Role Of Geostrophic Currents In Future Changes Of Coastal Upwelling In The California Current System

Given the importance of coastal upwelling in the California Current System (CCS), there is a considerable interest in predicting its response to global warming. However, upwelling changes are often treated as synonymous with changes in upwelling-favorable winds, while the role of geostrophic transport is unaccounted for. Here, we examine the respective roles of Ekman and geostrophic transports using the Community Earth System Model Large Ensemble. In some parts of the CCS, the contribution of geostrophic transport to long-term changes in upwelling is equal or greater than the contribution from Ekman transport. The combination of the two transports nearly close the momentum budget, and thus reproduce the mean state, interannual variability, and long-term changes in upwelling. These results highlight the importance of accounting for ocean circulation when quantifying upwelling and its variability and change.Plain Language Summary The California Current System (CCS) is an eastern boundary current system along the west coast of North America. The CCS hosts a diverse marine ecosystem and a high level of production for commercially valuable fish. Upwelling delivers deep, nutrient-rich water to the sunlit surface, which provides nutrients that stimulate phytoplankton growth, fueling the marine ecosystem. Thus, potential future changes in coastal upwelling are of concern for the health of the CCS ecosystem, the livelihood of fishers, and food security. To date, projections of upwelling change have focused primarily on modifications of the wind-driven (Ekman) transport. However, a second component of the ocean circulation, the geostrophic transport, can also impact coastal upwelling. In this study, we investigate the roles of both transport components in coastal upwelling change under global warming. We find that changes in the geostrophic currents, which have been neglected in many previous studies, make a substantial contribution to overall changes in the CCS coastal upwelling.