New production in the equatorial Paci®c: a coupled dynamical±biogeochemical model A. STOENS,* C. MENKES, Y. DANDONNEAU AND L. MEMERY LODyC, CNRS, ORSTOM, UniversiteÂParis VI, Tour 14, 2eÁme eÂtage, 4, Place Jussieu, 75252 Paris cedex 05, France ABSTRACT A simple 3-D biogeochemical model is coupled to a dynamical model forced by weekly winds deduced from ERS1 scatterometer data, to simulate new production in the equation Paci®c from April 1992 to June 1995. The biogeochemistry is modelled as a nitrate sink modulated by chlorophyll, using nitrate/chlorophyll regressions derived from ®eld data. The ®rst simulation was carried out assuming that remineralization below the euphotic layer is totally controlled by sinking particles. In the second simulation, it is shown that the simulation of nitrate and new production by the bio- geochemical model is improved, in comparison with ®eld data, by adding an explicit dissolved organic ni- trogen compartment. In the equatorial band, the model simulates a nitrate-poor region (low new pro- duction) in the fresh warm pool separated from richer waters of the upwelling region by a salinity front. The zonal displacement of this salinity/nitrate front is as- sociated with the El Nin Ä o±Southern Oscillation (ENSO). The modelled new production and physics, both in good agreement with the ®eld data, represent useful tools for the study of skipjack tuna (Katsuwonus pelamis) forage distribution in the Paci®c. Key words: dissolved export production, ENSO, equatorial Paci®c, model, new production, salinity front INTRODUCTION The equatorial Paci®c is one of the main productive areas of the world. The upwelling of nutrients main- tains intense biological activity that could account for 50% of global new production (Chavez and Barber, 1987). The inorganic carbon and nitrate contents of surface seawater are controlled by new production, de®ned by Dugdale and Goering (1967) as the sum of inorganic nitrogen inputs to the photic zone. New production and related transport of organic carbon and associated nutrients are key elements in oceanic car- bon cycle models (Bacastow and Maier-Reimer, 1991). The equatorial Paci®c is also subject to the El Nin Äo± Southern Oscillation (ENSO) that affects the world climate on an interannual time scale and the equa- torial Paci®c therefore constitutes a key region for studying the carbon cycle. To understand new pro- duction and carbon ¯uxes on basin and ENSO time scales, a 3-D ocean general circulation model (OGCM) of the Paci®c Ocean has been coupled to a simple biological model. The results from this model are described in this paper. Until recently, it was thought that the ¯ux of new production was carried mostly by the vertical ¯ux of particulate organic nitrogen (PON). In this respect, we made a ®rst simulation, in which the equivalent of new production in the photic layer is instantaneously remineralized below the photic layer as a vertical ¯ux of sinking particles. This produces, in the upwelling region, an excess accumulation of nitrate at depth, referred to as `nutrient trapping' (Najjar et al., 1992). However, recent results from JGOFS (Joint Global Ocean Flux Study) process studies show that new production results partly from dissolved organic ni- trogen (DON) and accounts for as much of the new production as does the downward export of particles (Buesseler et al., 1995; Luo et al., 1995; Libby and Wheeler, 1997). A second simulation was then made in which new production is fractionated into sedimenting particles, and into DON (50%). Indeed, previous modelling approaches (Bacastow and Maier- Reimer, 1991; Najjar et al., 1992) show that including DON produces better agreement between predicted and observed nutrient concentrations. * Correspondence. Fax: +33 1 44 27 71 59; e-mail: ast@lodyc.jussieu.fr Received for publication 4 April 1998 Accepted for publication 24 July 1998 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 7:3/4, 311±316, 1998 Ó 1998 Blackwell Science Ltd. 311