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Ryuichiro Inoue and Satoshi Osafune

Abstract

A part of near-inertial wind energies dissipates locally below the surface mixed layer. Here, their role in the climate system is studied by adopting near-inertial near-field wind-mixing parameterization to a coarse-forward ocean general circulation model. After confirming a problem of the parameterization in the equatorial region, we investigate effects of near-field wind mixing due to storm track activities in the North Pacific. We found that, in the center of the Pacific Decadal Oscillation (PDO) around 170°W in the mid latitude, near-field wind mixing transfers the PDO signal into deeper layers. Since the results suggest that near-field wind mixing is important in the climate system, we also compared the parameterization with velocity observations by a float in the North Pacific. The float observed abrupt and local propagation of near-inertial internal waves and shear instabilities in the main thermocline along the Kuroshio Extension for 460 km. Vertical diffusivities inferred from the parameterization do not reproduce the enhanced diffusivities in the deeper layer inferred from the float. Wave-ray tracing indicates that wave trapping near the Kuroshio front is responsible for the elevated diffusivities. Therefore, enhanced mixing due to trapping should be included in the parameterization.

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Ali Tamizi, Jose-Henrique Alves, and Ian R. Young

Abstract

A series of numerical experiments with the WAVEWATCH III spectral wave model are used to investigate the physics of wave evolution in tropical cyclones. Buoy observations show that tropical cyclone wave spectra are directionally skewed with a continuum of energy between locally generated wind-sea and remotely generated waves. These systems are often separated by more than 900. The model spectra are consistent with the observed buoy data and are shown to be governed by nonlinear wave-wave interactions which result in a cascade of energy from the wind-sea to the remotely generated spectral peak. The peak waves act in a “parasitic” manner taking energy from the wind-sea to maintain their growth. The critical role of nonlinear processes explains why one-dimensional tropical cyclone spectra have characteristics very similar to fetch-limited waves, even though the generation system is far more complex. The results also provide strong validation of the critical role nonlinear interactions play in wind-wave evolution.

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Xiaojie Yu, Xinyu Guo, Huiwang Gao, and Tao Zou

Abstract

Hydrographic surveys have revealed that the Yellow River plume propagates in the direction opposite to that of a Kelvin wave (upstream) under a low river discharge condition, but turns downstream as the river discharge increases. A numerical model reproduced the upstream extension of the plume under the low river discharge condition and the transition to the downstream direction under the high river discharge condition, and confirmed that the summer wind is not the necessary condition for upstream extension of the plume. With the condition of low river discharge, the model also indicated the dependence of the upstream extension of the plume on the tidal range: extending upstream in spring tide but turning downstream in neap tide. The upstream movement of the plume results from the upstream transport of freshwater that depends on the upstream tide-induced residual current around the river mouth and the downstream density-driven current around the offshore plume front. With the condition of high river discharge, the upstream tide-induced residual current cannot compete with the downstream density-driven current and the plume turns downstream. Momentum analysis confirms the important roles of advection term and viscosity term in the condition of low river discharge and the shift to a Coriolis force-dominated system under high river discharge condition. An idealized model study suggests a dimensionless number for the river discharge changing the river plume extension from upstream to downstream under a specific upstream ambient current around the river mouth.

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Han Wang and Oliver Bühler

Abstract

We present a new method to estimate second-order horizontal velocity structure functions, as well as their Helmholtz decomposition into rotational and divergent components, from sparse data collected along Lagrangian observations. The novelty compared to existing methods is that we allow for anisotropic statistics in the velocity field and also in the collection of the Lagrangian data. Specifically, we assume only stationarity and spatial homogeneity of the data and that the cross covariance between the rotational and divergent flow components is either zero or a function of the separation distance only. No further assumptions are made and the anisotropy of the underlying flow components can be arbitrarily strong. We demonstrate our new method by testing it against synthetic data and applying it to the Lagrangian Submesoscale Experiment (LASER) dataset. We also identify an improved statistical angle-weighting technique that generally increases the accuracy of structure function estimations in the presence of anisotropy.

