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

Abstract

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, doubleinertial 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]. IntroductionIt is well known that wind stress fluctuation is one of the major sources of internal waves in the ocean. In particular, traveling atmospheric disturbances such as hurricanes or midlatitude storms can generate energetic internal waves because of the large-amplitude wind stress, which changes its direction significantly over timescales of the order of local inertial period. Primary oceanic responses to atmospheric disturbances include generation of near-inertial waves in the mixed layer, which has been studied by many researchers on the basis of the linear theory. A representative study is that of Pollard and Millard [1970], who proposed a simple damped slab model that could reproduce many observed features of generation of nearinertial waves in the mixed layer. In general, however, the amplitude of the near-inertial waves excited by traveling hurricanes or storms becomes so large that nonlinear effects cannot be neglected. Actually, by using three-dimensional multilevel numerical model [Niwa and Hibiya, 1997], the present authors demonstrated that low-vertical-mode, superinertial waves with frequencies 2fo and 3fo (fo is the inertial frequency at the latitude of the hurricane track) were generated by a traveling hurricane through nonlinear interactions between high-vertical-mode, near-inertial waves that were originally excited in the mixed layer. In areas away from the hu...

On the Loss of Wind-Induced Near-Inertial Energy to Turbulent Mixing in the Upper Ocean

“…We use here forcing starting on 1 January 2001. Note that, even though our wind forcing has relatively high spatial resolution, the use of daily wind stress is likely to underestimate the wind near-inertial energy input to the ocean (e.g., Niwa and Hibiya 1999;Davies and Xing 2004;Furuichi et al 2008). Nevertheless, the overall pattern and magnitude of near-inertial energy input in our model is consistent with those obtained in the previous studies (see section 4a).…”

Sustained, Full-Water-Column Observations of Internal Waves and Mixing near Mendocino Escarpment

“…3, dashed lines). In addition, finite temporal sampling can lead to underestimation of higher-frequency motions, in the manner demonstrated by Niwa and Hibiya (1999). 3, solid lines) meet the desired criterion of v N .…”

On the spatial and temporal distribution of near‐inertial energy in the Southern Ocean

“…It is known that forcing an ocean model with a wind stress that is sampled at a finite frequency leads to an underestimation of the near‐inertial current amplitudes [ Niwa and Hibiya , ; Watanabe and Hibiya , ; Alford , ]. Niwa and Hibiya [] showed using an analytic approach that in the linear slab‐ocean model of Pollard and Millard [], a six‐hourly wind‐stress forcing leads to an underestimation of the resulting near‐inertial amplitudes by a factor of approximately 1/1.4 at and 1/2.2 at . Consequently, NIE in the linear slab‐ocean model is expected to be underestimated by 50% at and by even more south of that.…”