The Origin of the 

 Oceanic Microstructure 



G. I. Barenblatt and A. S. Monin 

 p. P. Shirshov Institute of Oceanology 

 Moscow, USSR 



ABSTRACT 



Microstructure of hydrodynamical fields, a well- 

 known phenomenon in the ocean, is attributed to the 

 formation and development of turbulent spots gener- 

 ated due to the loss of stability or breaking of 

 internal waves. Under some general assumptions the 

 relations are obtained governing the development of 

 turbulent spots at various 'stages of their evolution. 

 It is shown that the longest and slowest stage of the 

 extension of a turbulent spot is the final, viscous 

 one. Simple self-similar laws of the extension of 

 turbulent spots are obtained for this stage and com- 

 pared with experiment. Long-standing turbulent 

 layers of the "blini" shape, sharply bound by am- 

 bient non-turbulent stratified fluid, are identified 

 with turbulent spots of the above-mentioned origin 

 which are in the final viscous stage of their evolu- 

 tion. The relations are also obtained governing 

 viscous intrusion of the bottom seawater into the 

 body of the ocean. 



1 . INTRODUCTION 



Under strongly stable stratification, turbulent mix- 

 ing is inhibited due to large losses of the turbulent 

 energy for the work against the buoyancy forces. Un- 

 der natural conditions, therefore, turbulence cannot 

 be present in the whole body of the fluid during 

 rather long periods of time [Woods (1968) , Monin et 

 al. (1977), Federov (1976)]. In fact, it is concen- 

 trated only in separate turbulent layers having the 

 shape of "blini," vertically quasi-homogeneous due 

 to mixing, and separated by thin streaks with micro- 

 jumps of temperature, electrical conductivity, sound 

 velocity, salinity, density, refraction index, and 

 other thermodynamic parameters of sea water some- 

 times accompanied by micro jumps of flow velocity. 

 Such thin-layered vertical structure, which is ap- 

 parent from inhomogeneities ("steps") on the verti- 

 cal profiles of density and other thermodynamic 



parameters (see schematic drawing in Figure 1) or 

 even more sharply from multiple peaks on the pro- 

 files of vertical gradients of these parameters, is 

 called microstructure or fine structure of hydro- 

 dynamical fields. Numerous measurements performed 

 using the method of continuous vertical sounding in 

 the cruises of the research vessels of the Institute 

 of Oceanology, USSR Academy of Sciences, and re- 

 search vessels of other countries showed that the 

 microstructure exists always and everywhere in the 

 World Ocean (the lack of microstructure may be ex- 

 pected only for the regions of macroconvection which 

 occur rather seldom in the ocean, at least in the 

 low and temperate latitudes) . 



Smoothing over the microstructural "steps" on 

 the profile of a thermodynamic parameter, e.g., 

 density or temperature, we obtain a smooth curve 

 characterizing large-scale stratification of the 

 ocean (gross-stratification) . We have to emphasize 

 that from the point of view of the Richardson cri- 

 terion gross-stratification is nearly always stable 

 - the Richardson number computed for it, Ri(z), as 

 a rule, is essentially larger than its critical 

 value, 1/4. How can the turbulence be generated 

 under such conditions? Graphs of Ri(z), taking 

 into account the "steps" of microstructure, show 

 values of Ri < 1/4 in several layers of the micro- 

 structure - apprently in these very layers, at the 

 momeht of sounding, the generation of small-scale 

 turbulence took place (in other layers where Ri > 

 1/4 turbulence decayed with time) . The appropriate 

 conditions for local generation of turbulence at 

 stable gross-stratification may be created by in- 

 ternal waves. Indeed, in the field of internal 

 waves in the regions near their crests and hollows 

 the local values of the Richardson number can be 

 reduced lower than the critical value, 1/4, and the 

 turbulence spots would then be formed there. The 

 internal waves can also break. For the turbulent 

 spots formed after the breaking of internal waves, 

 the formation is characteristic of continuous spec- 

 trum, i.e., of developed turbulence immediately 



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