The Impact of Oscillatory Currents and Stratification on Turbulent Dissipation in the Marginally Stratified Celtic Sea, U.K.

We present full water column observations of turbulence microstructure, associated water properties and current velocities during eight tidal cycles at a marginally stratified shelf sea front in the southern Celtic Sea, U.K. The aim of the fieldwork, conducted at two sites (depth: 35m & 65m) in spring and late summer of 2012 and during both spring and neap tides, was to investigate the effect of stratification on the phase-dependent properties of turbulent dissipation within an oscillatory tidal regime. The results of this study demonstrate that, as opposed to tidal fronts that are governed by competition between restratification and mixing arising from tidal friction alone, second-order mixing processes are of importance in regulating the mixing regime in marginally stratified frontal regions. Dissipation profiles showed strong tidal asymmetry between the flood and ebb tides. Bottom boundary layer (BBL) growth was strongly limited by stratification both during the weakly stratified spring regime (N²_max = 3×10^-5 s^-2) and more pronounced during the strongly stratified summer regime (N²_max = 2×10^-4 s^-2). In accordance with numerical simulations of a stratified flow under an oscillatory current, stratification did not substantially affect dissipation rates within the BBL (~10^-5 Wkg^-1) but strongly supressed them above, forming regions of the lowest dissipation rates in the water column (~10^-9 Wkg^-1). Furthermore, a phase lag between maximum dissipation rates and currents was observed to increase with the strength of the stratification. Analysis of the Richardson number reveal a weak dependence of dissipation on local shear instability based on an assessment of the MacKinnon & Gregg scaling for mixing in stably stratified conditions. In addition to the modulation of bottom mixing due to the effects of stratification, we examine the tidal-phase dependence of surface mixed layer deepening associated with the opposing orientation of surface forcing versus tidal flow.