ADEOS observations of chlorophyll a concentration, sea surface temperature, and wind stress change in the equatorial Pacific during the 1997 El Nino onset

The atmosphere-ocean system in the equatorial Pacific transited to El Nino conditions from the spring to summer in 1997. The ADEOS comprehensively cap tured the onset of 1997-1998 El Nino through the combined observations of s urface wind stress vector (WS), sea surface temperature (SST), and concentr ation of the photosynthetic pigment chlorophyll a (CHL). Responses of SST a nd CHL to WS in the El Nino onset showed regional characteristics in the fo ur areas of the equatorial Pacific. In the western equatorial Pacific, CHL increased and SST decreased intermittently because of vertical mixing and c oastal upwelling around the islands caused by local WS bursts. In the centr al parts, CHL decreased by 40% and SST increased by 2 degrees C correspondi ng to the zonal movement of warm pool water, which seems to be controlled b y the WS field during the El Nino onset. In the eastern parts, WS decreased by about half and SST increased by 4 degrees C; however, CHL decreased onl y a little because of regulated processes of nutrients. In the eastern coas tal area, CHL decreased and SST increased according to deepening of the the rmocline. The thermocline depth seemed to be modulated by a dynamic balance with the basin-scale WS field and arrivals of Kelvin waves, which were cau sed by westerly WS bursts in the western equatorial Pacific and traveled in the thermocline to the eastern coastal area in the El Nino onset.