Propagation Of Near-inertial Waves Research Articles

Near-inertial response of a salinity-stratified ocean

Abstract We study the near-inertial response of the salinity-stratified north Bay of Bengal to monsoonal wind forcing using six years of hourly observations from four moorings. The mean annual energy input from surface winds to near-inertial mixed-layer currents is 10–20 kJ/m2, occurring mainly in distinct synoptic “events” from April to September. A total of fifteen events are analyzed: Seven when the ocean is capped by a thin layer of low-salinity river water (fresh) and eight when it is not (salty). The average near-inertial energy input from winds is 40% higher in the fresh cases than in the salty cases. During the fresh events, (A) mixed layer near-inertial motions decay about two times faster, and (B) near-inertial kinetic energy below the mixed layer is reduced by at least a factor of three relative to the salty cases. The near-inertial horizontal wavelength was measured for one fresh and one salty event; the fresh was about three times shorter initially. A linear model of near-inertial wave propagation tuned to these data reproduces (B); the thin (10 m) mixed layers during the fresh events excite high modes, which propagate more slowly than the low modes excited by the thicker (40 m) mixed layers in the salty events. The model does not reproduce (A); the rapid decay of the mixed layer inertial motions in the fresh events is not explained by linear wave propagation at the resolved scales; a different and currently unknown set of processes is likely responsible.

Effect of the different stratification changes along the two sides of a typhoon track on near-inertial energy propagation

The β-effect and mesoscale vorticity are considered the two important mechanisms guiding the downward propagation of near-inertial energy (NIE), while the influence of stratification is often neglected. In this study, the “heat pumping” and “cold suction” effects of Typhoon Kalmaegi on ocean stratification were analyzed. Enhanced stratification accelerated the vertical propagation of near-inertial waves (NIWs) from the mixed layer into the ocean interior along the left side of the typhoon track. To illustrate the impact of stratification changes on both sides of typhoon tracks, the Regional Ocean Modeling System was used to simulate the NIWs generated by Typhoon Kalmaegi in 2014. The results show that, despite higher wind energy input along the right side of the typhoon track, the NIE injection into the deep ocean along the left side was stronger owing to reduced dissipation and enhanced stratification. This study improves our understanding of the generation, propagation, and dissipation mechanisms of wind-induced NIWs during typhoons.

Downward Propagation and Trapping of Near-Inertial Waves by a Westward-Moving Anticyclonic Eddy in the Subtropical Northwestern Pacific Ocean

Abstract Mesoscale eddies can alter the propagation of wind-generated near-inertial waves (NIWs). Different from previous studies, the subsurface mooring observed NIWs are generated outside an anticyclonic eddy (ACE) and then interact with the arriving ACE. It is found that with the arrival of the ACE, the NIWs accelerate to propagate downward and the maximum vertical wavelength and group velocity of NIWs reach ∼500 m and ∼35 m day−1, respectively. When entering the core of the ACE, the near-inertial energy is trapped and finally stalls at a critical depth, which basically corresponds to the base of the ACE located at around 750-m depth. Through a ray-tracing model and dynamic analyses, this critical depth is much deeper than that of NIWs generated directly inside an ACE. By using depth–time varying stratification and relative vorticity, ray-tracing experiments further demonstrate that NIWs generated outside and passed over by an ACE can propagate to deep depths. Furthermore, energy budget analyses indicate that the net energy transfer from the ACE to NIWs plays an important role in the enhancement of downward-propagating near-inertial energy and its long-term persistence (∼45 days) in the critical layer. Within the critical layer, the enhancement of shear instability and nonlinear interactions among internal waves account for the loss of the trapped near-inertial energy and provide energy for furnishing deep ocean mixing. Significance Statement The interactions between near-inertial waves and a westward-moving anticyclonic eddy are investigated in this study. Knowledge about the propagation of near-inertial waves continues to be a topic of interest because near-inertial waves transfer energy from the mixed layer to the interior ocean, which is an important source of turbulent mixing. While much is known about how near-inertial energy propagates inside an anticyclonic eddy, few studies have examined how near-inertial energy propagates when it is generated outside an anticyclonic eddy and then enters the arriving anticyclonic eddy. In this study, the deep propagation and trapping of near-inertial energy by a westward-moving anticyclonic eddy is observed, which contributes greatly to the energy budget and the deep-ocean mixing.

