Numerical Simulations of Buoyant Ekman Layers in the Presence of Variable Stratification. Part I: Constant Interior Forcing

Abstract

Two-dimensional numerical simulations of the turbulent buoyant Ekman layer in the presence of variable stratification help explain a transport mechanism for the detachment of the boundary layer flow around the front. Initially, a constant, spatially uniform, along-the-isobaths interior current and constant interior stratification are assumed. Buoyant inhibition of the boundary layer Ekman flow in the stratified region leads to shut down processes that are absent in the unstratified region. Depending on the direction of the interior flow, two dynamically distinct possibilities exist: (i) convergence around the front and subsequent export (detrainment) of boundary fluid toward the interior (southward forcing current, i.e., with shallow water on the right), and (ii) divergence around the front and subsequent import (entrainment) of interior fluid (northward interior forcing, i.e., with shallow water on the left or y positive). The position and strength of the convergence/divergence are controlled by the background diffusivity and the strength of the stratification. The area of detrainment (entrainment) represents a region of increased (decreased) bottom stress. In some of the studied cases, the bottom boundary layer on the unstratified region was modified by the presence of the front. An analogy between the case presented and the case of flow over variable topography is made.