Decay of semi‐diurnal internal‐tide beams due to subharmonic resonance

Abstract

Results of a numerical study on internal‐tide generation are presented, using the nonlinear nonhydrostatic MIT‐GCM. In the model runs, a lunar semidiurnal (M2) internal tide beam is generated over the continental shelf break. Its further nonlinear evolution is analysed, in particular the generation of M1 internal tides by Subharmonic Resonance (SR). This is done for three different latitudes: at a mid‐latitude, slightly below (i.e., equatorward of) the critical latitude (poleward of which the M1 internal tide cannot propagate as a free wave), and at the equator. In the second case the M2 beam loses much energy to M1 already over the continental slope, yielding a pronounced spectral peak at M1, as well as a distinct M1 signal in the current velocity field. At the equator, the transfer to M1 is less pronounced, but still noticeable. Analysis of the growth rate of M1 yields a timescale of two days.