Near-inertial waves generated by typhoon MITAG under the influence of anticyclonic eddy east of Taiwan

Based on subsurface mooring observations and HYCOM data, a complete investigation was conducted of the near-inertial waves (NIWs) caused by Typhoon MITAG to the east of Taiwan. HYCOM data were mainly used to reveal the role played by anticyclonic eddies in the propagation of NIWs. The results show that most typhoon-generated NIWs propagate towards negative vorticity, and NIWs near the edge gradually accumulated towards the eddy center and down to 800 m. NIWs propagating through the thermocline to the deep ocean were mainly concentrated in the eddy, and the near-inertial energy flux showed a significant enhancement from 400 to 600 m. Moreover, the downwards propagation of NIWs in the eddy enhanced the kinetic energy of background flow. NIWs outside the anticyclonic eddy dissipated quickly, while inside the eddy, there were high value areas of e-folding time. Dynamic mode decomposition illustrates that the anticyclonic eddy mainly captures higher modes of NIWs, and the state of continuous energy growth of higher modes can be maintained for more than a week. In addition, NIWs can also be carried westwards by the advection of the mean background flow at the eddy’s edge.

Near-inertial wave propagation between stratified and homogeneous layers

Near-inertial wave propagation between stratified and homogeneous layers

Enhanced Near-Inertial Waves and Turbulent Diapycnal Mixing Observed in a Cold- and Warm-Core Eddy in the Kuroshio Extension Region

Abstract Shipboard observations of upper-ocean current, temperature–salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in a cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6f. The mode regression method was proposed to extract NIWs from the shipboard ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean. Significance Statement We provide observational evidence for the downward propagation of near-inertial waves enhanced by mesoscale eddies. This is significant because the down-taking of wind energy by the near-inertial waves is an important energy source for turbulent mixing in the interior ocean, which is essential to the shaping of ocean circulation and climate. The anticyclonic eddies are widely regarded as a conduit for the downward near-inertial energy propagation, while the cyclonic eddies activity influencing the near-inertial waves propagation lacks clear cognition. In this study, enhanced near-inertial waves and turbulent dissipation were observed inside both cyclonic and anticyclonic eddies in the Kuroshio Extension region, which has significant implications for improving the parameterization of turbulent mixing in ocean circulation and climate models.

Observed Near-Inertial Waves in the Northern South China Sea

Characteristics of near-inertial waves (NIWs) induced by the tropical storm Noul in the South China Sea are analyzed based on in situ observations, remote sensing, and analysis data. Remote sensing sea level anomaly data suggests that the NIWs were influenced by a southwestward moving anticyclonic eddy. The NIWs had comparable spectral density with internal tides, with a horizontal velocity of 0.14–0.21 m/s. The near-inertial kinetic energy had a maximum value of 7.5 J/m3 and propagated downward with vertical group speed of 10 m/day. Downward propagation of near-inertial energy concentrated in smaller wavenumber bands overwhelmed upward propagation energy. The e-folding time of NIWs ranged from 4 to 11 days, and the larger e-folding time resulted from the mesoscale eddies with negative vorticity. Modified by background relative vorticity, the observed NIWs had both red-shifted and blue-shifted frequencies. The upward propagating NIWs had larger vertical phase speeds and wavelengths than downward propagating NIWs. There was energy transfer from the mesoscale field to NIWs with a maximum value of 8.5 × 10−9 m2 s−3 when total shear and relative vorticity of geostrophic currents were commensurate. Our results suggest that mesoscale eddies are a significant factor influencing the generation and propagation of NIWs in the South China Sea.

Generation and Propagation of Near-Inertial Waves in a Baroclinic Current on the Tasmanian Shelf

AbstractNear-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency feff, while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves ωmin. On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where feff was less than the Coriolis frequency f. If everywhere feff > f then NIWs were generated where ωmin < f and feff was smallest. The background current not only affected the location of generation, but also the NIWs’ propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly toward the equator because ωmin > f on the continental slope for the entire sample period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency ω within the anomalously low-frequency band (i.e., ωmin < ω < feff), which only exists if the baroclinicity is nonzero. We observed two periods when ωmin < f on the shelf, but the relative vorticity was positive (i.e., feff > f) for one of these periods. The process of NIW propagation remained consistent with the local ωmin, and not feff, emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs.

