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Mixing Efficiency in the Presence of Stratification: When Is It Constant?
Author(s) -
Monismith Stephen G.,
Koseff Jeffrey R.,
White Brian L.
Publication year - 2018
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2018gl077229
Subject(s) - richardson number , buoyancy , dissipation , turbulence , physics , kinetic energy , mixing (physics) , stratification (seeds) , covariance , turbulence kinetic energy , reynolds number , constant (computer programming) , meteorology , mechanics , atmospheric sciences , thermodynamics , statistics , mathematics , classical mechanics , seed dormancy , germination , botany , quantum mechanics , dormancy , biology , computer science , programming language
The efficiency of the conversion of mechanical to potential energy, often expressed as the flux Richardson number, Ri f , is an important determinant of vertical mixing in the ocean. To examine the dependence of Ri f on the buoyancy Reynolds number, Re B , we analyze three sets of data: microstructure profiler data for which mixing is inferred from rates of dissipation of turbulent kinetic energy ( ε ) and temperature variance ( χ ) measured in the open ocean, time series of spectrally fit values of ε and covariance‐derived buoyancy fluxes measured in nearshore internal waves, and time series of spectrally fit values of ε and χ measured in an energetic estuarine flow. While profiler data are well represented by Ri f ≈ 0.2 for 1 < Re B < 1,000, the covariance data have much larger values of Re B and, consistent with direct numerical simulation results, show that Ri f ~ Re B −0.5 . The estuarine data have values of Re B that fall between those of the other two data sets but also shows Ri f ≈ 0.2 for Re B < 5000. Overall, these data suggest that Ri f is in general not constant and may be substantially less than 0.2 when Re B is large, although the value at which the transition from constant to Re B ‐dependent mixing may depend on additional parameters that are yet to be determined. Nonetheless, for much of the ocean, Re B < 100 and so Ri f is constant there.