Southwestward extent of chlorophyll-enriched waters from the peruvian and equatorial upwellings between Tahiti and Pananama

The transition zone between waters deriving from the Peruvian and Equatorial upwellings, and oligotrophic waters of the central South Tropical Pacific, was examined by sea-surface chlorophyll sampling carried out by merchant ships on the Tahiti-Panama track. The results from 118 transects (December 1979 to September 1985) show that this transition is generally associated with convergences between the Westward South Equatorial current and eastward flows, as indicated by contemporaneous temperature sections obtained from expendable bathythermographs. The transition position varies between 5 and 17°S and variations occur mostly on time scales of a month or less. The Tuamotu atolls (15 to22°S, 135 to 150°W) were observed to be reached by waters deriving from the upwellings on 14 % of the transects; this might help to explain the anomalous abundance of life on these atolls. The 1982-1983 El Nino resulted in a fall in chlorophyll concentrations in the eastern tropical Pacific, but dit not cause a significant change of the position of the transition. (Résumé d'auteur