Subtropical catastrophe: Significant loss of low‐mode tidal energy at 28.9°

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Abstract
An idealized numerical study of a northward propagating internal tide reveals a dramatic loss of energy to small‐scale subharmonic instabilities near 28.9°N. Inspired by observations of the internal tide radiating northward from the Hawaiian Ridge, a three‐dimensional numerical model is initialized with a northward baroclinic tidal flux of approximately 1.7 kW/m. After an initial spinup period, energy is quickly transferred from the baroclinic tide to subharmonic motions, with half the horizontal tidal wavenumber and small vertical scales, through nonlinear advection of horizontal tidal velocity gradients. Potential oceanic implications are twofold. First, once a steady‐state has been reached, the instability acts as a partial filter of northward tidal flux between 27.5 and 29.5°N, in rough agreement with some altimetric tidal observations. Second, elevated shear of the subharmonic motions suggests the potential for elevated near‐surface dissipation rates near the critical latitude that may be important for upper ocean mixing.