The Surface Boundary Layer in Coastal Upwelling Regions

Observations from the Oregon, northwest Africa, Peru, and northern California shelves are used to examine the characteristics of the surface boundary layer in coastal regions during the upwelling season. The observations from these four regions yield a consistent picture of the structure of the surface boundary layer. Both CTD and moored observations reveal the presence of surface mixed layers that are typically 0–20 m thick with variability at diurnal and subtidal (periods longer than 36 hours) frequencies. The subtidal surface mixed-layer depth variability scales as u*/(NIf)½, where u* = (τS/ρ0)½ is the shear velocity, NI is the buoyancy frequency below the surface mixed layer, and f is the Coriolis frequency. Surprisingly, this relationship indicates that the subtidal variability of surface mixed-layer depth does not depend strongly on either the surface heat flux or advection of heat, both of which are large in coastal upwelling regions. Within the surface mixed layer the cross-shelf current is vertically uniform. Below the surface mixed layer there is a transition layer characterized by a subsurface maximum in the vertical shear of the cross-shelf velocity. The wind-driven, cross-shelf transport in the surface boundary layer agrees well in magnitude and variability with the expected Ekman transport τS/ρ0f), with a substantial fraction (50%–25%) of the transport occurring in the transition layer below the surface mixed layer. The alongshelf current contains a strong, near-surface, vertical shear. When this vertical shear is in the surface mixed layer (unstratified flow) it is linearly related to the shear velocity u* and is consistent with a near-surface wind-driven log-layer, though the magnitude of the shear is larger than expected. Below the surface mixed layer, during moderate to strong wind stresses, the vertical shear is related to the stratification such that the gradient Richardson number (estimated over vertical scales of a few meters) is near critical (Ri∼0.25). Well below the surface mixed layer, in the interior, the flow tends to be more stable (Ri≥0.25), though near-critical Richardson numbers do occur.