Observed Near Inertial Waves in the Wake of Typhoon Linfa (2015) in the Northern South China Sea

This study examined the characteristics and vertical propagation of near inertial waves (NIWs) induced by Typhoon Linfa (2015), based on in situ observations conducted southeast of Dongsha Islands in the South China Sea. The results demonstrate that the near inertial currents induced by Linfa had velocities up to 35cm s−1 in the mixed layer and 20cm s−1 in the ocean interior. The near inertial currents were polarized with predominantly clockwise-rotating components, the magnitudes of which were about 10 times larger than the counter-clockwise rotating components. The energy density spectrum showed that the emergence of NIWs resulted in energy redistribution from the diurnal band to the near inertial band. The wavenumber spectrum and the downward/ upward current decomposition demonstrated that the NIWs and energy flux propagated mainly downward. The estimated vertical phase velocity and group velocity are 1.44 and 0.48m h−1, respectively, corresponding to a vertical wavelength of 49.7m. The e-folding time scale was 7.5 d based on the near inertial kinetic energy in the ocean interior. We found no obvious wave-wave interaction during the decay process of the NIWs. The frequency was blue-shifted, being 0.03f0 higher than the local inertial frequency, which was caused by the background vorticity. The normal mode analysis suggests that the higher mode plays a dominant role in the propagation stage of the NIWs.

An improved model of near-inertial wave dynamics

The YBJ equation (Young & Ben Jelloul, J. Marine Res., vol. 55, 1997, pp. 735–766) provides a phase-averaged description of the propagation of near-inertial waves (NIWs) through a geostrophic flow. YBJ is obtained via an asymptotic expansion based on the limit $\mathit{Bu}\rightarrow 0$, where $\mathit{Bu}$ is the Burger number of the NIWs. Here we develop an improved version, the YBJ+ equation. In common with an earlier improvement proposed by Thomas, Smith & Bühler (J. Fluid Mech., vol. 817, 2017, pp. 406–438), YBJ+ has a dispersion relation that is second-order accurate in $\mathit{Bu}$. (YBJ is first-order accurate.) Thus both improvements have the same formal justification. But the dispersion relation of YBJ+ is a Padé approximant to the exact dispersion relation and with $\mathit{Bu}$ of order unity this is significantly more accurate than the power-series approximation of Thomas et al. (2017). Moreover, in the limit of high horizontal wavenumber $k\rightarrow \infty$, the wave frequency of YBJ+ asymptotes to twice the inertial frequency $2f$. This enables solution of YBJ+ with explicit time-stepping schemes using a time step determined by stable integration of oscillations with frequency $2f$. Other phase-averaged equations have dispersion relations with frequency increasing as $k^{2}$ (YBJ) or $k^{4}$ (Thomas et al. 2017): in these cases stable integration with an explicit scheme becomes impractical with increasing horizontal resolution. The YBJ+ equation is tested by comparing its numerical solutions with those of the Boussinesq and YBJ equations. In virtually all cases, YBJ+ is more accurate than YBJ. The error, however, does not go rapidly to zero as the Burger number characterizing the initial condition is reduced: advection and refraction by geostrophic eddies reduces in the initial length scale of NIWs so that $\mathit{Bu}$ increases with time. This increase, if unchecked, would destroy the approximation. We show, however, that dispersion limits the damage by confining most of the wave energy to low $\mathit{Bu}$. In other words, advection and refraction by geostrophic flows does not result in a strong transfer of initially near-inertial energy out of the near-inertial frequency band.

