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Layered mixing on the New England Shelf in summer
Author(s) -
Wang Jianing,
Greenan Blair J. W.,
Lu Youyu,
Oakey Neil S.,
Shaw William J.
Publication year - 2014
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2014jc009947
Subject(s) - stratification (seeds) , buoyancy , geology , mixed layer , hydrography , ekman layer , internal wave , richardson number , oceanography , water column , turbulence , thermocline , wind stress , boundary layer , mechanics , physics , seed dormancy , botany , germination , dormancy , biology
The 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 ( N 2 ) and shear variance ( S 2 ), S 2 and dissipation rate ( ε ), N 2 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.