Buoyancy Effects in a Stratified Ekman Layer

Abstract

Abstract The K-profile parameterization scheme is used to investigate the stratified Ekman layer in a “fair weather” regime of weak mean surface heating, persistently stable density stratification, diurnal solar cycle, and broadband fluctuations in the surface stress and buoyancy flux. In the case of steady forcing, the boundary layer depth typically scales as h ∼ u*/Nf, where u* is the friction velocity, f is the Coriolis frequency, and N is the interior buoyancy frequency that confirms empirical fits. The diurnal cycle of solar forcing acts to deepen the boundary layer because of net interior absorption and compensating surface cooling. Parameterized mesoscale and submesoscale eddy-induced restratification flux compresses the boundary layer. With transient forcing, the mean boundary layer profiles are altered; that is, rectification occurs with a variety of causes and manifestations, including changes in h and in the Ekman profile u(z). Overall, stress fluctuations tend to deepen the mean boundary layer, especially near the inertial frequency. Low- and high-frequency surface buoyancy-flux fluctuations have net shallowing and deepening effects, respectively. Eddy-induced interior profile fluctuations are relatively ineffective as a source of boundary layer rectification. Rectification effects in their various combinations lead to a range of mean velocity and buoyancy profiles. In particular, they lead to a “rotated” effective eddy-viscosity profile with misalignment between the mean turbulent stress and mean shear and to a “flattening” of the velocity profile with a larger vertical scale for the current veering than the speed decay; both of these effects from rectification are consistent with previous measurements.