Journal of Geophysical Research: Oceans

The Congolese upwelling system (CoUS), located along the West African coast north of the Congo River, is one of the most productive and least studied systems in the Gulf of Guinea. The minimum sea surface temperature in the CoUS occurs in austral winter, when the winds are weak and not particularly favorable to coastal upwelling. Here, for the first time, we use a high‐resolution regional ocean model to identify the key atmospheric and oceanic processes that control the seasonal evolution of the mixed layer temperature in a 1°‐wide coastal band from 6°S to 4°S. The model is in good agreement with observations on seasonal timescales, and in particular, it realistically reproduces the signature of the surface upwelling during the austral winter, the shallow mixed layer due to salinity stratification, and the signature of coastal wave propagation. The analysis of the mixed layer heat budget for the year 2016 reveals a competition between warming by air‐sea fluxes, dominated by the incoming shortwave radiation throughout the year, and cooling by vertical mixing at the base of the mixed layer, as other tendency terms remain weak. The seasonal cooling is induced by vertical mixing, where local wind‐driven dynamics play a secondary role compared to subsurface processes. A subsurface analysis shows that remotely forced coastal‐trapped waves raise the thermocline from April to August, which strengthens the vertical temperature gradient at the base of the mixed layer and leads to the mixing‐induced seasonal cooling in the Congolese upwelling system.

Properties of the Fraser River plume in the Strait of Georgia, BC, are significantly influenced by the tide. However, the dynamics and magnitude of this tidal influence on the plume area are not known. Here, we use 17 years of daily satellite observations of suspended particulate matter to understand the tidal variability of the plume area. A consistent inverse relationship between the Fraser River plume area and the tidal elevation with a phase lag of 1–2 hr is revealed from two independent analyses: one by correcting for temporal aliasing and extracting tidal signal from the whole image set and the other using only same‐day image pairs. The plume area increases/decreases by about 20% after ebb/flood tides, and a lower river discharge typically leads to a more dramatic tidal variation in the plume area. A simple analytical model based on the volume conservation and salinity balance equations is developed to analyze the mechanism of the tidal variability in the plume size. The observed tidal patterns of the plume area are largely reproduced using tidally modulated plume salinity (observed from instrumented ferries) and river discharge (from numerical model outputs). Tidal flux in both river discharge and entrainment rate is found to be important in explaining plume area variability.

The advent of under‐ice profiling float and biologging techniques has enabled year‐round observation of the Southern Ocean and its Antarctic margin. These under‐ice data are often overlooked in widely used oceanographic datasets, despite their importance in understanding seasonality and its role in sea ice changes, water mass formation, and glacial melt. We develop a monthly climatology of the Southern Ocean (south of 40°S and above 2,000 m) using Data Interpolating Variational Analysis, which excels in multi‐dimensional interpolation and consistent handling of topography and horizontal advection. The climatology successfully captures thermohaline variability under sea ice, previously hard to obtain, and outperforms other observation‐based products and state estimate simulations in data fidelity, with smaller root‐mean‐square errors and biases. To demonstrate its multi‐purpose capability, we present a qualitative description of the seasonal variation, including (a) the surface mixed layer, (b) the water mass volume census, (c) the Antarctic Slope Front, and (d) shelf bottom waters. The circumpolar variation in the extent of dense shelf water—including its presence outside the four major formation sites—and the annual volume overturning that reaches deep waters are revealed for the first time. The present work offers a new monthly climatology of the Southern Ocean and the Antarctic margin, which will be instrumental in investigating the seasonality and improving ocean models, thereby making valuable winter observations more accessible. We further highlight the quantitative significance of under‐ice data in reproducing ocean conditions, advocating for their increased use to achieve a better Southern Ocean observing system.

