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Equatorial waves and warm pool displacements during the 1992-1998 El Niño Southern Oscillation events: Observation and modeling

Delcroix T., Dewitte B., duPenhoat Y., Masia F., Picaut J.

In the equatorial Pacific, zonal displacements of the eastern edge of the warm pool represent an intrinsic manifestation of El Nino Southern Oscillation (ENSO) events, with numerous dynamical and biogeochemical consequences. Following a previous work dedicated to the 1986-1989 Geosat period, we focus on the 1992-1998 zonal displacements of the warm pool using mainly TOPEX/Poseidon data. We also used a simple linear model forced by monthly ERS winds to help in the interpretation of the results. We found that the 1992-1998 zonal displacements of the warm pool resulted mainly from horizontal advection by zonal current anomalies, through a combination of interannual equatorial Kelvin and first meridional mode Rossby waves. The interannual equatorial Kelvin waves were essentially wind forced in the western and central equatorial Pacific, with some minor contribution from reflected Rossby waves on the western Pacific boundary. In particular, westerly wind anomalies and the resulting downwelling Kelvin waves (entailing eastward surface current anomalies and thermocline deepening) contributed strongly to the onset of the 1993, 1994-1995 and 1997-1998 Fl Nino events. In contrast, easterly wind anomalies and the resulting upwelling Kelvin waves (with westward surface current anomalies and thermocline shoaling) played a role in stopping the 1993 El Nino and in shifting the 1994-1995. and 1997-1998 El Nino into La Nina events. Consistently with the 1987-1988 El Nino-La Nina scenario, two main downwelling Rossby wave packets, originating from eastern boundary reflections and wind forcing, crossed the entire basin in 1993 and 1994-1995. These waves favored the decay of the corresponding El Nino events, in the sense that their associated current anomalies contributed to shifting the displacements of the eastern edge of the warm pool from eastward to westward. Unlike what happened for the termination of the 1993 and 1994-1995 El Nino events, downwelling Rossby wave packets, mostly reflected from impinging Kelvin waves, did not propagate all the way to the western Pacific during the 1997-1998 Fl Nino. They stopped propagating in the central basin where they met unfavorable eastward migrating westerly wind anomalies which generated upwelling Rossby waves. Hence reflected downwelling and wind-forced upwelling Rossby waves opposed each other for shifting the eastern edge of the warm pool. The rapid demise of the 1997-1998 Fl Nino and its shift into La Nina in mid-1998 are interpreted as resulting mainly from the effect of upwelling Kelvin waves forced by easterly wind anomalies occurring in the west from the end of 1997. The associated thermocline shoaling was further enhanced by the wind-forced upwelling Rossby waves in the central basin in mid-1998, strongly influencing the fast sea surface temperature (SST) cooling at times when the thermocline was very close to the surface at the end of the mature phase of the 1997-1998 El Nino.

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