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Mooring data collected at a flat‐topped seamount suggest the generation of pure inertial waves (PIWs; waves with a dominant frequency equal to the local inertial frequency f) by low‐frequency flows over large‐scale topography. Energetic PIWs were observed within a narrow depth range (∼100 m) near the seafloor at the edge of the summit. These waves could be associated with low‐frequency flows. A two‐dimensional nonhydrostatic model was used to show that the observed PIWs are most likely internal wave beams generated by low‐frequency flows over the seamount. Two types of PIWs were identified via observation and model. The first is a PIW that can only travel horizontally. The other propagates upward, with super‐inertial intrinsic frequency that is Doppler‐shifted by the flows to f. Nonlinear triadic interactions among waves with the frequencies [0, f, f] may transfer energy from mean flows to PIWs, promoting energy decay of geostrophic flows over large‐scale topography. Plain Language Summary Near‐inertial waves (NIWs) are intrinsic motions of the ocean caused by Earth’s rotation. Understanding their generation and evolution is key to parameterizing NIWs and the associated mixing in global ocean circulation models. This study presents novel observations suggesting that energetic pure inertial waves (PIWs; i.e., NIWs with a dominant frequency appearing at the local inertial frequency) are generated by low‐frequency flows over large‐scale seamount topography in the abyssal ocean. The nonlinear forcing of low‐frequency flows due to their acceleration near the seamount summit is suggested to be essential for generating PIWs. These findings provide further insight into NIWs in the abyssal ocean, which is important for understanding the distribution of NIWs and the energy decay of geostrophic flows over topography. Key Points Energetic pure inertial waves (PIWs) were observed within a narrow depth range near the seafloor at a large abyssal seamount Near‐bottom‐enhanced PIWs stem from nonlinear forcing of low‐frequency flows over the seamount Low‐frequency flows over the seamount generate two types of PIWs: nonvertically propagating and upward‐propagating

The influence of turbulent ocean mixing transcends its inherently small scales to affect large scale ocean processes including water‐mass transformation, stratification maintenance, and the overturning circulation. However, the distribution of ocean mixing is not well described by sparse ship‐based observations since this mixing is both spatially patchy and temporally intermittent. We use strain information from Argo float profiles in the upper 2,000 m of the ocean to generate over 400,000 estimates of the energy dissipation rate, indicative of ocean mixing. These estimates rely on numerous assumptions, and do not take the place of direct measurement methods. Temporally averaged estimates reveal clear spatial patterns in the parameterized dissipation rate and diffusivity distribution across all the oceans. They corroborate previous observations linking elevated dissipation rates to regions of rough topography. We also observe heightened estimated dissipation rates in areas of high eddy kinetic energy, as well as heightened diffusivity in high latitudes where stratification is weak. The seasonal dependence of mixing is observed in the Northwest Pacific, suggesting a wind‐forced response in the upper ocean. Key Points Argo floats can be used to estimate the turbulent mixing in the global ocean Spatial patterns of mixing are apparent (e.g., elevation over rough topography) Temporal patterns of mixing are also apparent (e.g., seasonal cycles)

Near‐inertial waves (NIWs) are essential energy sources for deep diapycnal mixing. However, the mechanisms for generating deep and abyssal NIWs are largely unknown. Here, using 3 year full‐depth mooring data at 130°E/15°N, we demonstrate that downward propagating tropical cyclone (TC)‐induced NIWs and parametric subharmonic instability (PSI) can contribute to the intensification of near‐inertial kinetic energy (NIKE) over 1,000–3,000 m. The deep warm core eddy (WCE) is found to facilitate the downward propagation of TC‐induced NIWs with fast vertical group velocity and the intensified PSI efficiency that improves the energy transfer from diurnal internal tides (DITs) to NIWs at the site poleward of critical latitude for DITs. Quantitative regression analysis suggests that compared to the PSI, downward penetrating TC‐induced NIWs contribute more to the deep NIKE. Our study emphasizes that accurately representing deep eddy activities and TC's wind intensity in numerical models is crucial to simulating the deep NIKE. Plain Language Summary Near‐inertial waves (NIWs) contain a pronounced portion of energy in the ocean interior and play an important role in maintaining the global meridional overturning circulations. However, sources of deep and abyssal NIWs are less known due to the lack of in‐situ observations. Here, we demonstrate that TC‐induced NIWs and parametric subharmonic instability (PSI) are conducive to generating deep near‐inertial kinetic energy (NIKE) based on a 3 year mooring measurement at 130°E/15°N. The warm core eddy (WCE) can facilitate the downward propagation of TC‐induced NIWs, which can be inferred from the positive correlation between eddy kinetic energy and vertical group velocity. Although the mooring location is poleward of the critical latitude for diurnal internal tides (DITs), the PSI of DITs can efficiently generate NIKE when the negative vorticity of WCE is presented. The linear regression analyses suggest that under WCE modulation, the downward propagating TC‐induced NIWs can explain more than 50% of the total variance of deep NIKE, while the PSI of DITs can only explain less than 30%. Key Points Mooring measurements reveal that downward propagating TC‐induced NIWs and PSI contribute to intensified near‐inertial energy below 1,000 m Deep warm core eddies improve NIW's vertical group velocity and PSI efficiency at site north of diurnal tidal critical latitude TC‐induced NIWs and PSI of internal tide explain more than 50% and less than 30% of variance of deep near‐inertial energy, respectively

