Sen Jan

The driving mechanism for the multidecadal sea surface height (SSH) variability with a period of about 50‐year in the East Sea/Japan Sea and related dynamics in the northwestern Pacific marginal seas are investigated using a multi‐millennial pre‐industrial control climate model simulation. The model results suggest that multidecadal variability is associated with the meridional shift of zero‐wind stress curl in the central North Pacific. The SSH anomalies driven by open ocean wind stress curl anomalies propagate westward via baroclinic Rossby waves and drive an anomalous sea level setup around the Japanese Islands; this leads to variability in the Tsushima Warm Current (TWC) transport, with this linkage explicitly demonstrated using the island rule. Sea surface height anomalies propagating from the open ocean also affect the Kuroshio transport in the East China Sea by modulating the onshore side of the SSH across the Kuroshio. Although the changes in the transport of the Kuroshio and TWC have opposite responses to wind stress curl changes in the central North Pacific, their direct correlation is insignificant and weakly positive. With a multi‐millennial climate model simulation, this study allows a statistically robust finding on the mechanism for the multidecadal internal variability, complementing previous studies based on short term observation and reanalysis data.

Dunes with lengths of hundreds of meters and heights up to 16 m have been discovered on the upper slope in the South China Sea, at a site exposed to forcing by large internal solitary waves and energetic internal tides. Similar dunes occur beneath internal solitary waves elsewhere in the South China Sea and in other ocean basins. Previous work has characterized dune scales, provided evidence for formation by internal solitary waves, and indicated dependence of dune characteristics on the underlying seafloor slope. Here we report an analysis of multibeam measurements, sediment cores, and water column measurements at the dune discovery site. The calculated dune statistics are wavelength, height, skewness, and asymmetry. The results confirm that subtle but statistically significant changes in dune metrics coincide with a change in underlying seafloor slope from supercritical to critical for the predominantly diurnal internal tide. Dune orientations and wave directions coincide, and the mean dune wavelength is nearly equal to the mean wave excursion amplitude. However, the modest variations in dune scales are unrelated or at most weakly related to the scales of the overlying forcing by internal tides and waves. Sediments range in size from clay to gravel and are coarse on dune crests and fine on troughs. Estimates of bottom stress and sediment transport indicate that tides alone cannot move coarse sediments on dune crests, but waves and tides combine to cause offshore sediment transport and dune migration.

Sponsored by the National Science and Technology Council of Taiwan, the programs Sailing to the Blue Sea (SBS), Kuroshio to Turbulence Exchange (KTEX), and the U.S.-Taiwan collaboration program Island Arc Turbulent Eddy Regional Exchange (ARCTERX) jointly conducted a multidisciplinary field campaign aboard the R/V New Ocean Researcher 1 from March 25 to April 19, 2024. The cruise completed a round trip from Kaohsiung to Palau, executing coordinated oceanic and atmospheric observations in the western North Pacific. The scientific objectives of this campaign were to: (1) investigate the physical, biological, and biogeochemical properties of a targeted cyclonic mesoscale eddy in the western North Pacific and its impact on oceanic weather through air-sea interactions; (2) quantify water mass transformation and nutrient flux variability from the North Equatorial Current (NEC) to the Kuroshio; (3) examine the diurnal warm layer and its coupling with atmospheric boundary layer variability in the subtropical and tropical regions; (4) detect potential ground motion in the Philippine Sea basin at the eastern flank of the Gagua Ridge, east to Taiwan; and (5) measure along-track gravity anomaly across the Philippine Sea basin. The campaign successfully sampled a cyclonic eddy centered at (131°E, 18°N), hydrographic transects across the Kuroshio and the NEC, and high-resolution upper ocean temperature profiles in the tropical ocean. To facilitate detailed analyses of the complex cruise data, the ship-based and autonomous observations are introduced here.

The influence of extreme climate events on regional water mass variability in the western North Pacific (NP) remains poorly understood. In 2017, Seaglider and Argo float observations revealed an anomalously high-salinity (S > 35) North Pacific Tropical Water (NPTW) in the western boundary region. To investigate the origin of this anomaly, we analyzed in situ hydrographic data, satellite-derived surface fluxes, and model-based ocean reanalysis. Results indicate that the high-salinity NPTW was linked to anomalous atmospheric and oceanic conditions associated with the 2015–2016 extreme El Niño. During this period, an expanded region of excess evaporation over precipitation (E–P) in the eastern–central NP likely enhanced surface salinity, which was subsequently subducted and advected westward via zonal currents. This mechanism led to the westward migration of saline NPTW over a two-year period, culminating in the replacement of the less saline West Philippine Sea Tropical Water by 2017. These findings highlight a delayed and basin-scale impact of extreme El Niño events on regional subsurface salinity distribution in the western North Pacific.

The spatial distribution of the flow field inferred from shipboard measurements often suffers from a spatial‐temporal aliasing effect. This study examined a detailed view of the horizontal divergence and relative vertical vorticity captured by a two‐vessel survey to characterize velocity gradients and the resulting process in the presence of nonlinear lee waves. The three‐dimensional structure of the horizontal velocity gradients and turbulent mixing within a nonlinear internal lee wave behind a shallow seamount was investigated. Synchronous two‐vessel Acoustic Doppler Current Profilers provide in‐situ measurements of velocity vectors that significantly minimize the spatial and temporal aliasing effect. The magnitude of horizontal divergence and relative vertical vorticity normalized by the planetary vorticity (, ∼ O (10)) is one order of magnitude greater than prior observations in the typical oceanic sub‐mesoscale flow field using a two‐vessel survey. Our analysis indicates that the spatial variations of horizontal divergence and relative vertical vorticity over the seamount are associated with flow‐topography interactions. Owing to the bottom Ekman effect, the deflected Kuroshio enhances the relative vertical vorticity, , and horizontal components of relative vorticity, and , resulting in vertical shear instability and symmetric instability. Instability hotspots are identified by the negative potential vorticity (PV) at the rear half of the nonlinear internal wave, where depressed isopycnals rebound. In situ observational surveys conducted on the lee of pinnacles further indicated that the enhanced turbulent eddy diffusivity and vertical nitrate gradient are collocated with the negative PV within a nonlinear internal lee wave.

