Displacements and transformations of nitrate-rich and nitrate-poor water masses in the tropical Pacific during the 1997 El Nino

Type Article
Date 2005-04
Language English
Author(s) Radenac Marie-Helene1, Dandonneau Yves2, Blanke Bruno3, 4, 5
Affiliation(s) 1 : CNES, Lab Etudes Geophys & Oceanogr Spatiale, IRD,UPS, F-31401 Toulouse, France.
2 : UPMC, IRD, CNRS, Lab Oceanog Dynam & Climatol,IPSL, F-75252 Paris, France.
3 : UBO, IFREMER, CNRS, Lab Phys Oceans,UFR Sci & Tech, F-29285 Brest, France.
Source Ocean Dynamics (1616-7341) (Springer), 2005-04 , Vol. 55 , N. 1 , P. 34-46
DOI 10.1007/s10236-005-0111-5
WOS© Times Cited 9
Keyword(s) Lagrangian analysis, El Nino, Tropical Pacific, Nitrate uptake
Abstract A Lagrangian analysis was applied to the outputs of a coupled physical-biogeochemical model to describe the redistribution of nitrate-rich and nitrate-poor surface water masses in the tropical Pacific throughout the major 1997 El Nino. The same tool was used to analyze the causes of nitrate changes along trajectories and to investigate the consequences of the slow nitrate uptake in the high nutrient low chlorophyll (HNLC) region during the growth phase of the event. Three patterns were identified during the drift of water masses. The first mechanism is well known along the equator: oligotrophic waters from the western Pacific are advected eastward and retain their oligotrophic properties along their drift. The second concerns the persistent upwelling in the eastern basin. Water parcels have complex trajectories within this retention zone and remain mesotrophic. This study draws attention to the third process which is very specific to the HNLC region and to the El Nino period. During the 1997 El Nino, horizontal and vertical inputs of nitrate decreased so dramatically that nitrate uptake by phytoplankton became the only mechanism driving nitrate changes along pathways. The study shows that because of the slow nitrate uptake characteristic of the tropical Pacific HNLC system, nitrate in the pre-El Nino photic layer can support biological production for a period of several months. As a consequence, the slow nitrate uptake delays the gradual onset of oligotrophic conditions over nearly all the area usually occupied by upwelled waters. Owing to this process, mesotrophic conditions persist in the tropical Pacific during El Nino events.
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