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

Abstract

The 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.

Semiannual variability of the California Undercurrent along the Southern California Current System: A tropical generated phenomenon

“…Previous research evidences the relevance of the local wind on the generation of coastal undercurrents [McCreary, 1981;Philander and Yoon, 1982;Yoon and Philander, 1982]. Nonetheless, in agreement with subsequent studies that exhibit the relevance of remote forcing on the CU dynamics [Connolly et al, 2014;Thomson and Krassovski, 2015], our results show that the tropical dynamics dominates the seasonal variability of the Southern CU. Yoshida [1967] proposes that the equatorial dynamics can influence the dynamics of eastern boundary undercurrents, we found that the seasonal CU variability along the SCCS is dominated by the equatorial dynamics through the generation of semiannual Kelvin waves that reach the coast and travel poleward as Semiannual Coastal-Trapped Waves.…”

“…Previous research based on observations with limited resolution, or coverage, to detect the propagation of SCTW has associated the semiannual CU variability to the local dynamics off Southern California [Hickey, 1979;Chelton, 1984;Lynn and Simpson, 1987;Gay and Chereskin, 2009]. Subsequent studies, however, reveal that remote wind-induced anomalies propagate as coastal-trapped waves and play a key role on the CU dynamics [Connolly et al, 2014;Thomson and Krassovski, 2015]. In addition, [Frischknecht et al, 2015] evidence that, by the same propagating mechanism, the equatorial dynamics dominates the near-shore temperature, salinity, and biogeochemical variability along the subsurface California Current System during El Niño periods.…”

Seasonal and interannual variability in along‐slope oceanic properties off the US West Coast: Inferences from a high‐resolution regional model

“…It becomes unstable and feeds jets and eddies into the coastal transition zone (CTZ) [Brink and Cowles, 1991] merging with CCS. Along the coast in WA and BC upwelling winds are relatively weaker than in OR and Northern CA [Foreman et al, 2011] and shelf and slope dynamics on temporal scales of several days and longer are strongly influenced by remotely generated coastally trapped waves (CTW) [Connolly et al, 2014;McCabe et al, 2015;Thomson and Krassovski, 2015]. In Southern CA, winds are generally weak and coastal ocean variability on subtidal time scales can be influenced notably by CTW propagating from Mexico [Enfield, 1987].…”

Remote forcing of subsurface currents and temperatures near the northern limit of the California Current System

“…Part of the reason for this inconsistency is likely the occurrence of the major El Niño event in 1982 and the relatively short data record used. A recent analysis by Thomson and Krassovski [2015] using an intermediatelength current record (2010)(2011)(2012)(2013)(2014) from southern Vancouver Island found evidence for CTW variability at 10-40 day periods but little evidence of direct CTW variability beyond a period of 40 days. Pringle and Riser [2003] presented one of the very few observationally based works relating ocean temperatures and remote wind stress with regard to CTWs.…”

“…In contrast to the central CalCS, the northern extent of the CalCS, along southern Vancouver Island, has received less attention with respect to CTW influence. Noteworthy exceptions include Crawford and Thomson [1982], Yao et al [1984], Denbo and Allen [1987], Connolly et al [2014], and Thomson and Krassovski [2015]. Crawford and Thomson [1982] showed that the continental shelf and slope off southern Vancouver Island allow first-mode diurnal frequency barotropic shelf waves and attributed fluctuations in diurnal currents to them.…”