Modification of Parametric Subharmonic Instability in the Presence of Background Geostrophic Currents

Abstract

Although various numerical experiments have been carried out to investigate the cascade of internal tide energy associated with parametric subharmonic instability (PSI), most of them assume idealized oceanic situations, ignoring the existence of background relative vorticity (ζ). Here we show, for the first time, that the existence of ζ can significantly modify the behavior of PSI by changing the local effective Coriolis frequency (f eff = f+ζ/2). A set of numerical experiments for 25.4°N show that the PSI efficiency significantly increases at the position where positive ζ shifts f eff close to the M 1 critical frequency (ω = ω M1 ). Furthermore, numerical experiments for 31°N, poleward of the M 1 critical latitude, show that significant PSI energy transfer is possible when negative ζ is low enough. The presented results provide important implications for understanding the spatial distribution of diapycnal mixing in the ocean where background geostrophic currents prevail.