Annual and interannual variations of phytoplankton pigment concentration and upwelling along the Pacific equator

The following variables along the Pacific equator from 145°E to 95°W were employed: surface layer phytoplankton pigment concentrations derived from Nimbus 7 coastal zone color scanner (CZCS) measurements of ocean color radiances; vertical velocities simulated at the 90‐m bottom of the euphotic layer from a wind‐driven ocean general circulation model; and nitrate concentrations estimated from model‐simulated temperature. The upward flux of nitrate into the euphotic layer was calculated from the simulated vertical motion and nitrate concentration. The CZCS‐derived phytoplankton pigment concentration was uniform from 175° to 95°W. Longitudinal profiles of upwelling, phytoplankton biomass, and 90‐m nitrate flux were of different shapes. The small annual cycles of the phytoplankton pigment concentration and nitrate flux were in phase: increased phytoplankton biomass was associated with increased upward nitrate flux, but the phase was not consistent with the annual cycles of the easterly wind or of the upwelling intensity. Variation of phytoplankton pigment concentration was greater during El Niño than during the annual cycle. The substantially reduced phytoplankton pigment concentration observed during El Niño was associated with smaller upward nitrate flux. Phytoplankton biomass during non‐El Niño conditions was not related to nitrate flux into the euphotic layer.