Partial decoupling of primary productivity from upwelling in the California Current system

Coastal winds and upwelling of deep nutrient-rich water along subtropical eastern boundaries yield some of the ocean's most productive ecosystems. Simple indices of coastal wind strength have been extensively used to estimate the timing and magnitude of biological productivity on seasonal and interannual timescales and underlie the prediction that anthropogenic climate warming will increase the productivity by making coastal winds stronger. The effect of wind patterns on regional net primary productivity is not captured by such indices and is poorly understood. Here we present evidence, using a realistic model of the California Current system and satellite measurements, that the observed slackening of the winds near the coast has little effect on near-shore phytoplankton productivity despite a large reduction in upwelling velocity. On the regional scale the wind drop-off leads to substantially higher production even when the total upwelling rate remains the same. This partial decoupling of productivity from upwelling results from the impact of wind patterns on alongshore currents and the eddies they generate. Our results imply that productivity in eastern boundary upwelling systems will be better predicted from indices of the coastal wind that account for its offshore structure.