What do we need to Probe Upper Ocean Stratification Remotely?
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Radiophysics and Quantum Electronics, Vol. 63, No. 1, June, 2020 (Russian Original Vol. 63, No. 1, January, 2020)
WHAT DO WE NEED TO PROBE UPPER OCEAN STRATIFICATION REMOTELY? V. I. Shrira1 and R. B. Almelah2
UDC 551.46
We consider whether it is possible in principle to retrieve the key parameters of the mixed layer in the upper ocean (its thickness, bulk eddy viscosity and the pycnocline stratification below) using a theoretical model, which assumes the surface velocity and wind stress to be known from observations. To this end we examine the dynamics of the Ekman current in the novel two-layer model of the upper ocean made of two layers with greatly differing constant eddy viscosities. The presence of stratification manifests itself through suppression of turbulence and, hence, in much smaller value of the eddy viscosity compared to the bulk eddy viscosity νe1 in the mixed layer. Within this two-layer model the general solution in terms of Green’s function has been derived and analyzed. It was found that a spectral component of frequency ω of the Ekman current on the surface “feels” the presence of the stratified layer when the mixed layer depth d is less then or comparable to the Ekman scale 2νe1 /f + ω, where f is the Coriolis parameter. Thus, under conditions of strong wind resulting in large eddy viscosity νe1 , the depth of the mixed layer could be (in principle) inferred from the observations of wind and surface velocity. We conclude by stating that to retrieve the mixed layer parameters from the wind and surface velocity data, the theoretical model has to be extended by taking into account the effects of the Stokes drift due to surface waves and the possibility of intense mixing at the bottom of the mixed layer.
1.
INTRODUCTION
The mixed layer and seasonal pycnocline are the most prominent features in the upper ocean (e.g., [1, 2]). In the mixed layer between the ocean free surface and the seasonal pycnocline the temperature and salinity is nearly uniform, which led to the term “mixed” or “quasi-uniform” layer. The mixed layer depth is influenced by a variety of physical processes affecting the stratification at its bottom, including winds, turbulent mixing, and radiative heating and cooling. The thickness of the mixed layer varies depending on the external forcing. In spring and summer there exists strong seasonal stratification, the mixed layer is relatively shallow, while it is deeper and less prominent in autumn and much deeper in winter. The seasonal pycnocline disappears in winter and the mixed layer extends to the main pycnocline. The typical depth of the seasonal pycnocline ranges between 20 m and 200 m. At higher latitudes, the mixed layer depth increases as a result of stronger winds and poleward cooling. The mixed layer is the layer in direct contact with the atmosphere and most active in the airsea interaction. There is a very significant momentum and heat exchange between this layer and the atmosphere. Its correct modeling and monitoring of its actual state are crucial for weather prediction and
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