Fifty-seven days of moored current records are examined, focusing on the sequential passage of Typhoons Nesat and Nalgae separated by 5 days in the northwestern South China Sea. Both typhoons generated strong near-inertial waves (NIW) as detected by a moored array, with the near-inertial velocity to the right of the typhoon path significantly larger than to the left. The estimated vertical phase and group velocities of the NIW induced by Typhoon Nesat are 0.2 cm s −1 and 0.85 m h −1 , respectively, corresponding to a vertical wavelength of 350 m. Both the vertical phase and group velocities of the NIW induced by Typhoon Nalgae are lower than those of Typhoon Nesat, with the corresponding vertical wavelength only one-half that of Nesat. The threshold values of induced near-inertial kinetic energy (NIKE) of 5 J m −3 reach water depths of 300 and 200 m for Typhoons Nesat and Nalgae, respectively, illustrating that the NIKE induced by Typhoon Nesat dissipated less with depth. Obvious blueshifts in the induced NIW frequencies are also detected. The frequency of NIW induced by Typhoon Nesat significantly increases at water depths of 100–150 m because of Doppler shifting, but decreases significantly at water depths of 100–150 m for Nalgae because of the greater influence of the background vorticity during the passage of Typhoon Nalgae.

Seasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the Coastal and Regional Ocean Community high‐resolution numerical model with realistic background stratification and fluxes. In good agreement with remote‐sensing and in‐situ observations, our numerical simulations capture the seasonal cycle of the anomalies induced by the anticyclone, both in the sea surface temperature (SST) and in the mixed layer depth (MLD). The eddy signature on the SST shifts from warm‐core in winter to cold‐core in summer, while the MLD deepens significantly in the core of the anticyclone in late winter. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (∼4 m in near surface) and if the atmospheric forcing contains high‐frequency. In summer with this configuration, the vertical mixing parameterized by the k − ϵ closure scheme is three times higher inside the eddy than outside the eddy, and leads to an anticyclonic cold core SST anomaly. This differential mixing is explained by near‐inertial waves, triggered by the high‐frequency atmospheric forcing. Near‐inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter in the anticyclone core. On the other hand, eddy MLD anomaly appears more sensitive to horizontal resolution, and requires SST retroaction on air‐sea fluxes. These results detail the need of high frequency forcing, high vertical and horizontal resolutions to accurately reproduce the evolution of a mesoscale eddy. Plain Language Summary Mesoscale eddies are turbulent structures present in every regions of the world ocean, and accounting for a significant part of its kinetic energy budget. These structures can be tracked in time and recently revealed a seasonal cycle from in situ data. An anticyclone (clockwise rotating eddy in the northern hemisphere) is observed in the Mediterranean to be predominantly warm at the surface and to deepen the mixed layer in winter, but shifts to a cold‐core summer signature. This seasonal signal is not yet understood and studied in ocean models. In this study we assess the realism of an anticyclone seasonal evolution in high resolution numerical simulations. Eddy surface temperature seasonal shift is retrieved and is linked to an increased mixing at the eddy core spontaneously appearing at high vertical resolution (vertical grid size smaller than 4 m) in the presence of high frequency atmospheric forcing. This increased mixed is due to the preferred propagation of near‐inertial waves in the anticyclone due to its negative relative vorticity. Eddy‐induced mixed layer depth anomalies also appear to be triggered by sea surface temperature retroaction on air‐sea fluxes. These results suggest that present‐day operational ocean forecast models are too coarse to accurately retrieve mesoscale evolution. Key Points Enhanced mixing in anticyclones explain inverse eddy sea surface temperature (SST) signature Vertical resolution is crucial to model eddy core mixing triggered by near‐inertial waves Mixed layer anomaly is mainly driven by SST retroaction on air‐sea fluxes

The dynamics of near‐inertial motions, and their relation to mixing, is investigated here with an extensive data set, including turbulence and high‐resolution velocity observations from two cruises conducted in 2008 (summer) and 2010 (winter) in the Bornholm Basin of the Baltic Sea. In the absence of tides, it is found that the basin‐scale energetics are governed by inertial oscillations and low‐mode near‐inertial wave motions that are generated near the lateral slopes of the basin. These motions are shown to be associated with persistent narrow shear‐bands, strongly correlated with bands of enhanced dissipation rates that are the major source of mixing inside the permanent halocline of the basin. In spite of different stratification, near‐inertial wave structure, and atmospheric forcing during summer and winter conditions, respectively, the observed dissipation rates were found to scale with local shear and stratification in a nearly identical way. This scaling was different from the Gregg‐Henyey‐type models used for the open ocean, but largely consistent with the MacKinnon‐Gregg scaling developed for the continental shelf. Key Points First detailed observations are made of internal wave mixing during different seasons High dissipation in narrow shear‐bands is major source of mixing in halocline Internal wave mixing in the Baltic Sea follows scaling by MacKinnon and Gregg

