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The Western Boundary of the Equatorial Pacific Upwelling: Some Consequences of Climatic Variability on Hydrological and Planktonic Properties

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Abstract

The longitude of the western limit of the equatorial Pacific upwelling is a key parameter for studies of carbon budget and pelagic fisheries variability. Although it is well defined at the surface on the equator by a salinity front and a sharp variation of the partial pressure of CO2, data from two equatorial cruises make it clear that this hydrological limit does not necessarily coincide with the boundary of the nitrate and chlorophyll enriched area. In January-February 1991 during a non-El Niño period, when trade winds and the South Equatorial current (SEC) were favorable to upwelling, the two limits were at the same longitude. Conversely, in September-October 1994 during El Niño conditions, when the equatorial upwelling had stopped, the nitrate and chlorophyll enriched zone was found a few degrees of longitude east of the hydrological boundary (5.5° at the surface and 2.5° for the 50 m upper layer), whereas no such offset was observed for zooplankton biomass. A simple model, based on the HNLC (High Nutrient - Low Chlorophyll) ecosystem functioning, was initialized with nitrate uptake measurements and estimates of upwelling break duration. The model results support the hypothesis that zonal separation of the limits arises from biological processes (i.e. nitrate uptake and phytoplankton grazing) achieved during that upwelling break.

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Authors and Affiliations

  1. Station Marine d'Endoume, Chemin de la Batterie-des-Lions, IRD, Centre d'Océanologie de Marseille, 13007, Marseille, France

    Martine Rodier

  2. IRD, LEGOS/GRGS, 18 Av. Edouard Belin, 31401, Toulouse Cedex, France

    Gérard Eldin

  3. Station Marine d'Endoume, Chemin de la Batterie-des-Lions, IRD, Centre d'Océanologie de Marseille, 13007, Marseille, France

    Robert Le Borgne

Authors
  1. Martine Rodier
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  2. Gérard Eldin
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  3. Robert Le Borgne
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Rodier, M., Eldin, G. & Le Borgne, R. The Western Boundary of the Equatorial Pacific Upwelling: Some Consequences of Climatic Variability on Hydrological and Planktonic Properties. Journal of Oceanography 56, 463–471 (2000). https://doi.org/10.1023/A:1011136608053

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