Modeled Dynamics of Physical and Biological Processes in the Central California Current System From 1993 to 2016

A three-dimensional physical-biological model was used to study the dynamics of physical and biological processes in the central California Current System (CCS) from 1993 to 2016 and investigate its response to different climate modes. The variations of new production (NP) and nitrate fluxes in the central CCS showed robust seasonal and interannual variability. In the coastal upwelling region (0-150 km offshore), the variability of NP was associated with both coastal upwelling and alongshore nitrate advection. In the coastal transition zone (150-300 km offshore), it was related to nitrate fluxes that were determined primarily by the physics rather than nitrate concentration, with vertical mixing and offshore transport being dominant factors. In the offshore oceanic region (California Current; 300-1,000 km offshore), nitrate concentrations played a more important role in the variations of nitrate fluxes, and the NP variability was dominated by the nutrient supplies of vertical mixing and lateral transports. Although El Nino-Southern Oscillation can affect coastal upwelling, alongshore currents, and nitrate fluxes, the coastal NP responded to El Nino-Southern Oscillation mostly in the frequencies of <1 and >6 years. Pacific Decadal Oscillation and North Pacific Gyre Oscillation may affect the CCS ecosystem through large-scale winds and stratification, modifying the circulation in the upper layer and leading to changes in nitrate concentration and NP. Plain Language Summary The central California Current System (CCS) is one of the most productive ecosystems in the global ocean, because strong coastal upwelling brings a large amount of subsurface nutrients to the sunlit layer and supports abundant phytoplankton growth. Understanding the response of this system to varying physical forcing is of great importance in predicting how marine ecosystem will evolve with global change. By using a three-dimensional model, this study demonstrates that biological properties in the central CCS show strong interannual variability. The dominant driver leading to this variability varies in different subregions moving from the coast to offshore. Over the CCS, climate variability can cause variations in surface wind, stratification, sea level, and circulations at different time scales, which contribute to changes in nutrient supply and phytoplankton production.