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

Abstract

It has been demonstrated in a recent numerical experiment that double‐inertial frequency internal waves may play a crucial role in diapycnal mixing processes in the deep ocean, with the energy effectively transferred across the internal wave spectrum down to small dissipation scales by nonlinear wave‐wave interactions [Hibiya et al., 1998]. To examine whether or not such double‐inertial frequency waves are actually generated in the real deep ocean, current meter data from long‐term moorings in the northwest Pacific basin are analyzed together with global sea surface wind data. By incorporating the wind data into a simple damped slab model, predominant inertial currents are shown to be excited in the mixed layer in the northwest Pacific basin by traveling midlatitude storms during fall and winter. The multiple filter analysis demonstrates that double‐inertial frequency waves as well as near‐inertial frequency waves are significantly amplified in the deep ocean internal wave field during the periods strong inertial currents are excited in the mixed layer. This suggests that in addition to near‐inertial frequency waves, double‐inertial frequency waves are actually excited by strong atmospheric disturbances through nonlinear effects as demonstrated in the numerical experiment by Niwa and Hibiya [1997]. Double‐inertial frequency waves 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 internal wave field, consistent with the theoretical prediction based on the magnitudes of group velocity and nonlinear interaction time [Olbers, 1983].