Xiao-Hua Zhu

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.

Characterizing tidal circulation at a high spatiotemporal resolution is essential for understanding submesoscale transport and mixing in marine protected areas (MPAs). This study presents the first application of high‐frequency Coastal Acoustic Tomography (CAT) to synoptically resolve transient eddy‐like structures at 2–3 km scales in an open‐island coastal environment. A six‐station CAT array deployed off Nanji Island, China, reconstructed depth‐averaged currents every 3 min at ∼1.5 km resolution over a ∼26‐hr observation period. CAT velocities closely matched independent Acoustic Doppler Current Profiler (ADCP) measurements ( R > 0.95; RMSE < 0.09 m s ⁻¹ ), confirming the accuracy of the tomographic inversion. The flow was dominated by the M2 constituent, with northwestward flood and southeastward ebb currents superimposed on a persistent northeastward residual flow. During flood–ebb transitions, short‐lived eddies formed and persisted for ∼40 min. Deformation and kinetic energy flux convergence diagnostics suggest that tidal phase‐dependent strain intensification favored eddy formation, producing localized retention zones that favor particle retention and reduce lateral dispersion. Horizontal eddy diffusivities estimated from strain‐based and Lagrangian methods range from 2.56 to 10.54 m ² s ⁻¹ and peak during flood tides. Using an assumed representative horizontal nitrate gradient (∂ C /∂ x = 1.0 × 10 ⁻⁶ mmol m ⁻⁴ ), these estimates imply lateral nutrient flux magnitudes of 0.24–0.92 mmol m ⁻² d ⁻¹ , suggesting strong tidal phase modulation of biogeochemical exchange. These findings highlight the roles of tidal phase‐dependent flow variability and transient eddy activity in regulating transport and mixing in island‐shelf systems and show that high‐frequency CAT can resolve physical processes relevant to adaptive MPA management.

A tidal model based on altimeter observations reveals that first‐mode diurnal internal tides (DITs) propagate approximately 2,100 km eastward from the Luzon Strait (LS) into the Pacific Ocean. As they radiate over long distances, the DITs refract equatorward due to the beta effect. In this study, we utilize in situ round‐trip acoustic echo time measurements between the seafloor and the sea surface, obtained from an array of 10 pressure‐recording inverted echo sounders (PIES), to investigate the variability of DITs in the eastern Philippine Sea (EPS). The observations conducted over 1‐year and 1.5‐year periods during 2020–2021 reveal a clear weakening of DIT amplitudes in summer, in contrast to the barotropic diurnal tides, which show maximum spring tide amplitudes at the solstices and minimum amplitudes at the equinoxes. The observed seasonal variation in DIT energy flux shows a significant correlation with the relative vorticity averaged over regions of energetic warm eddies. Ray‐tracing using HYCOM ocean model outputs indicates that the warm eddies in the upstream region of the ray path during summer (July to September) enhance the equatorward refraction of DITs. This study suggests that the superposition of the K ₁ and P ₁ constituents induces a pronounced semi‐annual cycle in the DITs, even over considerable propagation distances. In addition, warm eddies exert a substantial influence on the DIT propagation path. Our results imply that the pronounced temporal variability of DITs should be considered to improve the parameterization of internal‐wave‐induced ocean mixing in oceanic and climate models.

This study investigated the summer subsurface marine heatwaves (SSMHWs) in the East/Japan Sea (EJS) using the Japan Coastal Ocean Predictability Experiment 2 ocean reanalysis datasets during 1993–2023. Climatologically, the cumulative (maximum) intensity area-averaged over the EJS ranges from 20°C days (2.5°C) at surface to its maximum of 36°C days (4°C) at approximately 70 m, confirming significant subsurface intensification of summer MHWs. Compared with the almost uniform spatial patterns at surface, the intensities of MHWs at subsurface layers show substantial spatial variations. The SSMHWs in the EJS are caused not by surface atmospheric forcings, but primarily by internal oceanic processes. During summer, the strong stratification in the EJS inhibits the vertical mixing and traps more heat fluxes at surface. As the main heat sources of the upper ocean, the Tsushima Warm Current (TWC) plays a crucial role in triggering SSMHWs in the EJS. Anomalous increased volume transport of TWC carries more warm water into the EJS, favoring the basin-scale intense SSMHWs. This warm water is redistributed by the East Korean Warm Current and mesoscale eddies, leading to regional features of SSMHWs in the EJS. Our results highlighted the roles of oceanic circulation in forming intense SSMHWs in the EJS.