Poleward propagation of near-inertial waves induced by fluctuating winds over a baroclinically unstable zonal jet

We investigate the excitation and radiation of near-inertial internal gravity waves continuously excited by a latitudinally confined temporally fluctuating wind in a numerical model of a stratified ocean on a $\unicode[STIX]{x1D6FD}$-plane at mid-latitude. The surface wind forcing contains both high- and low-frequency components which excite propagating waves and a baroclinically unstable zonal jet respectively. Wentzel–Kramers–Brillouin (WKB) ray theory implies that near-inertial waves propagate strictly towards the equator. We seek to refine this view here by (i) adding the non-traditional Coriolis force (accounting for the horizontal component of the Earth’s rotation) into the equations of motion, in order to allow poleward sub-inertial propagation to occur, and (ii) relaxing the conceptual constraint of no zonal variability, to allow the zonal jet to undergo instability, to meander and to sustain an active field of mesoscale eddies, potentially impacting the excitation of near-inertial waves. The key results are that, while (i) permits weakly stratified waveguides with sub-inertial poleward wave propagation to develop in accord with theory, the sub-inertial energy flux observed is very small compared with the equatorward flux. Thus, in terms of energy radiated from the storm track, non-traditional effects are small for wind-driven near-inertial waves. The consequences of (ii) are much more pronounced. Refinement (ii) produces a radiating wave field that is bidirectional, i.e. with both poleward and equatorward components. We show that the presence of regions of significant background vorticity with horizontal scales significantly smaller than the width of the storm track provides the scale selection mechanism to excite waves with sufficiently super-inertial frequencies to propagate poleward distances of the order of 1000 km.

PSI of the Internal Tide on a β Plane: Flux Divergence and Near-Inertial Wave Propagation

Abstract The dynamics of a forced, low-mode oceanic internal tide propagating poleward on a β plane are investigated numerically. The focus is on the transfer of energy from the tide to near-inertial oscillations (NIOs) initiated by a weakly nonlinear interaction known as parametric subharmonic instability (PSI). It is shown that PSI is a mechanism for generating NIOs in the upper ocean, which subsequently radiate to depth. The exponentially growing NIOs eventually reach finite amplitude, and further interaction with the tide leads to a quasi-steady state in which dissipation is balanced by a reduction in the poleward tidal flux. The results are sensitive to the prescribed value of the vertical eddy viscosity νe that serves to parameterize the background turbulence. This sensitivity suggests that independent processes leading to turbulence in the upper ocean are able to control the rate of energy transfer from the tide to NIOs. For νe = O(10−5 m2 s−1), the poleward tidal flux decreases approximately 15%. This value is much smaller than was found in previous numerical studies, but it is in reasonable agreement with recent estimates from observations taken near the M2/2 inertial latitude in the Pacific.

Inertial oscillations as deep ocean response to hurricanes

We discuss the deep ocean response to passing hurricanes (aka typhoons), which are considered as generators of near-inertial, internal waves. The analysis of data collected in the northwestern parts of the Pacific and Atlantic oceans in the hurricane season permit us to assess the deep ocean response to such a strong atmospheric forcing. A large number of moorings (more than 100) in the northwestern Pacific have allowed us to characterize the spatial features of the oceanic response to typhoons and the variable downward velocity of near-inertial wave propagation. The velocity of their downward propagation varies in the range 1–10 m/hour. It is higher in the regions of low stratification and high anticyclonic vorticity. The inertial oscillations generated by a hurricane last for 10–12 days. The mean anticyclonic vorticity in the region increases the effective frequency of inertial oscillations by 0.001–0.004 cyc/hour.

Seasonal and Spatial Variability of Near-Inertial Kinetic Energy from Historical Moored Velocity Records

