Basin-scale water biogeochemistry and stratification changes are decisive factors in the response of the California Current System to climate change

Coastal winds in the California Current System (CCS) are credited with the high productivity of its planktonic ecosystem, and the hypoxic and corrosive waters that structure diverse macrofaunal habitats. These winds are thus considered the leading mediator of climate change impacts in Eastern Boundary Upwelling systems. We used an eddy-permitting regional model to downscale the response of the CCS to the major distinct climate changes - regional winds, ocean stratification, and remote water properties – projected by several global Earth System Models. Contrary to expectations, we found that basin scale changes, and not coastal wind forcing, were the decisive factor in the response of marine ecosystem properties including nitrate concentrations, net primary productivity, and dissolved oxygen. Changing winds were projected to shift the seasonal intensity of upwelling across the CCS, consistent with prior analyses; however increasing stratification offset the response to winds, particularly in summer. Changes in downscaled nitrate concentrations and net primary production were small compared to the range of responses across the Earth System Models. In contrast, in the downscaled models temperature increases and dissolved oxygen decreases were large and broadly consistent with the global resolution models. These results imply that global models with poor resolution of coastal processes nevertheless yield important information about the dominant climate impacts on coastal ecosystems.