Biogeochemical modelling of the tropical Pacific Ocean. I: Seasonal and interannual variability

A coupled physical–biogeochemical model has been developed in order to study physical–biological interactions in the tropical Pacific Ocean on seasonal-to-interannual timescales. The model incorporates both iron- and nitrogen-limited phytoplankton growth, and succession of phytoplankton size classes in accordance with the “ecumenical” iron hypothesis. The model shows a strong El Niño–Southern Oscillation component to phytoplankton variability in the central equatorial Pacific over the period 1980–1998. It is possible that this mode is more dominant in the model than in nature, although the correlation of modelled and observed chlorophyll in this region is strong. The model results show that interannual variability dominates over the mean seasonal cycle for both physical and biogeochemical fields, with the exception of undercurrent transport west of the dateline. Physical and biogeochemical fields show consistent seasonal phasing among the four El Niño events simulated, although there is variability in the magnitudes and exact timing. Nutrient concentration anomalies at constant temperature in the thermocline of the central equatorial Pacific appear to be largely advected from the west, and the strong seasonal cycle of the equatorial undercurrent in the west introduces a significant annual component to the variability of nutrient concentrations at longitudes where the mean seasonal cycle of the local physical forcing is negligible. The biogeochemical model maintains realistic nutrient pools over the time scales required for interannual simulation, and responds in a realistic fashion to changing upper ocean hydrography and circulation. The model is quite sensitive to the temporal resolution of the wind forcing, which introduces additional uncertainty into the validation and prediction of biogeochemical fields.