Spatial and Temporal Variability of Upwelling and Downwelling Along the British Columbia Coast During 1993–2021

Abstract

This study analyzes the spatial and temporal variability in the phenology and magnitude of upwelling and downwelling in the Northern California Current System based on high-resolution wind data and a realistic 29-year hindcast simulation of a regional ocean model. In coastal sectors of approximately 1 degree of latitude, vertical volume transport at a mixed layer depth (MLD) is shown to be highly correlated with local wind forcing, which includes both the divergence of Ekman transport at the coastal boundary and offshore Ekman suction/pumping. The strongest upwelling occurred off the west coast of central Vancouver Island, while the weakest upwelling was observed off the west coast of northern Haida Gwaii. In Queen Charlotte Sound and Hecate Strait, wind stress generated a persistent spatial pattern of alternating strong local upward and downward water movement. Downwelling magnitudes and upwelling onset times were correlated with the winter North Pacific Index (NPI) at several sectors, whereas weaker upwelling and stronger downwelling at northern sectors were associated with the positive phase of the North Pacific Gyre Oscillation (NPGO). The variability in the duration of both seasons was variability of the spring transition. Significant temporal trends were found in the upwelling magnitudes in the southern half of the BC coast. In recent years, the onset of upwelling has occurred earlier, the duration of the summer upwelling season has shown an increasing trend, and downwelling duration has been decreasing.