Conventional methods of measuring water discharge and suspended sediment concentration (e.g., water sampling and moving acoustic Doppler current profiler [ADCP]) present challenges in large tidal rivers due to temporal and spatial constraints. This study introduces a novel approach to monitor water discharge and suspended sediment discharge (SSD) in large tidal rivers. Total water discharge and SSD exhibit notable variability in tidal rivers due to the river–tidal interactions; understanding this variability and its causes is essential for effective tidal river management. From June to November 2023, a field study was conducted at Nanjing (NJ) to continuously monitor water discharge, suspended sediment concentration (SSC), and SSD in the tidal reaches of the Yangtze River using coastal acoustic tomography (CAT). Total water discharge ranged from 8,765 to 43,356 m³/s, with a mean of 27,825 m³/s, while tidal discharge varied between −11,998 and 9,983 m³/s, with a mean of 69 m³/s. SSC ranged from 0.02 to 0.09 kg/m³, and SSD ranged from 110 to 3,823 kg/s. Tidal variations in SSC and SSD were within ±0.04 kg/m³ and −1,252 to 1,410 kg/s, respectively. Over short timescales, tides caused instantaneous fluctuations in velocity, water discharge, and SSD, with tides contributing −40% to instantaneous water discharge and SSD at NJ. Over seasonal timescales, no significant wet/dry variations were observed in water discharge, SSC, or SSD during a few months of 2023. Long‐term CAT application (e.g., decades) is required to reveal trends in tidal river dynamics.

In this study, for the first time, we found a Subsurface Low Potential Vorticity Water (SLPVW) to the east of Taiwan Island (122–124, 21.67–23) via an array comprising 12 current and pressure‐recording inverted echo sounders (CPIESs) from 25 June 2018 to 29 July 2019. This SLPVW exhibits remarkable intraseasonal variability, with an ∼100‐day period east of Taiwan Island, corresponding with the variability in mesoscale eddies. Compared with large‐scale climatological mean circulation, mesoscale eddy can trap SLPVW as a highway westward transport. In addition, the SLPVW impingement results in a significant subsurface velocity variation (about 10 cm/s) in the 200–400 m layer. These findings shed new light on the mechanisms of the intraseasonal variation in the Kuroshio subsurface layer.

The El Niño‐Southern Oscillation (ENSO)‐related interannual variability of the Kuroshio in the East China Sea (ECS) was revisited based on reanalysis outputs during 1993–2018. Unlike the synchronized variations from 2006 to 2018, the period of 1993–2005 showed regional differences in how the ECS‐Kuroshio responded to ENSO events. Specifically, from the upstream region to the midway of the continental slope, the ECS‐Kuroshio exhibited distinct six‐year interannual modulation during 1993–2005. In contrast, downstream Kuroshio variability primarily followed a four‐year cycle, aligning with ENSO variability during the same period. Further analysis suggested that the sea surface height anomaly (SSHA) east of the Kerama Gap, near the midpoint of the Ryukyu Island chain, extended inside the ECS until the southern side of the Tokara Strait along the ECS‐Kuroshio path and was well correlated with the Kuroshio in the Tokara Strait during 1993–2005. The cause of this SSHA signal was attributed to forcing by ENSO‐related wind stress curl changes in the interior region. There was an obvious difference in the ENSO‐related atmospheric circulation before and after 2005. The wind stress curl pattern in the North Pacific during 1993–2005, characterized by a maximum in the Kerama Gap latitude band, shifted northward compared to that during 2006–2018. The relative northward shift of the ENSO‐related wind stress curl, which stimulates the long baroclinic Rossby wave propagating westward and arriving east of the Kerama Gap, affect the interannual variabilities of both the upstream and downstream Kuroshio.

