Layered mixing on the New England Shelf in summer

Abstract

AbstractThe layered structure of stratification and mixing on the New England Shelf (NES) in summer is examined by analyzing a comprehensive set of observations of hydrography, currents and turbulence. A clear distinction in mixing characteristics between the midcolumn water (consisting of subsurface stratification, middepth weak stratification and lower‐layer stratification) and a well‐mixed bottom boundary layer (BBL) is revealed. The combination of subtidal Ekman onshore bottom transport and cross‐shore density gradient created a lower‐layer stratification that inhibited the upward extension of the BBL turbulence. The BBL mixing was related to strong shear generated by bottom stress, and the magnitude and periodic variation of BBL mixing was determined by both the tidal and subtidal flows. Mixing in the midcolumn water occurred under stably stratified conditions and showed correspondence with the occurrence of near‐inertial and semidiurnal internal waves. Positive correlations between buoyancy frequency squared (N2) and shear variance (S2), S2 and dissipation rate (ε), N2 and ε are established in the midcolumn, but not in the BBL. The midcolumn ε was reasonably described by a slightly modified MacKinnon‐Gregg (MG) model.