Mechanisms of the 1997-1998 El Niño-La Niña, as inferred from space-based observations

The intensity of the 1997 El Niño and the 8°C sudden drop in sea surface temperature (SST) around 0°-130°W during the turn into La Niña in 1998 were a surprise to the scientific community. This succession of warm and cold events was observed from start to finish with a comprehensive set of remotely sensed and in situ observations. In this study we employ space-based observations to demonstrate, for the first time, their maturity in capturing the preconditioning, onset, evolution, and decay of the 1997 El Niño and its transition into the 1998 La Niña. An accumulation of warm water in the west and equatorial wave reflection on the western ocean boundary appeared favorable for the development of El Niño. However, the action of a series of westerly wind bursts from December 1996 to June 1997, notably in March 1997, was instrumental in setting up this huge El Niño. The westerly wind bursts excited equatorial downwelling Kelvin waves and advected the eastern edge of the warm pool eastward, which triggered a distinct warming over the central and eastern parts of the equatorial basin. Once these warmed regions joined, the coupling between the SST and surface winds was fully effective, and El Niño reached its mature phase. By that time much of the warm waters of the western equatorial Pacific was transferred toward the east by surface eastward currents. The demise of El Niño and its turn into La Niña in spring 1998 were due to the arrival in the east of various interrelated phenomena. Upwelling was brought from the west by favorable off-equatorial wind stress curl and equatorial Kelvin waves generated by easterly winds and wave reflection on the western ocean boundary. Additional upwelling was brought from the east by equatorial Rossby waves generated by westerly winds. These various upwelling signals were added to the general uplifting of the thermocline because of the slow discharge of the upper layer of the equatorial basin by diverging surface currents. A series of equatorial Kelvin and Rossby waves, characterized by upwelling and opposite surface currents, led to the breakup of the warm waters, the surfacing of the thermocline, and the drastic drop in SST in May 1998 around 0°-130°W. With the arrival of cold water in the east the easterly winds expanded from the west, and La Niña turned into a growing mode. This view of the 1997-1998 El Niño-La Niña, afforded from space, enables the testing of various El Niño theories.