Rapid Spinup and Spindown of Flow along Slopes

Abstract

The near-bottom mixing that allows abyssal waters to upwell tilts isopycnals and spins up flow over the flanks of midocean ridges. Meso- and large-scale currents along sloping topography are subjected to a delicate balance of Ekman arrest and spindown. These two seemingly disparate oceanographic phenomena share a common theory, which is based on a one-dimensional model of rotating, stratified flow over a sloping, insulated boundary. This commonly used model, however, lacks rapid adjustment of interior flows, limiting its ability to capture the full physics of spinup and spindown of along-slope flow. Motivated by two-dimensional dynamics, the present work extends the one-dimensional model by constraining the vertically integrated cross-slope transport and allowing for a barotropic cross-slope pressure gradient. This produces a closed secondary circulation by forcing Ekman transport in the bottom boundary layer to return in the interior. The extended model can thus capture Ekman spinup and spindown physics: the interior return flow is turned by the Coriolis acceleration, leading to rapid rather than slow diffusive adjustment of the along-slope flow. This transport-constrained one-dimensional model accurately describes two-dimensional mixing-generated spinup over an idealized ridge and provides a unified framework for understanding the relative importance of Ekman arrest and spindown of flow along a slope.