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Lei Liu, Huijie Xue, and Hideharu Sasaki

Abstract

Using the extended “interior + surface quasigeostrophic” method from the 2019 study by Liu et al. (hereafter L19), subsurface density and horizontal velocities can be reconstructed from sea surface buoyancy and surface height. This study explores the potential of L19 for diagnosing the upper-ocean vertical velocity w field from high-resolution surface information, employing the 1/30° horizontal resolution OFES model output. Specifically, we employ the L19-reconstructed density and horizontal velocity fields in a diabatic version of the omega equation that incorporates a simplified parameterization for turbulent vertical mixing. The w diagnosis is evaluated against OFES output in the Kuroshio Extension region of the North Pacific, and the result indicates that the L19 method constitutes an effective framework. Statistically, the OFES-simulated and L19-diagnosed w fields have a 2-yr-averaged spatial correlation of 0.42–0.51 within the mixed layer and 0.51–0.67 throughout the 1000-m upper ocean below the mixed layer. Including the diabatic turbulent mixing effect has improved the w diagnoses inside the mixed layer, particularly for the cold-season days with the largest correlation improvement reaching 0.31. Our encouraging results suggest that the L19 method can be applied to the high-resolution sea surface height data from the forthcoming Surface Water and Ocean Topography (SWOT) satellite mission for reconstructing 3D hydrodynamic conditions of the upper ocean.

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Zhongya Cai and Jianping Gan

Abstract

We investigated the mean kinetic energy (MKE) and eddy kinetic energy (EKE) in the South China Sea to illustrate the dynamics of the vertically rotating cyclonic–anticyclonic–cyclonic (CAC) circulation in the upper, middle, and deep layers. We found that strong MKE along the basin slope and the associated EKE arising from the vertical shear and stratification of the mean current characterize the circulation. In the upper layer, the external MKE input from the Kuroshio intrusion and wind forcing drive the cyclonic circulation, with the wind forcing providing most of the EKE. External forcing, however, does not directly provide the MKE and EKE of the CAC circulation in the semi-enclosed middle and deep layers, where the internal pressure work near Luzon Strait and the vertical buoyancy flux (VBF) in the southern basin and along the western slope maintain the MKE and EKE. The internal pressure work is formed by ageostrophic motion and pressure gradient field associated with circulation. The VBF is generated by vertical motion induced by the geostrophic cross-isobath transport along the slope where variable density field is maintained by the external flow and the internal mixing. The kinetic energy pathway in the CAC circulation indicates that the external forcing dominates upper-layer circulation and the coupling between internal and external dynamics is crucial for maintaining the circulation in the middle and deep layers. This study provides a new interpretation to the maintenance of CAC circulation from energy prospect.

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Renhao Wu, Shimei Wu, Tianhua Chen, Qinghua Yang, Bo Han, and Han Zhang

Abstract

Lekima was a devastating super typhoon hitting China in 2019. Here, we use a high-resolution wave–current coupling model to investigate the impacts of wave–current interaction during Lekima on wave height, storm surge, ocean currents, and momentum balance. The model results were in good agreement with observations. It was found that, in the open waters, the strong currents generated by the typhoon winds reduced the typhoon-induced maximum significant wave heights (MSWHs) by 6%–15%. The baroclinicity of seawater also slightly reduced the MSWHs by approximately 3%. In the coastal waters, the MSWHs were increased by 6%–15% when feedbacks from water levels were considered. The typhoon-induced highest storm surge occurred in the coastal waters right of the typhoon’s landing position. The nonconservative wave forces contributed by approximately 0.1–0.4 m to the most severe storm surge (3 m), with this effect being most prominent in coastal waters. The baroclinicity of seawater generally increased the storm surge but had little influence on very shallow waters. Tides tend to exacerbate storm surge in most nearshore waters, except in a small bay. Waves generally increased the velocity of offshore ocean currents via the wave-breaking-induced acceleration. A cross-shore momentum balance analysis shows that when the typhoon was near the shore, the dominant terms in the momentum equation were the horizontal pressure gradient force and the surface wind stress, and the contribution of wave breaking had similar pattern to that of the wind stress but a lower magnitude. Our findings have significant implications for the numerical modeling of typhoons and the prediction of their impacts in the coastal environment.

