Stability of gyral waves

The gyral resonance can occur only if the ageostrophic current speed (anticyclonic) is higher than the phase velocity of the Rossby wave (cyclonic). As concerns the short-period gyral waves, the length of Rossby waves adapts so that their natural period of gyral waves coincides with the forcing period. The problem of stability is posed for long-period gyral waves for which a sub-harmonic mode locking occurs. Their natural period being thus imposed, generally it does not coincide with the forcing period.

Phenomena of regulation, mainly involving the average latitude of the gyre whose role in the tuning between the Rossby wave and forcing is essential (a decrease in the average latitude of 0.1°, i.e. 11.1 km, generates an increase in the circumference of the gyre of the North Atlantic of 122 km), maintain the conditions of resonance despite the wind changes (whose impact on the ageostrophic movement is weak owing to the long-periods involved) but mostly changes in the period of the oscillations of solar irradiance.

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