Reynolds Stresses and Secondary Circulation in a Stratified Rotating Shear Flow

Abstract

A transverse secondary circulation should result from vertical mixing of momentum in a stratified shear flow influenced by the earth's rotation. A rigidly mounted acoustic Doppler current profiler was deployed at the bottom of Juan de Fuca Strait, a stratified estuary on the west coast of North America, for 21 days, to examine this dynamical process and its variation over a spring?neap tidal cycle. The data demonstrate a significant fortnightly modulation of the estuarine exchange flow, with stronger along-channel flow, less dynamic stability, and much greater cross-strait currents at neap tide than at spring tide. Reliable estimates of the Reynolds stresses in the stratified shear flow away from boundaries were also obtained. They were small except for several days around neap tide when they were associated with times of reduced Richardson number. These stresses are not related in any simple way (such as with a smooth, positive, eddy viscosity) to the shear of the mean flow but do have a pattern of vertical divergence that is similar to that of the cross-strait flow. A limited time series of microstructure profiles also shows enhanced mixing during a period of large Reynolds stresses.