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Dehai Song, Wen Wu, and Qiang Li

Abstract

Bay–shelf exchange is critical to coastal systems because it promotes self-purification or pollution dilution of the systems. In this study, the effects of wave–current interactions on bay–shelf exchange are explored in a micromesotidal system—Daya Bay in southern China. Waves can enlarge the shear-induced seaward transport and reduce the residual-current-induced landward transport, which benefits the bay–shelf exchange; however, tides work oppositely and slow the wave-induced bay–shelf exchange through vertical mixing and reduced shear-induced exchange. Five wave–current interactions are compared, and it is found that the depth-dependent wave radiation stress (WRS) contributes most to the bay–shelf exchange, followed by the wave dissipation as a source term in the turbulence kinetic energy equation, and the mean current advection and refraction of wave energy (CARWE). The vertical transfer of wave-generated pressure to the mean momentum equation (also known as the form drag) and the combined wave–current bottom stress (CWCBS) play minor roles in the bay–shelf exchange. The bay–shelf exchange is faster under southerly wind than under northerly wind because the bay is facing southeast; synoptic events such as storms enhance the bay–shelf exchange. The CARWE terms are dominant in both seasonal and synoptic variations of the bay–shelf exchange because they can considerably change the distribution of significant wave height. The WRS changes the bay–shelf exchange mainly through altering the flow velocity, whereas the wave dissipation on turbulence alters the vertical mixing. The form drag and the CWCBS have little impact on the bay–shelf exchange or its seasonal and synoptic variations.

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Stephanie Contardo, Ryan J. Lowe, Jeff E. Hansen, Dirk P. Rijnsdorp, François Dufois, and Graham Symonds

Abstract

Long waves are generated and transform when short-wave groups propagate into shallow water, but the generation and transformation processes are not fully understood. In this study we develop an analytical solution to the linearized shallow-water equations at the wave-group scale, which decomposes the long waves into a forced solution (a bound long wave) and free solutions (free long waves). The solution relies on the hypothesis that free long waves are continuously generated as short-wave groups propagate over a varying depth. We show that the superposition of free long waves and a bound long wave results in a shift of the phase between the short-wave group and the total long wave, as the depth decreases prior to short-wave breaking. While it is known that short-wave breaking leads to free-long-wave generation, through breakpoint forcing and bound-wave release mechanisms, we highlight the importance of an additional free-long-wave generation mechanism due to depth variations, in the absence of breaking. This mechanism is important because as free long waves of different origins combine, the total free-long-wave amplitude is dependent on their phase relationship. Our free and forced solutions are verified against a linear numerical model, and we show how our solution is consistent with prior theory that does not explicitly decouple free and forced motions. We also validate the results with data from a nonlinear phase-resolving numerical wave model and experimental measurements, demonstrating that our analytical model can explain trends observed in more complete representations of the hydrodynamics.

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Anne Takahashi, Toshiyuki Hibiya, and Alberto C. Naveira Garabato

Abstract

The finescale parameterization, formulated on the basis of a weak nonlinear wave–wave interaction theory, is widely used to estimate the turbulent dissipation rate ε. However, this parameterization has previously been found to overestimate ε in the Antarctic Circumpolar Current (ACC). One possible reason for this overestimation is that vertical wavenumber spectra of internal wave energy are distorted from the canonical Garrett–Munk spectrum by a spectral hump at low wavenumbers (~0.01 cpm). Such distorted vertical wavenumber spectra were also observed in other mesoscale eddy-rich regions. In this study, using eikonal simulations, in which internal wave energy cascades are evaluated in the frequency–wavenumber space, we examine how the distortion of vertical wavenumber spectra impacts the accuracy of the finescale parameterization. It is shown that the finescale parameterization overestimates ε for distorted spectra with a low-vertical-wavenumber hump because it incorrectly takes into account the breaking of these low-vertical-wavenumber internal waves. This issue is exacerbated by estimating internal wave energy spectral levels from the low-wavenumber band rather than from the high-wavenumber band, which is often contaminated by noise in observations. Thus, to accurately estimate the distribution of ε in eddy-rich regions like the ACC, high-vertical-wavenumber spectral information free from noise contamination is indispensable.

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