Feilong Lin

A comprehensive understanding of the regional wave conditions has become increasingly important for structural design, operational safety, and marine hazard prevention. Continuous directional wave measurements in the deeper offshore waters east of Hainan Island remain limited. This study examined the wave characteristics and tropical cyclone-induced spectral evolution east of Hainan Island using nine months of directional wave observations collected at a water depth of approximately 120 m. The monthly mean significant wave height H_1/3 ranged from 0.80 to 2.81 m, with a maximum observed H_1/3 of 4.56 m, while the monthly mean significant wave period T_1/3 ranged from 5.01 to 8.61 s; waves from the ENE and E waves were dominant. The baseline ERA5-based classification indicated a predominance of swell-dominated mixed seas (58%), although ±1 RMSE wind-speed perturbations substantially altered the class proportions. In contrast, pure wind seas consistently showed the strongest wind–wave directional agreement across all sensitivity scenarios, with 68.4–75.5% of cases within 22.5° and 95.2–97.1% within 45°. Among the two tropical cyclones affecting the study region, Tropical Storm Maliksi produced a stronger local wave response than that of Severe Tropical Storm Prapiroon, with mean H_1/3 values of 2.06 and 1.49 m, respectively. During Maliksi, the spectral density increased markedly near 0.10–0.12 Hz, while the pre-storm double-peaked spectrum evolved into a stronger, directionally concentrated dominant component. These findings provide site-specific observational evidence of wave variability and tropical cyclone-induced spectral evolution east of Hainan Island, offering a reference for regional wave characterization and wave-model evaluation.

Under suitable dynamic conditions, mode‐2 internal waves (IWs) may arise when the stratified water of a river plume region interacts with steep topography. However, the dynamics and effect of mode‐2 IWs in river plume systems remain largely unexplored. In this study, the presence of mode‐2 IWs was confirmed by observations during the 2016 flood season on the landward slope of the submerged canyon off the Yangtze River Estuary (YRE). The isopycnal bulges in the mid‐depth layer enclose the center of the velocity jet during peak ebb tides, featuring a convex mode‐2 internal wave. Beneath the jet core, intense turbulence dissipation rates on the order of O (10⁻⁴) W kg⁻¹ were observed. Mode‐2 IWs with an amplitude of ∼3.0 m and a buoyancy frequency of O (10⁻²) s⁻² can theoretically increase the current velocity by ∼0.5 m s⁻¹. The estimated velocity increase aligns with the observations, confirming the development of mode‐2 IWs. The current jet promoted shear instability beneath it, causing intense turbulent dissipation. The interaction between the stratified current and the supercritical slope off the YRE forms internal lee waves. The cross‐shore ebb current can be supercritical relative to mode‐2 IWs, thereby forming mode‐2 internal lee waves. The background shear in the river plume can amplify the velocity jet of mode‐2 IWs, indicating its effect on turbulent mixing. Our findings reveal that in river plume systems characterized by supercritical topography and pronounced barotropic tides, mode‐2 internal waves may frequently develop and increase turbulent mixing.

Anticyclonic mesoscale eddies are known to trap and modulate near-inertial kinetic energy (NIKE); however, the spatial distribution of NIKE within the eddy core and periphery, as well as the mechanisms driving its energy cascade to smaller scales, remains inadequately understood. This study analyzed the evolution of NIKE in anticyclonic eddies using satellite altimetry and field observations from four mooring arrays. By extracting near-inertial oscillations (NIOs) and subharmonic wave kinetic energy across mooring stations during the same period, we characterized the spatial structure of NIKE within the eddy field. The results revealed that NIKE was concentrated in the eddy core, where strong NIOs (peak velocity ~0.23 m/s) persisted for ~7 days, with energy primarily distributed at depths of 200–400 m and propagating inward from the periphery. Subharmonic waves fD1 generated by interactions between NIOs and diurnal tides highlighted the role of the vertical nonlinear term in energy transfer. A further analysis indicated that under vorticity confinement, NIKE accumulated in the core of the eddy and dissipated through shear instability and nonlinear wave interactions. The migrating anticyclonic eddy thus acted as a localized energy source, driving mixing and energy dissipation in the ocean interior.

Internal tides are pivotal dynamic processes enhancing the mixing of oceanic waters and facilitating energy transfer across various scales within the ocean. In recent years, the proliferation of satellite altimetry observations has enabled global predictions of the elevation and phase of internal tides. This study, leveraging the advanced global internal tide prediction model known as the Multivariate Inversion of Ocean Surface Topography-Internal Tide Model (MIOST-IT), employs a two-dimensional Fourier bandpass filtering approach to decompose the internal tides in the Luzon Strait, thereby addressing the east–west directional blind zones inherent in along-track satellite altimetry-based modal decomposition. To further elucidate the propagation trajectories of individual tidal modes in different directions, we introduce the directional Fourier filter method to characterize the spatial distribution features of each modal internal tide in the vicinity of the Luzon Strait. This work significantly enhances the accuracy and reliability of extracting parameters for distinct modal internal tides, furnishing a scientific basis for subsequent studies on internal tide dynamics and model refinement.