Not-yet-known not-yet-known not-yet-known unknown A substantial community variation of free-living bacterioplankton and their main predator heterotrophic nanoflagellates (HNFs) is often unexplained because their mutual dependency through presumed trophic interactions is often overlooked. Here, we collected bacterioplankton and HNFs from the surface layer and depth of chlorophyll-a maximum (DCM) along 13 degrees of latitude in the Kuroshio from Taiwan to Japan. We performed three-way variation partitioning analyses to unravel their reciprocal influences, in addition to dispersal limitation and environmental dissimilarity. Our analyses reveal that bacterioplankton and HNFs mutually and uniquely explain over 10% of compositional variation when both layers are analyzed collectively, underscoring the significance of trophic interactions. The vertical compositional variations of bacterioplankton and HNFs also reciprocally explain each other. Whereas, considering the surface layer alone, bacterioplankton uniquely explains 12.9% of HNF composition, but HNF makes a non-significant unique contribution to bacterial composition. In the DCM layer, the two trophic levels do not mutually explain each other. Dispersal limitation uniquely accounts for more than 20% of the compositional variation in each layer but does not significantly explain vertical variations. Environmental dissimilarity makes a minor contribution to both trophic levels. Our results suggest that influences of trophic interactions are more pronounced when considering vertical rather than horizontal variations. The horizontal dispersal limitation dominates the compositional variation within a water layer, while trophic interactions between bacterioplankton and HNFs mediate their compositional variation across depths. By considering two trophic levels, this study advances our understanding of metacommunity dynamics of free-living bacterioplankton and HNFs.

Measurements in the South China Sea reveal the structure of the bottom boundary layer beneath onshore propagating highly nonlinear internal solitary waves of depression. Offshore directed free stream velocities beneath 13 waves with durations of 10–20 min and velocities up to 1.4 m/s are consistent with the solitary wave solution to the Korteweg‐de Vries equation as are phase velocities estimated from pressures and free stream velocities. The measurements indicate a thin layer during free stream acceleration, a thicker layer with an inflected velocity during deceleration, and a long‐lived sediment laden wake, after wave passage in the free stream, with velocities opposite to those at maximum flow strength. Reynolds‐averaged Navier‐Stokes simulations with the k‐ε turbulence model reproduce the measured velocities and turbulent Reynolds shear stresses accurately during acceleration and early deceleration. However, differences between the simulations and the measurements during late deceleration and in the wake suggest energetic large‐scale turbulence not represented by the simulations. This turbulence might be similar in origin to coherent vortices that have been observed in laboratory experiments and direct numerical simulations at much smaller Reynolds numbers, which have been attributed to absolute and global instabilities resulting from inflected velocity profiles.

We study two sea surface temperature (SST) warming events and upper ocean stratification changes in the northern South China Sea in 2022 using data from an EM‐APEX float and satellite observations. The diurnal warm layers (DWLs) and the increasing buoyancy frequency N² above the top of the thermocline can restrict the penetration depth of nighttime convection and wind‐driven mixing, which prevents cooler water from mixing upward, allowing solar heating to increase the SST by more than 1°C in a few days. The stratification budget approach is used to reproduce observations below 40 m despite some uncertainties in estimating variables such as horizontal gradient. After the first SST warming event, the stratification changes in the subsurface layers constituted by an increase in N² above 70 m and a decrease below this depth can be attributed to the combined effects of turbulent diffusion and vertical advection rather than to horizontal advection or penetrative solar radiation. This ocean interior mixing is likely caused by the shear of near‐inertial waves at ∼50 m, when the nighttime convection could not penetrate through the DWL's base around 20 m. The stratification budget approach fails to simulate the changes above 40 m after the second SST warming event partly due to the presence of a near‐surface freshwater layer. Our observations offer insights into the effect of inertial wave‐induced mixing in the ocean interior when near‐surface stratified layers are present, which can lead to changes in upper ocean stratification and SST.

Measurements in the South China Sea reveal the structure of the bottom boundary layer beneath onshore propagating highly nonlinear internal solitary waves of depression. Offshore directed free stream velocities beneath thirteen waves with durations of 10-20 minutes and velocities up to 1.4 m/s are consistent with the solitary wave solution to the Korteweg de Vries equation, as are phase velocities estimated from pressures and free stream velocities. The measurements indicate a thin layer during free stream acceleration, a thicker layer with an inflected velocity during deceleration, and a long lived sediment laden wake, after wave passage in the free stream, with velocities opposite those at maximum flow strength. Reynolds averaged Navier Stokes simulations with the k-ε turbulence model reproduce the measured velocities and turbulent Reynolds shear stresses accurately during acceleration and early deceleration. However, differences between the simulations and the measurements during late deceleration and in the wake suggest energetic large scale turbulence not represented by the simulations. This turbulence might be similar in origin to coherent vortices that have been observed in laboratory experiments and direct numerical simulations at much smaller Reynolds numbers, which have been attributed to absolute and global instabilities resulting from inflected velocity profiles.