Interannual Transport Variations in the California Undercurrent Off Vancouver Island: Roles of Remote Coastal Sea Level Variability and El Niño

Abstract The California Undercurrent is a dominant flow feature and has large impacts on regional ecosystems off the west coast of Canada. So far there is limited knowledge on their interannual transport variations. In this paper a high‐resolution ocean circulation model in the northeast Pacific has been established to investigate seasonal and interannual transport variability in the California Undercurrent off Vancouver Island (VI) over 1993–2020. The model forcing includes winds, heat flux, ocean tides, and river runoffs. The model monthly temperature, salinity, and currents are in good agreement with observations at two long‐term monitoring site off West VI. Seasonally the California Undercurrent transport increases from spring to fall and decreases in winter. Interannually the transport anomalies of the California Undercurrent are positively correlated with the inflow through the model southern boundary off Oregon and with the Oceanic Niño Index. It is argued that the interannual changes of the California Undercurrent off VI are likely associated with the sea level variability off South California propagating poleward, providing the poleward longshore pressure gradient along the upper continental slope. El Niño enhances the California Undercurrent mainly due to the equatorial coastal sea level variation propagating poleward.