The Surface Boundary Layer in Coastal Upwelling Regions

Abstract

Abstract 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 ...