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.