In this study, we examine intensive observational measurements from a 12‐glider array in the South China Sea, and reveal that tropical cyclone “Haitang” created the conditions for the development of several types of forced submesoscale instabilities within a mesoscale anticyclonic eddy. The anticyclonic eddy shed from the Kuroshio loop current in the Luzon Strait and propagated toward the South China Sea. Fine‐scale temperature and salinity observation from gliders captured the complex mesoscale frontal structure induced by mesoscale strain around anticyclonic eddy (AE). Various favorable conditions for submesoscale instabilities show significantly different spatial distributions as well as temporal evolution characteristics in the AE. Analyses indicate that the occurrence probability of forced symmetric instability (SI) and gravitational instability (GI) during the tropical cyclone (TC) period (∼5 days) is found to be 2 times higher than that during the non‐TC period (∼25 days). Heat loss creates conditions for GI in the upper part of the negative potential vorticity (PV) layer, and GIs tend to be distributed inside the eddy. Strong wind stress induced by the TC promotes the injection of negative PV through cross‐front Ekman buoyancy flux, leading to the occurrence of SI at the edge of the eddy. During the TC, stable wind fields are more favorable for the development of submesoscale instability compared to rotating wind fields. The effect of strong winds breaks the normal diurnal cycle of SI, creating conditions for active submesoscale instabilities at midday. These findings help us to understand submesoscale air‐sea interaction processes.

The Southern Ocean spring phytoplankton bloom impacts regional food webs and the marine carbon cycle, but we do not fully understand which drivers—environmental, ecological, or biological—control the timing of the onset of the spring bloom. Nutrients, particularly iron, are likely replete in the austral winter, but the importance of underwater light availability and grazing pressure are topics of ongoing discussion. Furthermore, in the extreme polar winter, phytoplankton physiology may impart additional constraints on the bloom onset. We analyzed biogeochemical (BGC) Argo profiles from the Pacific sector of the Southern Ocean, and a one‐dimensional water column turbulence model forced by reanalysis data. Though the surface mixed layer defines where density is homogenous, the presence of enhanced turbulence and the active mixing of constituents, such as chlorophyll fluorescence, is better estimated by the depth of active mixing that we estimate from the turbulence model. We identified two regimes: one north of the subantarctic front where bloom onsets occur around July, before the seasonal maximum in mixing depth and when light availability remained near its annual minimum value. It is likely that changes in the phytoplankton loss rate control the bloom onset in this region. South of the subantarctic front, bloom onsets occur closer to austral spring following enhanced light availability, suggesting that bloom onset is primarily controlled by phytoplankton growth rather than loss terms. Our analysis shows that new insights can be gained into spring bloom phenology from the combination of float and model data.

The interannual relationships between the cross‐equatorial cell (CEC) in the Indian Ocean (IO) and the El Niño‐Southern Oscillation (ENSO) are examined through observational data and numerical simulations. The findings indicate a notable intraseasonal variation in the boreal summer IO CEC responses to preceding ENSO events, showing a weakening in early summer (May–June) and a strengthening in late summer (August–September) subsequent to an El Niño events. These contrasting responses are primarily driven by opposite meridional Ekman transport anomalies, characterized by anomalous northward Ekman transport in early summer and southward transport in late summer. Further analysis reveals that ENSO‐induced surface zonal wind anomalies predominantly influence these anomalous meridional Ekman transport in the upper layer, accounting for over 80% of the variation. In early summer, an antisymmetric wind pattern over the IO, induced by El Niño in the decaying spring, along with the westward extension of the anomalous western North Pacific anticyclone (WNPAC), generates anomalous easterlies over the North IO (NIO), leading to weakened meridional Ekman transport and a diminished CEC. Simultaneously, those anomalous easterlies and the associated weakened CEC in early summer trigger a wind‐CEC‐SST (WCS) negative feedback mechanism. The resulting anomalous northward transport and reduced upwelling lead to sea surface temperature (SST) warming in the northern NIO, creating a northward surface ocean temperature gradient in the July–August period. This gradient, along with the eastward retreat of the WNPAC, causes anomalous NIO westerlies in late summer, enhancing meridional Ekman transport and ultimately strengthening the CEC.

