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arXiv · 2002.12425

Near-resonant instability of geostrophic modes: beyond Greenspan's theorem

Thomas Le Reun·Basile Gallet·Benjamin Favier·Michael Le Bars
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Abstract

We explore the near-resonant interaction of inertial waves with geostrophic modes in rotating fluids via numerical and theoretical analysis. When a single inertial wave is imposed, we find that some geostrophic modes are unstable above a threshold value of the Rossby number $kRo$ based on the wavenumber and wave amplitude. We show this instability to be caused by triadic interaction involving two inertial waves and a geostrophic mode such that the sum of their eigen frequencies is non-zero. We derive theoretical scalings for the growth rate of this near-resonant instability. The growth rate scaled by the global rotation rate is proportional to $(kRo)^2$ at low $kRo$ and transitions to a $kRo$ scaling for larger $kRo$. These scalings are in excellent agreement with direct numerical simulations. This instability could explain recent experimental observations of geostrophic instability driven by waves.

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Thomas Le Reun, Basile Gallet, Benjamin Favier, Michael Le Bars. 2020-06-01. Near-resonant instability of geostrophic modes: beyond Greenspan's theorem. https://doi.org/10.1017/jfm.2020.454

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