displacement of the Pacific warm- pool (Picaut et al. 1996; Boulanger et al. 2001; Lenga- igne et al. 2002). Indeed, during the 1997–1998 El Nin˜o, the largest on record (McPhaden 1999a), a series of westerly wind events (e.g., December 1996 and March 1997, Fig. 1a, d) has been suggested (McPhaden and Yu 1999; McPhaden 1999b; Boulanger and Menkes 1999; Pe´ rigaud and Cassou 2000; Wang and Weisberg 2000) to play a major role in its onset. However, while El Nin˜o was growing (from December 1996 to June 1997), reaching its mature stage (from July 1997 to February 1998) and then decaying (from March 1998 to May 1998; Fig. 1b), strong westerly and easterly high-frequency wind activ- ity in the central Pacific was observed (Fig. 1d) on top of the low-frequency wind variability (Fig. 1c). Even the termination of that El Nin˜o has been suggested as associated with a strong Madden-Julian Oscillation event (Takayabu et al. 1999). Such a strong high-fre- quency wind activity raises the general question of how the small-scale high-frequency wind variability interacts with the large-scale and low-frequency El Nin˜o charac- teristics. Understanding these interactions is probably a key question for ENSO predictability and its impacts. In fact, while El Nin˜o has been suggested to be as pre- dictable up to 12 months in advance (Latif et al. 1994; Chen et al. 1995), the amplitude and timing of the 1997– J.-P. Boulanger (&) C. Menkes M. Lengaigne Laboratoire d’Oce´anographie Dynamique et de Climatologie, UMR CNRS/IRD/UPMC, IPSL, Tour 45–55/Etage 4/Case100, 4 Place Jussieu, 75252 Paris cedex 05, France E-mail: jpb@lodyc.jussieu.fr Climate Dynamics (2004) 22: 267–280 DOI 10.1007/s00382-003-0383-8