A simulation of biological processes in the equatorial Pacific Warm Pool at 165 degrees E

A 9-year simulation (1984-1992) of biological processes in the equatorial P acific Warm Pool is presented. A modified version of the four-component (ph ytoplankton, zooplankton, nitrate, and ammonium) ecosystem model by McClain et al. [1996] is used. Modifications include use of a spectral model for c omputation of photosynthetically available radiation and inclusion of fecal pellet remineralization and ammonium nitrification. The physical parameter s (horizontal and vertical velocities and temperature) required by the ecos ystem model were derived from an improved version of the Gent and Cane [199 0] ocean general circulation model [Murtugudde and Busalacchi, 1998], Surfa ce downwelling spectral irradiance was estimated using the clear-sky models of Frouin et al. [1989] and Gregg and Carder [1990] and cloud cover inform ation from the International Satellite Cloud Climatology Project. The simul ations indicate considerable variability on interannual timescales in all f our ecosystem components. In particular, surface chlorophyll concentrations varied by an order of magnitude with maximum values exceeding 0.30 mg m(-3 ) in 1988, 1989, and 1990 and pronounced minima during 1987 and 1992. The d eep chlorophyll maximum (DCM) ranged between 75 and 125 m with values occas ionally exceeding 0.40 mg m(-3). With the exception of periods during 1984 and 1988, surface nitrate was always near depletion. Ammonium exhibited a s ubsurface maximum just below the DCM with concentrations as high as 0.5 mg- atN m(-3), where "at" is atoms. Total integrated annual primary Production varied between 40 and 250 gC m(-2) yr(-1) with an annual average of 140 gC m(-2) yr(-1). Finally, the model is used to estimate the mean irradiance at the base of the mixed layer, i.e., the penetration irradiance, which was 1 8 W m(-2) over the 9-year period or about 8% of the incident total irradian ce. The average mixed layer depth was 42 m.