The atmosphere-ocean system in the equatorial Pacific transited to El Niño conditions from the spring to summer in 1997. The ADEOS comprehensively captured the onset of 1997-1998 El Niño through the combined observations of surface wind stress vector (WS), sea surface temperature (SST), and concentration of the photosynthetic pigment chlorophyll a (CHL). Responses of SST and CHL to WS in the El Niño onset showed regional characteristics in the four areas of the equatorial Pacific. In the western equatorial Pacific, CHL increased and SST decreased intermittently because of vertical mixing and coastal upwelling around the islands caused by local WS bursts. In the central parts, CHL decreased by 40% and SST increased by 2°C corresponding to the zonal movement of warm pool water, which seems to be controlled by the WS field during the El Niño onset. In the eastern parts, WS decreased by about half and SST increased by 4°C; however, CHL decreased only a little because of regulated processes of nutrients. In the eastern coastal area, CHL decreased and SST increased according to deepening of the thermocline. The thermocline depth seemed to be modulated by a dynamic balance with the basin-scale WS field and arrivals of Kelvin waves, which were caused by westerly WS bursts in the western equatorial Pacific and traveled in the thermocline to the eastern coastal area in the El Niño onset.
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