Wavenumber integration (WI) theory is typically employed in numerical simulations of underwater acoustic propagation, for which the solution of depth-dependent kernel functions is the core issue of integral transform modeling. In the current widely used WI models, the depth equation is discretized using the finite element method or a spectral method. The finite element model is characterized by slow error convergence but high computational efficiency, whereas the spectral model is characterized by low computational efficiency but high accuracy. We introduce a spectral element approach for evaluating kernel functions by discretizing the depth-separated wave equation in weak form. The domain is partitioned into elements, and within each, Lagrange interpolants constructed at Gauss–Legendre–Lobatto nodes serve as basis functions. This yields a global matrix that is both block-diagonal and symmetric. The results of the numerical scheme and simulation experiments show that the newly developed spectral element algorithm and model not only combine the advantages of both methods but can also flexibly highlight the strengths of either method via optimal parameter setting, demonstrating high application value.

Conventional altimeter satellites, such as TOPEX/Poseidon and Jason series, can identify ocean mesoscale eddies (MEs) but cannot effectively distinguish submesoscale eddies (SMEs) due to horizontal resolution limitations. The emergence of the Surface Water and Ocean Topography (SWOT) satellite has enabled the resolution (or detection) of SMEs. At present, Data Unification and Altimeter Combination System (DUACS) (MEs-resolving) and SWOT (SMEs-resolving) satellites operate concurrently in orbit, however a systematic comparison and analysis of their observational outputs has yet to be conducted. Using a closed-contour scalar analysis method, this study identifies SMEs in the tropical western Pacific Ocean and compares the results with those from the dataset. The latitude-dependent Rossby deformation radius is employed to differentiate MEs from SMEs. For MEs, SWOT detects 176 per 10.5-day sub-cycle, while DUACS detects 162, which are roughly equivalent. For SMEs, SWOT identifies 273 per sub-cycle, far exceeding the 13 detected by DUACS. For amplitudes, DUACS measures 5.22 cm and 3.67 cm for MEs and SMEs, respectively, while the values reported by the SWOT satellite are 6.13 cm and 4.49 cm. In both datasets, cyclonic eddies are more prevalent in all cases except for the SMEs detected by SWOT, where anticyclonic eddies slightly outnumber cyclonic eddies. Additionally, during the trial operation and scientific orbit phases, SWOT is able to resolve 29 SMEs per orbit. The results indicate that high-resolution data can distinguish phenomena that conventional satellite altimeters cannot capture, providing valuable references for the analysis and application of SME characteristics.

Mesoscale eddies induce significant variations in the temperature and salinity of the upper ocean, thereby exerting a substantial impact on sound propagation. Specifically, they can form or dissipate surface sound channels (SCs) and subsurface sound channels (SSCs). However, the specific impact of mesoscale eddies remains unclear at present. In this paper, META2.0, Argo, and SODA are employed to analyze the sound propagation characteristics of mesoscale eddies in the Kuroshio Extension (KE) by using a composite analysis method and a ray-tracing model. Results demonstrate that cyclonic eddies (CEs) cause the disappearance of the original SCs in winter, while simultaneously generating SSCs. Conversely, anticyclonic eddies (AEs) induce SCs in spring and winter, while in summer they induce SSCs and in autumn create dual-duct sound channels (DCs). This study quantitatively reveals the influence of mesoscale eddies on the genesis and demise of SCs and SSCs, providing technical support for sonar equipment to utilize mesoscale eddies.

An internal solitary wave (ISW) significantly affects acoustic propagation; however, its detailed characteristics are poorly understood. Simulation experiments of sound propagation in a shallow water environment are presented to examine the effects of the source conditions and characteristics of the ISW on transmission loss (TL). The results show that the TL decreases as the depth of the source increases and the frequency of the source decreases and that the different characteristics of the ISW are highly important for estimating sound propagation when a SONAR system is in an ISW environment. Coupled normal mode theory is further employed to analyse the variations in coupling between sound field modes in an ISW environment. Further analysis reveals that the magnitude of the TL is affected by the direction and fluctuation of energy propagation between different modes, and in different ISW environments under the deep and low-frequency source conditions, the sound field energy is mainly in lower-order modes.

As a semi-enclosed marginal sea, the Japan Sea has long been hindered in tidal studies by sparse tide-gauge observations and limited satellite altimetry coverage. Although Sentinel-3A provides high-quality sea-level observations, its 27-day exact-repeat sun-synchronous orbit introduces severe tidal aliasing into conventional harmonic analysis—most critically, the S₂ constituent degenerates into a constant and cannot be recovered independently. In this study, we apply the Modified Harmonic Analysis model based on the Credo of Smoothness (MHACS) with ridge regression to eight years (2016–2024) of Sentinel-3A altimetry in the Japan Sea for the first time, estimating tidal constants for the eight major constituents. By exploiting the smooth connections among major astronomical constituents, MHACS mitigates the aliasing problem, with severely aliased constituents such as S₂ inferred from the smooth admittance relationship rather than independently resolved. The inverted tidal fields show excellent agreement with the independent FES2014 global tide model, with a spatially averaged vector difference of only 1.39 cm, whereas the traditional tidal response method yields 4.52 cm—3.3 times larger. Independent validation against 22 GESLA-3 tide-gauge records further confirms this ranking (3.012 cm vs. 5.676 cm). As a consistency check, the MHACS-derived P₁/K₁ and K₂/S₂ amplitude ratios (0.316 and 0.278) agree closely with equilibrium tidal theory, although this test is not fully independent. These results demonstrate that MHACS is well suited for tidal inversion from 27-day repeat altimetry.