Remote alongshore winds drive variability of the California Undercurrent off the British Columbia‐Washington coast

Abstract

AbstractThe California Undercurrent transports warm, salty, nutrient‐rich, oxygen‐depleted water along the continental slope from the equatorial Pacific to the Aleutian Islands. We use multiyear acoustic Doppler current profiler records collected simultaneously at two mooring sites off Vancouver Island to detail the regional structure of the undercurrent and to show that much of its variability is attributable to the passage of remotely forced, coastal‐trapped waves. We also document two subsurface currents missed by earlier current measurements. The undercurrent becomes evident in spring, intensifies through summer and fall, and merges with the wind‐driven poleward surface flow in winter. During intensification at the southern mooring site (A1), the undercurrent shoals from 250 ± 50 m in early summer to 150 ± 50 m depth in late fall. At the northern site (BP2), 225 km to the northwest of A1, the current is weaker and maintains a year‐round depth of 150 ± 50 m. Temporal variability in the undercurrent velocity attains highest coherence with winds along the southern Oregon‐northern California coast, with peak coherence occurring for “synoptic” (10–40 day period) alongshore winds off Cape Blanco in southern Oregon. The undercurrent lag of 3 ± 2 days relative to the Cape Blanco winds at synoptic periods is consistent with low mode, poleward propagating, coastally trapped waves. For periods >40 days, the wind‐current coherence remains high for winds off the Oregon‐California coast but lags are often negative, indicating possible forcing by alongshore baroclinic pressure gradients. At interannual time scales, the undercurrent variations have links to climate‐scale processes in the equatorial Pacific.