Abstract Temporal and spatial patterns of near-inertial kinetic energy (KEmoor) are investigated in a database of 2480 globally distributed, moored current-meter records (deployed on 690 separate moorings) and compared with the distribution of wind-forced mixed-layer energy flux FML. By computing KEmoor using short (30 day) multitaper spectral windows, the seasonal cycle is resolved. Clear winter enhancement by a factor of 4–5 is seen in the Northern Hemisphere for latitudes 25°–45° at all depths <4500 m, in close agreement with the magnitude, phase, and latitudinal dependence of the seasonal cycle of FML. In the Southern Hemisphere, data coverage is poorer, but a weaker seasonal cycle (a factor of 2) in both KEmoor and FML is still resolvable between 35° and 50°. When Wentzel–Kramers–Brillouin (WKB) scaled using climatological buoyancy-frequency profiles, summer KEmoor is approximately constant in depth while showing a clear decrease by a factor of 4–5 from 500 to 3500 m in winter. Spatial coverage is sufficient in the Northern Hemisphere to resolve broad KEmoor maxima in the western portion of each ocean basin in winter, generally collocated with FML maxima associated with storm forcing. The ratio of depth-integrated KEmoor to FML gives a replenishment time scale, which is about 10 days in midlatitudes, consistent with 1) previous estimates of the dissipation time scale of the internal wave continuum and 2) the presence of a seasonal cycle. Its increase to ≈70–80 days at lower latitudes is a possible signature of equatorward propagation of near-inertial waves. The seasonal modulation of the magnitude of KEmoor, its similarity to that in FML, and the depth decay and western intensification in winter but not in summer are consistent with a primarily wind-forced near-inertial field for latitudes poleward of ≈25°.

The Effect of a Bottom Shelf Front upon the Generation and Propagation of Near-Inertial Internal Waves in the Coastal Ocean

Abstract A three-dimensional nonlinear baroclinic model in cross-sectional form is used to study the generation and propagation of wind-forced near-inertial internal waves in a coastal region in the presence of a bottom front. Initially calculations are performed with the front in an infinite domain region. By this means coastal effects are removed. The initial response is in terms of inertial oscillations in the surface layer. However, in the frontal area these are modified by interaction through the nonlinear momentum terms with regions of positive and negative vorticity associated with the alongfront flow. This leads to a change in amplitude, phase, and frequency of the inertial current, and a resulting Ekman pumping that drives near-inertial internal waves in the frontal region. On the positive vorticity side of the front these waves are at the superinertial frequency and rapidly propagate away. On the negative side they are at the subinertial frequency and are trapped and inertial energy leaks to depth. Calculations with a coastal boundary and no front show that the offshore propagation of near-inertial waves is similar to that found on the positive vorticity side of the front. This shows that in terms of superinertial internal wave generation and propagation the front acts in a similar manner to a coastal boundary. However, with a coastal boundary, inertial currents below the thermocline are phase shifted by 180° from those above. This phase shift is only found in the frontal case when the front is adjacent to a coast and is due to the no-normal-flow condition at the coast. For the case of a stratified region between the coast and the front, near-inertial energy is trapped in this region and is dissipated at a rate depending upon the local value of vertical eddy viscosity. The offshore spatial distribution of near-inertial internal waves is found to be independent of whether the coastal region is represented by a vertical wall or a more realistic sloping seabed. However, in the case of a weak coastal front, the near-frontal distribution of near-inertial energy is influenced by the horizontal spatial variability of stratification within the front.

Near-Inertial Wave Propagation in the Western Arctic

Abstract From October 1997 through October 1998, the Surface Heat Budget of the Arctic (SHEBA) ice camp drifted across the western Arctic Ocean, from the central Canada Basin over the Northwind Ridge and across the Chukchi Cap. During much of this period, the velocity and shear fields in the upper ocean were monitored by Doppler sonar. Near-inertial internal waves are found to be the dominant contributors to the superinertial motion field. Typical rms velocities are 1–2 cm s−1. In this work, the velocity and shear variances associated with upward- and downward-propagating wave groups are quantified. Patterns are detected in these variances that correlate with underlying seafloor depth. These are explored with the objective of assessing the role that these extremely low-energy near-inertial waves play in the larger-scale evolution of the Canada Basin. The specific focus is the energy flux delivered to the slopes and shelves of the basin, available for driving mixing at the ocean boundaries. The energy and shear variances associated with downward-propagating waves are relatively uniform over the entire SHEBA drift, independent of the season and depth of the underlying topography. Variances associated with upward-propagating waves follow a (depth)−1/2 dependence. Over the deep slopes, vertical wavenumber spectra of upward-propagating waves are blue-shifted relative to their downward counterparts, perhaps a result of reflection from a sloping seafloor. To aid in interpretation of the observations, a simple, linear model is used to compare the effects of viscous (volume) versus underice (surface) dissipation for near-inertial waves. The latter is found to be the dominant mechanism. A parallel examination of the topography of the western Arctic shows that much of the continental slope is close to critical for near-inertial wave reflection. The picture that emerges is consistent with “one bounce” rather than trans-Arctic propagation. The dominant surface-generated waves are substantially absorbed in the underice boundary layer following a single roundtrip to the seafloor. However, surface-generated waves can interact strongly with nearby (<300 km) slopes, potentially contributing to dissipation rates of order 10−6–10−7 W m−3 in a zone several hundred meters above the bottom. The waves that survive the reflection process (and are not back-reflected) display a measurable blue shift over the slopes and contribute to the observed dependence of energy on seafloor depth that is seen in these upper-ocean observations.

