Organization of stratification, turbulence, and veering in bottom Ekman layers

Abstract

Detailed observations of the Ekman spiral in the stratified bottom boundary layer during a 3‐month period in an upwelling season over the Oregon shelf suggest a systematic organization. Counter‐clockwise veering in the bottom boundary layer is constrained to the weakly stratified layer below the pycnocline, and its height is nearly identical to the turbulent boundary layer height. Veering reaches 13+/−4 degrees near the bottom and exhibits a very weak dependence on the speed and direction of the interior flow and the thickness of the veering layer. A simple Ekman balance model with turbulent viscosity consistent with the law‐of‐the‐wall parameterization modified to account for stratification at the top of the mixed layer is used to demonstrate the importance of stratification on the Ekman veering. The model agrees reasonably well with observations in the lower 60–70% of the bottom mixed layer, above which it diverges from the data due to the unaccounted physics in the interior. Neglect of stratification in an otherwise identical model results in far worse agreement with the data yielding veering in the bottom Ekman layer which is much smaller than measured, but distributed over a much thicker layer.