Future Changes of Nutrient Dynamics and Biological Productivity in California Current System

A leading hypothesis for ocean productivity and global warming is that warming increases upper ocean stratification and decreases vertical mixing, thus hampers vertical exchange of nutrient across the pycnocline and declines productivity in the upper ocean. Eastern boundary upwelling ecosystems such as the California Current System (CCS) are productive regions sustained by the supply of cool and nutrient-rich waters to the sunlit layer. How does CCS respond to the warming-induced stratification and possible changes of upwelling-favorable winds in terms of ocean productivity and ecosystem dynamics? To answer this question, we used a coupled and nested physical-biogeochemical model to examine and estimate changes in the physical and biogeochemical fields by the end of 2050. We set up a regional model based ROMS with high spatial resolution in order to examine eddy and front activities under future climate change scenarios, which has been generally ignored by previous global future projection models. Consistent with previous global modeling studies, the regional CCS model has predicted increased upwelling intensity associated with stronger alongshore winds in the coastal region, and enhanced upper stratification in the open ocean. Warming in the open ocean pushes isothermals down to make contact with water masses with higher nutrient concentrations, leading to increased nutrient concentrations in the deep source waters of the CCS. Increased alongshore winds and elevated eddy activities further facilitate the upward nutrient flux. Increases in nutrients do not linearly result in plankton biomass changes, as the food web dynamics tends to take effect in some areas in CCS. This study takes advantage of high-resolution models and highlights mechanisms of future ecosystem changes in the coastal upwelling region.