Near-inertial waves (NIWs), a special form of internal waves with a frequency close to the local Coriolis frequency, are ubiquitous in the ocean. NIWs play a crucial role in ocean mixing, influencing energy transport, climate change, and biogeochemistry. This manuscript briefly reviews the generation and propagation of NIWS in the oceans. NIWs are primarily generated at the surface by wind forcing or through the water column by nonlinear wave-wave interaction. Especially at critical latitudes where the tidal frequency is equal to twice the local inertial frequency, NIWs can be generated by a specific subclass of triadic resonance, parametric subharmonic instability (PSI). There are also other mechanisms, including lee wave and spontaneous generation. NIWs can propagate horizontally for hundreds of kilometers from their generating region and radiate energy far away from their origin. NIWs also penetrate deep into the ocean, affecting nutrient and oxygen redistribution through altering mixing. NIW propagation is influenced by factors such as mesoscale eddies, background flow, and topography. This review also discussed some recent observational evidence of interactions between NIWs from different origins, suggesting a complicated nonlinear interaction and energy cascading. Despite the long research history, there are still many areas of NIWs that are not well defined.

The propagation of inertio-gravity waves (IGW) into the deep sea is relevant for energy transfer to turbulence where waves break, and thus for redistribution of nutrients, oxygen, and suspended matter. In constant stratification, vertical IGW propagation is readily modelled. In varying stratification, where homogeneous layers alternate with stratified layers, transmission, and reflection cause complex patterns. Half-year long moored acoustic Doppler current profiler (ADCP) observations midway between the Balearic Islands and Sardinia in the 2800-m deep Western-Mediterranean Sea occasionally demonstrate a distinct transition, between weakly stratified ( N  ≥ 2 f ) and homogeneous ( N  ≤  f ) layers, of IGW at near-inertial frequencies. Here, N denotes the buoyancy frequency and f denotes the local inertial frequency (vertical Coriolis parameter). The transition in stratification is rather abrupt, within Δ z  = 25 m, and provides an amplitude reduction of 1.3 for super-inertial motions. Simulations with non-traditional momentum equations involving the horizontal Coriolis parameter f h qualitatively confirm observed IGW refraction. The observational area is marked by variations in hydrographic characteristics, with abundant mesoscale eddies to the south and dense-water formation to the north of the site during the previous winter. Thus, also transitions occur from deep homogeneous layers into deeper, recently formed stratified ones. Polarization spectra of shear are bound by IGW-limits related to N  =  f , while current polarization to 2 f h . These frequencies coincide with large-scale buoyancy frequencies independently observed in various layers using shipborne CTD profiling.

An investigation of equatorial near-inertial wave dynamics under complete Coriolis parameters is performed in this paper. Starting from the basic model equations of oceanic motions, a Korteweg de Vries equation is derived to simulate the evolution of equatorial nonlinear near-inertial waves by using methods of scaling analysis and perturbation expansions under the equatorial beta plane approximation. Theoretical dynamic analysis is finished based on the obtained Korteweg de Vries equation, and the results show that the horizontal component of Coriolis parameters is of great importance to the propagation of equatorial nonlinear near-inertial solitary waves by modifying its dispersion relation and by interacting with the basic background flow.

The shear in the upper ocean of the northern South China Sea (SCS) is examined based on current observations from six moorings in 2010–2011. Spectral analysis results indicate that the sub-inertial currents, near-inertial waves (NIWs), diurnal and semidiurnal internal tides (ITs) dominate the current and shear in the northern SCS. Through comparing current variance and shear caused by these motions, this study shows the great contribution of NIWs to shear: Although NIWs only account for 2–7% of the total current variance, the shear caused by NIWs is approximate one fifth to one-quarter of the total shear. Moreover, the NIWs are dominated by the component with upgoing phase and downgoing energy, whereas the upgoing and downgoing components are comparable in both diurnal and semidiurnal ITs. Because the incoherent component has a larger contribution to shear than the coherent component, the shear of both diurnal and semidiurnal ITs exhibits significant signals with frequencies larger than the spring-neap cycles of approximate 14 days. The larger contribution to shear and smaller proportion in current variance suggest that the incoherent component of ITs has a larger vertical wavenumber than the coherent component. In addition, a case study shows that the mesoscale eddy pair occurring between 22 October and 2 December 2010 does not significantly enhance the ocean shear at two moorings especially below 150 m depth, although it contributes a lot to the current variance.