Evolution of near-inertial waves during an upwelling event on the New Jersey inner shelf

A 1996 field program provided a nearly three-dimensional view of the temporal evolution of near-inertial motion toward the end of an upwelling event on New Jersey's inner shelf. The appearance of near-inertial motion is marked by a rapid rise in kinetic energy at the surface that is of the same magnitude and temporal structure of the work done by the wind, indicating that the inertial motions are forced by local winds. The incipient near-inertial motion is surface intensified and spatially coherent. It has a horizontal wavelength of 300 km and a mean kinetic energy of 0.04 m2 S-2, both of which decrease to less than 100 km and 0.01 m2 S-2 within two inertial periods. An energy budget suggests that the rapid decline in surface kinetic energy is primarily due to vertical propagation into the thermocline. Near-inertial motion in the thermocline is heterogeneous suggesting interaction between near-inertial motion and subinertial shears. The heterogeneous nature of near-inertial waves poses a practical problem for shipborne acoustic doppler current profiler surveys of the subtidal velocity field.