Reexamining Flow across the Continental Shelf

We present moored observations of flows crossing the continental shelf in the northern California Current System, a highly productive Eastern Boundary upwelling system. Our focus is on the seasonally changing vertical structure of upwelling return flows. Shallower onshore flows can potentially lead to less upwelled nutrients, and this, in turn, may impact the structure of coastal ecosystems. Historically, analyses of such upwelling flows have met with mixed success because the upwelling circulation is typically an order of magnitude or more weaker than coincident alongshelf flows. Our analysis employs a streamwise-normal coordinate system to eliminate meander-induced biases in the cross-shelf flow that are unaccounted for with an alternative, commonly-applied approach. The resulting flow develops an organized pattern midway through the upwelling season. Under upwelling-favorable conditions an onshore return layer occurs just beneath the offshore surface flow, and a third offshore-directed layer exists at depth. Both subsurface layers strengthen in time. Mechanisms to explain this mean structure are evaluated, and it is suggested that the timing of the development and strengthening of both the interior return flow and the offshore near-bottom layer are consistent with the seasonally-changing direction and magnitude of the large-scale along-shelf sea-level gradient. The change to a poleward sea-level gradient initiates a seasonal relaxation of upwelled isopycnals that likely leads to the near-bottom flow. Late-season enhancement of the interior return flow is related to along-shelf winds but appears to form as a consequence of offshore transport in the near-bottom layer and the need to satisfy coastal mass balance. We suggest that coastal alongshelf pressure gradients are essential components of the coastal response that should be more routinely taken into account when analyzing coastal flows and their relationship to biogeochemical and ecosystem responses.