Low Richardson number turbulence in the ocean surface layer
Date
1998
Authors
Holgate, Simon John
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Abstract
The ocean surface layer is the interface through which most of the transfers of heat and momentum which drive the ocean and atmosphere must pass. Parameterization of turbulent mixing in the ocean is necessary in order to account for these small scale processes which cannot be explicitly treated in large scale models. For parameterizations to be useful, the physics of the processes, and the contexts in which they are valid, must be clearly understood.
Observations of turbulent kinetic energy dissipation, temperature and salinity were made with the instrument TOMI, during the Marine Boundary Layer Experiment, which was carried out in Monterey Bay, California, from April 10 to May 5, 1995. Additional data were obtained from a ship mounted Acoustic Doppler Current Profiler, CTD casts, navigational and meterological instruments.
Measurements of a key indicator of mixing, the gradient Richardson number, Ri9, were found to be below the critical value for instability in a high shear region of the ocean surface layer, which was maintained close to thermal wind balance. Dissipation levels were high in this low Richardson number region. Despite the evidence for shear instability, temperature gradient skewnesses were not found to follow the expected sign convention, but the distribution of dissipation estimates in the highly turbulent region matched theoretical expectations of lognormality over length scales much greater than those of individual mixing patches. Values of Ri9 measured in highly stratified waters were much greater than the critical value and the distribution of dissipation estimates was found to deviate systematically from lognormal as stratification increased. Dissipation distributions from individual patches continued to conform to the lognormal distribution however.
Comparison was made between direct measurements of the flux Richardson number, Rf, and theoretical/laboratory based estimates. It is shown that theory and laboratory measurements are consistent with a peaking of the flux Richardson number at the point of transition between turbulence and wave-like motions caused by suppression by the stratification, but they are inconsistent with direct measurements of large values of Rf in the highly turbulent region which had a large separation between the energy containing and the dissipative scales. It is conjectured that the process which leads to these large values of Rf has very different physics from the suppression mechanism and is related to the 2-dimensionality of the energy containing scales in stratified shear flow.