Radenac, Marie-Hélène¹, Yves Dandonneau², Christophe Menkes² and Bruno Blanke³

¹LEGOS, CNES-CNRS-IRD-Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France, Tel: (33) 5 61 33 30 00, Fax: (33) 5 61 25 32 05, E-mail: marie-helene.radenac@cnes.fr, ²LODyC, CNRS-IRD-UPMC, Paris, France and ³LPO, CNRS-IFREMER-Université de Bretagne Occidentale, Brest, France

Nitrate distribution in the equatorial Pacific during the 1997 El Niño: biological processes vs. vertical and horizontal physical processes

Outputs of a physical/biological model show that the disruption of the east-west distribution of nutrients in the euphotic layer during the major 1997 El Niño results from a competition between biological processes and horizontal and vertical mechanisms. In the central Pacific, zonal advection of nitrate-poor waters from the western Pacific and reduced vertical supply drive the impoverishment during the growth phase. During summer, rich waters advected from the east and nitrate supply from the subsurface contribute to a brief replenishment. The nitrate uptake is the strongest in the upper layer of the euphotic zone. Then, nitrate vertical supply cannot overcome horizontal losses. In December 1997, conditions in the central Pacific are those usually found in the warm pool. The nitrate uptake at the bottom of the euphotic layer controls the modeled new production which has dropped by 40% compared to the mean 1993-1996 values. On the opposite, new production increases in the western Pacific because of strong nitrate vertical supply. Lagrangian experiments illustrate the origins of equatorial surface waters during the mature phase.