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Clayson, C.A., St. Laurent, L., and Schmitt, R. (27-Feb-14)

We evaluate the upper ocean stability structure and its effect on mixing processes during several regimes occurring during the SPURS experiment using a combination of modeling and measurements from buoy and glider measurements. We focus on the density ratio changes with depth from the surface to the upper thermocline, subject to the influence of mixing processes. The stabilizing effects of freshwater from rain as contrasted to conditions of high solar radiation and low winds will be shown, with observations providing surprising new insights into upper ocean mixing in these regimes. Previous observations of freshwater lenses have demonstrated a maximum of dissipation near the bottom of the stable layer; our observations provide a first demonstration of a similar maximum near the bottom of the solar heating-induced stable layer. The effects of high evaporation rates on the density ratio and possible effects of double-diffusive instabilities will also be explored, and capabilities and limitations of current mixing models will be demonstrated. Our findings suggest that parameterizations of near-surface mixing rates during stable stratification and low-wind conditions require considerable revision, in the direction of larger diffusivities.