The intensity of fine scale near-inertial shear is thought to control the diapycnal diffusivity in the deep ocean. Recent field observations using expendable current profilers (hereafter referred to as XCP) show that the intensity of fine-scale near-inertial current shear is strongly dependent on latitude. Since the pattern of numerical reproduced global thermohaline circulation is strongly controlled by the distribution of diapycanl diffusivity, this observed feat ure should be taken into account in the ocean generalcirculation model.
In order to see underlying physical mechanism for this observed feature, numerical experiments are carried out. First, the Garrett-Munk internal wave fields at 49°N and 28°N, respectively, are embedded in the whole model domain that includes a topographic feature simulating triangular seamount. First, the quasi-stationary internal wave field at each of 49°N and 28°N is reproduced by calculating the nonlinear interactions over 10 inertial periods among the embedded internal waves. After the internal wave field achieves a quasi-steady state, M2-tidal forcing is applied.
The responses to M2-tidal forcing are quite different between the two latitudes. At 28°N, the horizontally elongated pancake-like structures of high horizontal velocity are created throughout the water column as time goes on. This structure has horizontal scale of about 50 km and vertical scale of about 50 m. Such fine-scale shear structures become less evident at 49°N. These results are consistent with the XCP observations.
The results of numerical experiments can be reasonably explained in terms of parametric subharmonic instabilities; parametric subharmonic instabilities (hereafter referred to as PSI) transfer energy at low vertical wavenumber, twice the inertial frequency to high vertical wavenumber, near-inertial frequency portion of the spectrum. PSI occurs where the frequency of theexternal forcing (in this case, M2-tidal forcing) is more than twice the local inertial frequency. Accordingly, at 28°N where the local inertial period is about 26 hours (twice the M2-tidal period), PSI works to create horizontallyelongated pancake-like structures with strong near-inertial current shear inducing intensive diapycnal diffusivity. |
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