Lateral Reynolds Stress and Eddy Viscosity in a Coastal Strait

Abstract

Abstract The lateral Reynolds stress uυ, representing the transfer of along-strait momentum toward the sides of Juan de Fuca Strait, is measured with an array of ADCPs. The contributions to the stress from a number of frequency bands are analyzed to highlight the roles of different processes. Motion in a frequency band that includes internal waves and Doppler-shifted subinertial eddies gives a Reynolds stress that, when scaled by the observed shear, is consistent with an eddy viscosity of O(10 m2 s−1). This viscosity acts on the tides and the estuarine flow. The tides can also impart a viscous stress upon the low-frequency estuarine flow. These tidal currents, although strong, are largely nondivergent within the area of the array and thus appear to be less important for the cross-strait transfer of momentum. The long-term mean estuarine circulation can be acted upon by meanders of the estuarine flow, defined as features with periods of 3–5 days. These meanders are found to also have a horizontal eddy viscosity of O(10 m2 s−1). The measured Reynolds stress divergences are consistent with both the strongly curved profile of the estuarine mean flow and also the more slablike tidal current profile. This paper represents the first direct calculation of eddy viscosity on the medium-sized scale of the array, O(1 km).