Coastal Trapped Waves, Alongshore Pressure Gradients, and the California Undercurrent*

Abstract

The California Undercurrent (CUC), a poleward-flowing feature over the continental slope, is a key transport pathway along the west coast of North America and an important component of regional upwelling dynamics. This study examines the poleward undercurrent and alongshore pressure gradients in the northern California Current System (CCS), where local wind stress forcing is relatively weak. The dynamics of the undercurrent are compared in the primitive equation Navy Coastal Ocean Model and a linear coastal trapped wave model. Both models are validated using hydrographic data and current-meter observations in the core of the undercurrent in the northern CCS. In the linear model, variability in the predominantly equatorward wind stress along the U.S. West Coast produces episodic reversals to poleward flow over the northern CCS slope during summer. However, reproducing the persistence of the undercurrent during late summer requires additional incoming energy from sea level variability applied south of the region of the strongest wind forcing. The relative importance of the barotropic and baroclinic components of the modeled alongshore pressure gradient changes with latitude. In contrast to the southern and central portions of the CCS, the baroclinic component of the alongshore pressure gradient provides the primary poleward force at CUC depths over the northern CCS slope. At time scales from weeks to months, the alongshore pressure gradient force is primarily balanced by the Coriolis force associated with onshore flow.

The surface expression of radiocarbon anomalies near Baja California during deglaciation

“…A recent study suggested that previously documented benthic 14 C anomalies in core PC08 might be an artifact of tuning the sediment age model to GISP2 age (Davies-Walczak et al, 2014). Following Davies-Walczak et al (2014) we therefore constructed an alternative age model based on an assumption of constant reservoir age in the better equilibrated (younger) planktonic species as measured in the present study.…”

“…Following Davies-Walczak et al (2014) we therefore constructed an alternative age model based on an assumption of constant reservoir age in the better equilibrated (younger) planktonic species as measured in the present study. The alternative age model only slightly affects the timing and magnitude of the estimated 14 C anomalies in the benthic and other planktic species, while imposing an unlikely sedimentation rate history (see the Online Supplement for details).…”

Hindcasts of potential harmful algal bloom transport pathways on the Pacific Northwest coast

“…These waves directly influence the velocity structure over the shelf, including the surface velocity, as they drive strong (∼0.15 m s −1 ) quasi‐barotropic velocity fluctuations on time periods of 5–10 days [ Battisti and Hickey , ]. Additionally, they influence the alongshore pressure gradient over the slope and thus the CUC [ Connolly et al ., ]. The event‐scale variability in velocity structure over the midshelf mooring, RS (Figure ), shows that our model is properly propagating CTWs from the boundaries.…”

“…The CUC is a large‐scale feature that forms over the midlatitude Eastern Pacific Ocean continental slope [ Hickey , ] extending as far north as VI [ Thomson and Krassovski , ; Pierce et al ., ]. In the northern CCS, the CUC has been linked to the alongshore pressure gradient [ Hickey , ; Werner and Hickey , ; Connolly et al ., ]. It typically manifests itself as a depth‐intensified northward current centered below the shelf‐break (∼250–350 m) and is present throughout the summer/fall upwelling season.…”

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

“…Several studies have related coastal flow variability in the eastern Pacific to remotely forced CTWs [cf. Connolly et al ., , and references therein]. Shaffer et al .…”