In 2015, four typhoons traversed the regions surrounding the Ryukyu Island Chain, resulting in different near‐inertial waves (NIWs), the characteristics of which were investigated through in situ observations around Miyakojima Island and numerical simulations covering the East Asian marginal seas. The spatial distribution of typhoon‐induced near‐inertial motions was significantly correlated with the typhoon tracks and background currents. Typhoons Chan‐hom and Goni traversed the observation transect, resulting in different spatial patterns of NIWs owing to their different tracks. Observations on both the western and eastern sides of the Ryukyu Island Chain failed to capture NIWs after typhoon Chan‐hom, because they were positioned to the left of the typhoon track where near‐inertial motions were enhanced only in the upper 50 m. In contrast, NIWs were negligible on the left side and energetic on the right side of typhoon Goni. Despite being hundreds of kilometers from the observation transect, typhoons Soudelor and Dujuan induced NIWs with higher energy levels than those induced by typhoons Chan‐hom and Goni. The energy propagation of NIWs after typhoons Soudelor and Dujuan was significantly influenced by background currents. The western boundary currents, including the Kuroshio and Ryukyu Currents, create negative relative vorticity on their right, which works like a waveguide for the poleward advection of NIWs. However, the Ryukyu Current can be impeded by westward‐propagating cyclonic eddies from the North Pacific, which further disrupts the waveguide.

This study investigated an extreme sea surface warming in the midlatitude western North Pacific (MLWNP) during the summer of 2012. The 2012 extreme event was characterized by warm sea surface temperature anomaly (SSTA) extending from the East/Japan Sea to central North Pacific. The SSTA box–averaged over the MLWNP (130–180°E, 33–50°N) in 2012 ranked as the third warmest in recent four decades, which has caused intense marine heatwaves in this region. During the summer of 2012, a positive Indian Ocean Dipole event co-occurred with El Niño, favoring anomalous moisture transport between the two basins that caused enhanced convection in the South China and Philippine Seas and western–to–central subtropical Pacific. The enhanced convective activities triggered two meridional atmospheric Rossby wave trains to form strong atmospheric blocking high–pressure systems in the MLWNP. This reduced the total cloud cover and surface wind speed, enhancing insolation and reducing the release of latent heat flux. In addition, the weakened wind strengthened the stratification and shoaled the mixed layer. As a result, the increased net heat flux into the ocean accompanied by a shallower mixed layer contributed to the upper ocean warming in the MLWNP. Meanwhile, the North Pacific was dominated by a negative phase of Pacific Decadal Oscillation (PDO), significantly contributing to warm SSTAs in the MLWNP in 2012. Consequently, the 2012 extreme warming in the MLWNP was the results of the combination of atmospheric Rossby waves and PDO. Our study highlighted the roles of high–frequency atmospheric teleconnection and low–frequency PDO in extreme sea surface warming in the MLWNP.

Marine heatwaves (MHWs) are anomalously warm events that profoundly affect climate change and local ecosystem. During the summer of 2012 (June–September), intense MHWs occurred in the tropical Indian Ocean (TIO) concurrently with an unseasonable positive Indian Ocean Dipole (pIOD) event. The MHW metrics (duration, frequency, cumulative intensity and maximum intensity) were characterized by northwestward–slanted patterns from west Australia to the Somalia coast. The analysis confirmed that these MHWs were closely associated with the unseasonable pIOD 2012. The weakening of Western North Pacific Subtropical High and strengthening of Australian High in spring induced an interhemispheric pressure gradient that drove two anticyclonic circulation patterns over the eastern TIO. The first anticyclonic circulation featured cross–equatorial wind anomalies from south of Java to the South China Sea/Philippine Sea, which led to strong upwelling off Sumatra–Java during the subsequent summer. The second anticyclonic circulation excited downwelling Rossby waves that propagated from the southeastern TIO to the western TIO. Thus, downwelling in the western pole and upwelling in the eastern pole led to a strong pIOD event peaking in summer, namely, the unseasonable pIOD 2012. These downwelling Rossby waves deepened the thermocline by more than 60 m and caused anomalous surface warming, thereby contributing to the occurrences of MHWs. With the development and peak of the unseasonable pIOD 2012, anomalous atmospheric circulation transported moisture from the TIO to the subtropical Western North Pacific (WNP), favoring a strong cyclonic anomaly that profoundly affected the summer monsoon rainfall over the subtropical WNP. This study provides some perspectives on the role of pIOD events in summer climate over the Indo–Northwest Pacific region.