Internal-tide generation and destruction by shoaling internal tides

Internal-tide generation is usually predicted from local topography, surface tides, and stratification. However, internal tides are often observed to be unrelated to local spring-neap forcing, appearing intermittently in 3-5 day bursts. Here we suggest a source of this intermittency by illustrating how remotely-generated shoaling internal tides induce first-order changes in local internal-tide generation. Theory, numerical simulations, and observations show that pressure perturbations associated with shoaling internal tides can correlate with surface-tide velocities to generate or destroy internal tides. Where shoaling internal tides have random phase, such as on the New Jersey slope, time-averaged internal-tide generation is unaffected, but instantaneous internal-tide generation varies rapidly, altering internal-tide energy and possibly affecting nonlinear internal waves, across-shelf transport, and mixing. Where shoaling internal tides are phase-locked to the local surface tide, such as in double-ridge systems, time-averaged internal-tide generation is affected and may result in resonance.