Response of the deep ocean internal wave field to traveling midlatitude storms as observed in long-term current measurements

It has been demonstrated in a recent numerical experiment that double-inert ial frequency internal waves may play a crucial role in diapycnal mixing pr ocesses in the deep ocean, with the energy effectively transferred across t he internal wave spectrum down to small dissipation scales by nonlinear wav e-wave interactions [Hibiya er al., 1998]. To examine whether or not such d ouble-inertial frequency waves are actually generated in the real deep ocea n, current meter data from long-term moorings in the northwest Pacific basi n are analyzed together with global sea surface wind data. By incorporating the wind data into a simple damped slab model, predominant inertial curren ts are shown to be excited in the mixed layer in the northwest Pacific basi n by traveling midlatitude storms during fall and winter. The multiple filt er analysis demonstrates that double-inertial frequency waves as well as ne ar-inertial frequency waves are significantly amplified in the deep ocean i nternal wave field during the periods strong inertial currents are excited in the mixed layer. This suggests that in addition to near-inertial frequen cy waves, double-inertial frequency waves are actually excited by strong at mospheric disturbances through nonlinear effects as demonstrated in the num erical experiment by Niwa and Hibiya [1997]. Double-inertial frequency wave s thus excited seem to propagate over horizontal distances of the order of 1000 km from their source region while feeding their energy to the local in ternal wave field, consistent with the theoretical prediction based on the magnitudes of group velocity and nonlinear interaction time [Olbers, 1983].