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

Abstract

A 9‐year simulation (1984–1992) of biological processes in the equatorial Pacific Warm Pool is presented. A modified version of the four‐component (phytoplankton, zooplankton, nitrate, and ammonium) ecosystem model by McClain et al. [1996] is used. Modifications include use of a spectral model for computation of photosynthetically available radiation and inclusion of fecal pellet remineralization and ammonium nitrification. The physical parameters (horizontal and vertical velocities and temperature) required by the ecosystem model were derived from an improved version of the Gent and Cane [1990] ocean general circulation model [Murtugudde and Busalacchi, 1998]. Surface downwelling spectral irradiance was estimated using the clear‐sky models of Frouin et al. [1989] and Gregg and Carder [1990] and cloud cover information from the International Satellite Cloud Climatology Project. The simulations indicate considerable variability on interannual timescales in all four 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 deep chlorophyll maximum (DCM) ranged between 75 and 125 m with values occasionally exceeding 0.40 mg m−3. With the exception of periods during 1984 and 1988, surface nitrate was always near depletion. Ammonium exhibited a subsurface 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 18 W m−2 over the 9‐year period or about 8% of the incident total irradiance. The average mixed layer depth was 42 m.