AUGUST 2016 O L B E R S A N D E D E N 2335 Revisiting the Generation of Internal Waves by Resonant Interaction with Surface Waves DIRK OLBERS Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany CARSTEN EDEN Institut fur€ Meereskunde, Universitat€ Hamburg, Hamburg, Germany (Manuscript received 1 April 2015, in final form 29 December 2015) ABSTRACT Two surface waves can interact to produce an internal gravity wave by nonlinear resonant coupling. The process has been called spontaneous creation (SC) because it operates without internal waves being initially present. Previous studies have shown that the generated internal waves have high frequency close to the local Brunt–Väisälä frequency and wavelengths that are much larger than those of the participating surface waves, and that the spectral transfer rate of energy to the internal wave field is small compared to other generation processes. The aim of the present analysis is to provide a global map of the energy transfer into the internal 2 wave field by surface–internal wave interaction, which is found to be about 10 3 TW in total, based on a realistic wind-sea spectrum (depending on wind speed), mixed layer depths, and stratification below the mixed layer taken from a state-of-the-art numerical ocean model. Unlike previous calculations of the spectral transfer rate based on a vertical mode decomposition, the authors use an analytical framework that directly derives the energy flux of generated internal waves radiating downward from the mixed layer base. Since the radiated waves are of high frequency, they are trapped and dissipated in the upper ocean. The radiative flux thus feeds only a small portion of the water column, unlike in cases of wind-driven near-inertial waves that spread over the entire ocean depth before dissipating. The authors also give an estimate of the interior dis- sipation and implied vertical diffusivities due to this process. In an extended appendix, they review the modal description of the SC interaction process, completed by the corresponding counterpart, the modulation in- teraction process (MI), where a preexisting internal wave is modulated by a surface wave and interacts with another one. MI establishes a damping of the internal wave field, thus acting against SC. The authors show 2 that SC overcomes MI for wind speeds exceeding about 10 m s 1. 1. Introduction generation by mesoscale eddies or the mean flow (Nikurashin and Ferrari 2011) and dissipation of bal- Internal gravity waves have a major share of the en- anced flow (Molemaker et al. 2010), but there are sev- ergy contained in oceanic motions. Important sources eral other processes that can generate internal waves are waves radiating out of the mixed layer because of (e.g., Olbers 1983), among which is the forcing by non- near-inertial motions excited by wind fluctuations (see, linear coupling to wind-generated surface waves, the e.g., Alford 2001; Rimac et al. 2013) and waves generated topic of the present study. If internal waves break, their at the ocean bottom, originating from the conversion of energy is partly used to mix the mean stratification. barotropic into baroclinic tides at submarine topography Although this diapycnal mixing is weak in the interior, (see, e.g., Falahat et al. 2014). Other forcing functions such breaking internal waves generate large amounts of that have been studied in recent years are lee-wave potential energy that then drives large-scale oceanic motions. Unlike the internal wave energy generated by near-inertial motions in the mixed layer and the tides Corresponding author address: Carsten Eden, Institut für Meereskunde, Universität Hamburg, Bundesstrasse 53, Hamburg that have low frequencies and propagate through the 20146, Germany. entire water column, internal waves generated by reso- E-mail: [email protected]. nant interactions of surface waves are of high frequency DOI: 10.1175/JPO-D-15-0064.1 Ó 2016 American Meteorological Society Unauthenticated | Downloaded 10/01/21 05:50 AM UTC 2336 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 46 and thus likely to be trapped in the upper ocean. The all, such that we focus on the spontaneous creation interaction of surface and internal waves thus could process in the main part of the present study but con- contribute to mixing just below the mixed layer. sider both processes in appendix C. The relative im- Since the stress (vertical flux of horizontal momen- portance of SC and MI interactions can be assessed tum) induced by linear surface waves is zero (see, e.g., easily, realizing that the cross section of the interactions Hasselmann 1970), nonlinear effects need to be con- is identical when formulating the scattering in terms of sidered, and the evaluation of the transfer of energy the action spectra. We show in appendix C that the ratio 5 from surface to internal waves enters the field of non- of MI to SC is (Ucrit/U) , with a critical