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

Abstract

Coastal winds and upwelling of deep nutrient-rich water along subtropical eastern boundaries yield some of the ocean’s most productive ecosystems1. Simple indices of coastal wind strength have been extensively used to estimate the timing and magnitude of biological productivity on seasonal and interannual timescales2 and underlie the prediction that anthropogenic climate warming will increase the productivity by making coastal winds stronger3,4,5,6. 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.

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Figure 1: Impact of wind drop-off on total upwelling and NPP.
Figure 2: Wind drop-off control of the NPP by modulation of the eddy physical fluxes.
Figure 3: An upwelling index that considers wind structure, and perhaps eddy activity, would better predict interannual NPP variations.

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Acknowledgements

We appreciate support from the Office of Naval Research (N00014-12-1-0939), National Science Foundation (OCE-1419450 and OCE-1419323), Bureau of Ocean Energy Management, and California Ocean Protection Council, as well as computing resources from the Extreme Science and Engineering Discovery Environment and on the Yellowstone cluster (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation.

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L.R., J.C.M. and C.D. conceived and designed the experiments; L.R. performed the experiments; L.R., C.D., J.C.M., H.F. and F.C., analysed the data; L.R., H.F. and J.-H.L. contributed materials/analysis tools; L.R., C.D. and J.C.M. co-wrote the paper.

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Correspondence to Lionel Renault.

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The authors declare no competing financial interests.

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Renault, L., Deutsch, C., McWilliams, J. et al. Partial decoupling of primary productivity from upwelling in the California Current system. Nature Geosci 9, 505–508 (2016). https://doi.org/10.1038/ngeo2722

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