The northern Andaman Sea (AS) continental shelf is unique due to the diverse marine ecosystem and existences of both hypoxia and internal tides, but limited in situ observations restrict our understanding of the hydrographic dynamic process. Based on the turbidity, mean volumes of backscattering strength (MVBS), we qualitatively studied the mean distribution characteristics and temporal variation in suspended matters on the northern AS continental shelf and their relation to hypoxia and internal tides. The results of both MVBS and turbidity revealed that the suspended matters exhibited a three-layer vertical structure. The upper and lower layers exhibited high values, while the middle layer had low values. The upper boundary of the high-value region in the upper layer descended below the surface to a depth of 30 m after sunrise and returned to the surface after sunset, indicating a diel vertical migration of zooplankton and micronekton. Daytime migration depth was likely constrained by hypoxia’s upper boundary. In the lower layer, three MVBS enhancements and attenuations correlated with vertical upward and downward velocities, respectively, primarily driven by uplift or suppression. We proposed vertical velocity patterns resulted from internal bores, possibly triggered by shoaling semidiurnal internal tides.

The southern Bay of Bengal (BoB) is a region of rich mesoscale eddy activity that plays an important role in modulating water exchange between the BoB and the Arabian Sea. Although statistical surface characteristics of mesoscale eddies in the BoB have been sufficiently explored previously, most studies concentrated on the western and central BoB, but rarely in the southern BoB. In this study, an energetic mesoscale anticyclonic eddy (AE) was detected by long‐term mooring observations in the southern BoB during June 2020. The mooring observations revealed that the AE‐induced maximum temperature and salinity anomalies at 100 m were as large as 6.8°C and 0.3 psu, respectively. The available Argo profiles showed that the AE‐induced salinity anomalies had a three‐layer vertical structure, with a positive anomaly (maximum 0.28 psu) deeper than 100 m and (maximum 0.72 psu) shallower than 40 m, and a negative anomaly (maximum −0.86 psu) between 40 and 100 m, indicating that the AE could exert a significant effect on the redistribution of surface freshwater in the southern BoB. The AE formation was closely connected with remote forcing (equatorial Kelvin wave) and local processes (Rossby wave and local wind stress curl). Local energy conversion analysis demonstrated that baroclinic and barotropic instabilities, as well as wind stress work made comparable contributions to the generation of the AE. Given the frequent occurrence of the AEs to northwest of Sumatra, they could have significant effects on surface freshwater redistribution and regional ecosystem productivity in the southern BoB.

Bottom current at the equatorial gap of the Ninety East Ridge is revealed by long‐term bottom mooring observations from 24 December 2019 to 2 April 2021. It is found that the observed bottom current has significant intraseasonal variability (ISV) with a dominant time scale of ∼33 days, which becomes enhanced in the boreal summer and winter. Variability of the bottom current was explored by using the Mercator Ocean outputs, which revealed that the enhancement of ISV of the bottom current is closely related to the westward propagation of Rossby waves originating from the eastern boundary, with their ray paths vertically propagating into the deep layer. Considering the importance of water mass transport through the gaps in the Ninety East Ridge to the renewal of deep water and modification of deep circulation in the Central Indian Basin (CIB) and West Australian Basin (WAB), our study provides in‐situ evidence for previously unrevealed energetic variation of water mass exchange between the CIB and WAB, and it further helps us gain a comprehensive understanding of the pathway and variation of the deep meridional overturning circulation in the Indian Ocean.

The impacts of mesoscale eddies on the modulation of typhoon-induced oceanic responses are important for understanding ocean dynamics. Satellite observations identified prominent ocean surface temperature and chlorophyll changes over the regions with mesoscale eddies after two sequential typhoons, e.g., Linfa and Nangka, in the South China Sea. The impacts of typhoons on the ocean surface were more prominent within cyclonic eddies than within anticyclonic eddies. The wind speed (translation speed) of Linfa was much larger (slower) than that of Nangka; thus, the changes induced by Linfa were stronger. However, the second typhoon easily generated mixing through the weak stratification induced by the first typhoon and impacted the upper ocean. The strong chlorophyll enhancement induced by Nangka was identified at a cyclonic eddy. Using a combination of reanalysis data, the depth of water origin (DWO) was applied to quantify the depth to which a typhoon’s impact could be exerted. Prominent changes were identified when the DWO reached the depth at which the temperature and nutrients differed from those within the mixed layer. This method can overcome the impacts of cloud coverage when examining a typhoon’s influence with remotely sensed data and offers a quantitative approach to determine the mechanisms responsible for typhoon-induced ocean surface changes.

The quasi-biweekly oscillation (QBWO) is the second most dominant intraseasonal mode for circulation over the Northwestern Pacific (WNP) during boreal summer. In this study, we investigated how the QBWO modulates tropical cyclone (TC) activities over the WNP from dynamic and thermodynamic perspectives. The propagation of the QBWO can be divided into four phases through empirical orthogonal function analysis of the vorticity at 850 hPa, which was proven to be effective in extracting the QBWO signal. TC generation and landings are significantly enhanced during the active period (phases 1 and 2) relative to the inactive period (phases 3 and 4). Composite analyses show the QBWO could significantly modulate TC activity as it propagates northwestward by changing the atmospheric circulation at both high and low levels. Cumulus convection provides an important link between TCs and the QBWO. The major component of the atmosphere heat source is found to be the latent heat release of convection. The condensation latent heat centers, vertical circulation, and water vapor flux divergence cooperate well during different phases of the QBWO. The vertical profile of the condensation latent heat indicates upper-level heating (cooling) during the active (inactive) phases of the QBWO. Thus, the northwestward propagation of the QBWO can modulate TC activity by affecting the configuration of atmospheric heating over the WNP.