wind speed 2 linear resonant interactions among the wave branches. around 10–15 m s 1, depending on the Brunt–Väisälä The process has been investigated before, however, with frequency and the parameters of the surface and internal disperse and partly inconclusive results. Restricting the wave spectra. discussion to studies that deal with the interactions of Although the flux by interaction with surface waves is waves in random fields (see, e.g., Hasselmann 1966, lower than other sources of internal waves in the main 1967), only a few remain: Kenyon (1968) computed the pycnocline even for strong winds, it might still be im- transfer from an observed swell spectrum to the first portant since it is likely to be dissipated just below the baroclinic wave mode in shallow water and concluded mixed layer. We thus revisit the process with the aim to that the observed level of internal wave energy cannot provide a globally integrated estimate of the flux and its be explained by the process. Olbers and Herterich (1979, interior dissipation. hereinafter OH79) analyzed the transfer to deep-water In section 2 we start by outlining the surface layer baroclinic wave modes from wind-sea spectra as a model with nonlinear forcing due to surface waves with a function of wind speed and for different stratifications. spectral and statistical treatment of the interaction. We They find that high-frequency internal waves are excited derive the coupling conditions to the ocean interior and with wavelengths that are large compared to the ones of the spectral energy flux into the internal wave field at the the surface waves, but at a rate that is small in general. mixed layer base in general form. Section 3 shows the Only for the case of a shallow mixed layer, high wind dependency of the energy flux on the parameters of a speeds, and strong stratification below the mixed layer, simple factorized wind-sea spectrum, the mixed depth, 2 internal waves are generated at the level of 0.1 mW m 2. and the Brunt–Väisälä frequency below the mixed layer This level is typically a lower limit of the energy transfer base. We present a parameterization of the energy trans- by near-inertial motions in the mixed layer (see, e.g., fer rate in terms of the local wind speed, mixed layer Rimac et al. 2016) and far below the transfer rates by the depth, and the Brunt–Väisälä frequency, resulting from a tides (see, e.g., Falahat et al. 2014). numerical evaluation of the spectral transfer integral. A Watson (1990, hereinafter W90; 1994, hereinafter global estimate of the flux is provided, and its interior W94) has analyzed surface-internal wave interactions dissipation is based on realistic parameters. The last sec- in a more general setting. W90 applies the modal in- tion contains a summary and conclusions. teraction framework for idealized conditions, and W94 presents a calculation of energy transfers for the North 2. Theoretical framework Pacific. As OH79, Watson considers what he calls spontaneous creation (SC)—two surface waves generate We abandon here the baroclinic vertical mode frame- an internal wave—and he also discusses a modulation work used by OH79, W90, W94, and others and instead interaction process (MI), where a preexisting internal directly calculate the energy flux of internal waves radi- wave is modulated by a surface wave and interacts with ating from the surface mixed layer into the interior another surface wave. MI is included in the general stratified ocean. Radiation physics rather than modes, framework of OH79 but was not evaluated. It was first similar to our approach, are also considered by Moehlis studied in detail by Dysthe and Das (1981). By this in- and Llewellyn Smith (2001) for a wind-driven case. We teraction, following W90 and W94, the internal mode believe that this is a much simpler framework than the loses energy to the surface waves. The SC process, an- one by OH79 and W90, since the flux can be interpreted alyzed in the present study, cannot be separated in as being induced by ‘‘surface wave pumping,’’ generated Watson’s calculations from the MI process, but from his by the wave-induced vertical velocity in the surface layer, results we note that the MI dominates SC up to a critical much like ‘‘Ekman pumping’’ is induced by the di- wind speed. In strong wind regimes, the SC process vergence of the wind stress and ‘‘inertial pumping’’ is overcomes the MI process. Note that this regime (high induced by the divergence of wind-generated, near- winds, low mixed layer depths) is the one where the SC inertial waves in the mixed layer (Gill 1984). In fact, our surface-internal wave interactions become relevant at formulation would allow us to treat these surface-related Unauthenticated | Downloaded 10/01/21 05:50 AM UTC AUGUST 2016 O L B E R S A N D E D E N 2337 processes in a similar way to the surface wave forcing we where u is the horizontal velocity, p is the pressure consider here.