Near-Inertial Wave Propagation into the Pycnocline during Ocean Storms: Observations and Model Comparison

Abstract Observations of near-inertial oscillations collected during the Ocean Storms Experiment in the northeast Pacific Ocean are compared with results from a linear, numerical model on a β plane, developed by Zervakis and Levine. A slab mixed layer model, forced by the observed wind time series, is used to identify three isolated events of local generation in October, January, and March for detailed analysis. Synoptic storm track maps are used to estimate the initial horizontal wavenumber of the mixed layer currents that are used as initial conditions to the modal. A comparison of the modal with the observed currents reveals some differences and similarities. Overall the January and March events are better represented by the model than the October event. The timescale of the initiation of vertical propagation of energy from the mixed layer occurs almost immediately in October rather than after 8 days in January and March-this difference cannot be explained by the model. The observed vertical and tempor...

The Structure of Near-Inertial Waves during Ocean Storms

Abstract Current meter data from two sites were analyzed for near-inertial motions generated by storm during the ten-month period of the Ocean Storms Experiment in the northeast Pacific Ocean. The most striking feature of the inertial wave response to storms was the almost instantaneous generation of waves in the mixed layer, followed by a gradual propagation into the thermocline that often lasted many days after the initiation of the storm. The propagation of near-inertial waves generated by three storms in October, January, and March was studied by using group propagation theory based on the WKB approximation. It was found that wave frequencies were slightly superinertial, with inertial shifts 1%–3% in October and March and around 1% in January. The phase of near-inertial currents propagated upward below the mixed layer, confirming the downward radiation of energy by these waves. The average downward energy flux during the storm periods was between 0.5 and 2.8 mW m−2. The vertical wavelengths indicated ...

Dispersion of wind-induced inertial waves by a barotropic jet

This note attempts to reinterpret previous results on the dispersion of wind-induced inertial waves by a geostrophic barotropic jet in the ocean. The approach is to consider the jet vorticity influence on the different baroclinic modes using a vertical normal mode expansion. Numerical and analytical analysis of the linear equations shows that vorticity effects on a single baroclinic mode strongly depend on the ratio of its Rossby radius and the length scale of the geostrophic vorticity: trapping of the near-inertial energy occurs when this ratio is small. When this ratio is of order one, inertial waves are almost unaffected by the geostrophic vorticity because dispersion efficiently overcomes the jet vorticity effects. A 2-D primitive-equation model is used to examine the scattering of wind-induced inertial waves in realistic situations. Results indicate that contribution of the lowest baroclinic modes, unaffected by the jet vorticity, explain some striking features reported in previous studies as the downward phase propagation of near-inertial waves in the positive vorticity region

Near-Inertial Wave Propagation In Geostrophic Shear

Abstract An approximate dispersion relation for near-inertial internal waves propagating in geostrophic shear is formulated that includes straining by the mean flow shear. Near-inertial and geostrophic motions have similar horizontal scales in the ocean. This implies that interaction terms involving mean flow shear of the form (v·Δ)V as well as the mean flow itself [(V·Δ)v] must be retained in the equations of motion. The vorticity ζ shifts the lower bound of the internal waveband from the planetary value of the Coriolis frequency f to an effective Coriolis frequency feπ = f + ζ/2. A ray tracing approach is adopted to examine the propagation behavior of near-inertial waves in a model geostrophic jet. Trapping and amplification occur in regions of negative vorticity where near-inertial waves' intrinsic frequency &omega0 can be less than the effective Coriolis frequency of the surrounding ocean. Intense downward-propagating near-inertial waves have been observed at the base of upper ocean negative vorticity...