The timescales Analysis of long-term variability of oceanic circulation forced by the stationary wind in a two-layer ocean. Part 2. Spectral analysis

A.B. Fedotov

Institute of Natural and Technical Systems,Russian Federation, Sevastopol, Lenin St., 28

Email: fedotov57@mail.ru

DOI: 10.33075/2220-5861-2018-3-65-69

UDC 551.465.553

Abstract:

            The work is devoted to the study of the evolution of ocean currents in the framework of the classical quasi-geostrophic vortex-resolving numerical two-layer model of circulation under the influence of a stationary spatially inhomogeneous wind. The model of evolution of a potential vortex in a two-layer liquid was used for the case of the upper layer depths of 100 meters, the lower layer – 3866 meters, numerical experiments were performed in a square computational domain with a resolution of 513 grid nodes for each of the sides of a square, the size of the computational region was assumed to be 3840 km, the horizontal length scale L = 611 km, whereas, when β = 2,0·10-11 m-1s-1, the speed scale is equal to U = 7,46 m/s, the time scale T = 0.95 day, the spatial grid step of the computational domain at a given resolution was equal to 7.5 km, the values of the dimensionless coefficient biharmonic viscosity for both layers was chosen equal to 1.0·10-8.

            The results of numerical simulation of the ocean at the stage of evolution of the flow field under the influence of spatially inhomogeneous constant wind at constant dissipation parameters: bottom friction and biharmonic viscosity are presented. The quasi-periodic process of spontaneous intensification of zonal jet flow with subsequent decrease of its intensity is noteworthy.

            This quasiperiodic process is associated with the alternation of the generation of cyclonic and anticyclonic vortices as a manifestation of baroclinic instability in the region of the Eastern tip of the jet flow, which then move in the West direction. In the process of its movement to the West, these vortices are attenuated and absorbed near the West coast by a large-scale flow, this process is relatively high frequency, it does not lead to a change in the total energy of the system, and it can be attributed to the “local” character of instability, while it is well represented on the periodogram of the time course of the position of the center of mass of the enstrophy spectrum for the upper layer.

            The quasi-periodic process of amplification and attenuation of the zonal jet flow itself, accompanied by the accumulation of energy in the jet, is a long-term one, and is a reflection of complex nonlinear processes associated with changes in the structure of the spatial spectrum of the fields of the flow system. The nature of the spatial spectrum variability allows us to speak about the signs of self-organization processes in the system of large-scale circulation. The criterion of self-organization processes are the oscillations of the spectrum width of the enstrophy, diagnosed by assessing the position of the center of mass of the spectrum of the enstrophy. Namely, the narrowing of the spatial spectrum of the enstrophy blocks the entry of the enstrophy and energy into the dissipative interval, which leads to an increase of the total normalized energy of the flows, the maximum value of the total energy is achieved in the system approximately 70 days after the maximum contraction of the spatial enstrophy spectrum. Conversely, the expansion of the spectrum in the “free turbulence” mode as a phase of termination of the “coherent” mode provides an increase in the spectral energy flow and the entropy in the dissipative spectral range, followed by a decrease in the total energy of the flow system.

Кeywords: synoptic variability, large-scale variability, wind-forced currents.

Full text in PDF (RUS)

LIST OF REFERENCES

  1. Fedotov A. B. Analysis of time scales of long-period variability of wind ocean circulation in a two-layer ocean // Monitoring systems of environment. 2017. No. 10 (30). P. 96-101.
  2. Rhines P.B. Geostrophic turbulence   // Ann. Rev. Fluid Mech. 1979. V. 11. P. 401–441.
  3. Fedotov A. B. Spectral flows of enstrophy and energy in the self-organization of β-turbulence // Marine hydrophysical journal, 1988, no. 3, P. 9-14.
  4. Larichev V. D., Fedotov A. B. The phenomenon of self-organization in geostrophic turbulence on the β-plane // Reports of the USSR Academy of Sciences, 1988, Vol. 298, no. 4, P. 971-975.

 

 

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