Structure and evolution of the oceanic surface boundary layer during the Surface Waves Processes Program

The vertical structure of the oceanic surface boundary layer and the role of Langmuir circulation in its evolution have been elucidated by analysis of field observations from the Surface Waves Processes Program (SWAPP). The surface forcing from wind stress, surface wave Stokes drift, and buoyancy flux were determined. The oceanic response was documented by vertical profiles of temperature and velocity in the upper 100 m. The velocity profiles were mapped onto a vertical coordinate relative to the base of the mixed layer. This allowed a view of boundary layer structure uncontaminated by changes in stratification due to diurnal cycling. Mean velocity profiles in the new coordinate system indicated a relatively low-shear mixed layer overlying a weakly stratified ‘transition layerʹ where shear was concentrated. At near-inertial frequencies, the mixed-layer velocity was ‘slab-likeʹ, in agreement with the assumption of some bulk mixed-layer models. However, at sub-inertial frequencies, unstratified shear was observed in the upper half of the temperature-mixed layer. This shear was coherent with the wind, but its dynamical significance could not be conclusively determined. Doppler sonars were used to map the cross-wind velocity structure at the sea surface and estimate the strength of Langmuir circulation. Circulation strength was related to the product of the wind stress and the Stokes drift, and was sustained for substantial periods (tens of hours) after an abrupt decrease in wind stress if waves remained strong. Two such events were analyzed in detail. It was shown that the upper ocean did not restratify in response to diurnal heating in the manner predicted by a model driven by wind stress alone. Instead, restratification was inhibited during the period of sustained Langmuir circulation and heat input at the surface was redistributed vertically.