Ocean Turbulence And Climate History

Abstract eng:
The global oceans play a critical role in the climate system, not only in the transport of heat fiom the equator towards the poles but also in the global carbon cycle. Although the wind driven component of this circulation continues to be of interest, the therrnohaline or Meridional Overturning Circulation (MOC) component is of particular importance and interest from a fundamental fluid mechanical perspective. This is because its existence depends upon the continuous generation of small scale turbulence engendered primarily by the breaking of hydrodynamic waves. A primary source of excitation of these waves is the flow of the lunar barotropic tide over ocean bottom topography. The turbulence produced by wave breaking effects a vertical flux of mass that may be characterized by an effective turbulent diffusivity and it is this vertical flux of mass that enables the high density deep water that forms at the poles to return to the surface. Following a review of recent advances in the characterization of this diffusivity, it will be demonstrated that a previously unexplained phenomenon in climate system history, the so-called Dansgaard-Oeschger oscillation (see Figure 1), can be understood in terms of a global-scale relaxation oscillation of the Atlantic MOC. It is furthermore shown that this nonlinear oscillation is highly sensitive to the representation of stratified turbulence. This is an extreme example of a multi-scale dynamical phenomenon in which small scale turbulence controls not only the existence but also the millennium timescale of a truly global scale climate related oscillatory phenomenon.

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