The ability to empirically and accurately predict runup on natural beaches is made difficult by the random nature of waves and extreme runup events. In some cases, extreme runup events are the result of bore capture where one broken wave passes over the front of and merges with another broken wave or shoreline capture where capture occurs at the instantaneous shoreline. Here we use high resolution Lidar data to investigate potential drivers of bore and shoreline capture on a macro‐tidal dissipative beach. The proportion of runup events that are derived from capture(s) was identified within normalized runup elevation percentiles which increased from 15% in the lowest tenth percentile of runup elevations to 55% of runup events in the highest tenth percentile. Bore capture was found to occur primarily on the rising infragravity wave in both space and time, whereas shoreline capture occurred predominately during the rising and peak phases of the infragravity wave. The occurrence of bore capture was not, however, fully restricted to particular infragravity phases, suggesting multiple drivers of capture. Bore trajectories of pairs of captured bores and non‐captured bores were tracked showing that the probability of capture is also a function of normalized interwave proximity, the ratio of depths beneath consecutive wave crests, and normalized proximity to the mean shoreline. More dissipative beaches therefore not only have more infragravity energy within the surf and swash zones (thus infragravity modulation of bore capture), the wider surf zones provides greater time for bores to capture preceding bores.

In existing two‐dimensional (depth integrated) storm surge models coupled with wave models, the surface wave effect is traditionally included as the radiation stress gradient forcing, which accounts for momentum transfer from surface waves to currents. However, recent studies on wave‐current interactions indicate that radiation stress alone does not fully capture the impact of waves on ocean currents and sea surface elevation. In this study, we derive new governing equations for two‐dimensional storm surge models to incorporate more comprehensive wave‐current interactions. Instead of using vertically integrated Eulerian currents, our formulation is based on vertically integrated Lagrangian currents, which include the wave Stokes drift. The resulting momentum equations include a new wave‐induced forcing term that depends on both waves and currents, in addition to the radiation stress gradient. We incorporated the new term into the Advanced Circulation storm surge model, coupled with the Wavewatch III wave model, and simulated storm surges during Hurricanes Michael (2018) and Ian (2022). Our results indicate that, while the radiation stress forcing significantly increases water levels (by 0.4 m or more) over a large area to the right of the storm track, the new wave‐induced forcing causes a notable reduction in water levels (up to 0.3 m) near the storm track shortly before the storm makes landfall. This reduction is attributed to the alignment of strong currents and waves in the alongshore direction.

Transient rip currents drive cross‐shore transport of nutrients, larvae, sediment, and other particulate matter. These currents are driven by short‐crested wave breaking, which is associated with rotational wave‐breaking forces (vorticity forcing) that generate horizontal rotational motions (eddies) at small scales. Energy from small‐scale eddies is transferred to larger‐scale eddies that interact and enhance cross‐shore exchange. Previous numerical modeling work on planar beaches has shown that cross‐shore exchange increases with increasing wave directional spread, but this relationship is not established for barred beaches, and processes connecting the wavefield to cross‐shore exchange are not well constrained. We investigate surf‐zone eddy processes using numerical simulations (FUNWAVE‐TVD) and large‐scale laboratory observations of varying offshore wave directional spreads (0 to ∼25° ${\sim} 25{}^{\circ}$) and peak period (1.5–2.5 s) on an alongshore uniform barred beach. We find that mean breaking crest length decreases, while crest end density (number of crest ends in a given area) increases, with increasing directional spread. In contrast, vorticity forcing, offshore low‐frequency rotational motion, and cross‐shore exchange peak at intermediate directional spreads (∼10°) $(\sim 10{}^{\circ})$. Distributions of the strength of vorticity forcing per crest and across the surf zone suggest that the peak in vorticity forcing at intermediate spreads results from a combination of a larger total breaking area and relatively long crests with large forcing, despite a lower total number of crests. However, low‐frequency rotational motion within the surf zone does not peak at mid‐directional spread, instead plateauing at directional spreads greater than ∼10° ${\sim} 10{}^{\circ}$. Results suggest that eddy‐eddy interaction, the transformation of vorticity across the surf zone, and influence of bathymetry are fruitful topics for future work.

This study investigates near‐bottom currents and physical processes from simulations with the hydrodynamic model ROMS‐AGRIF at two seamounts of the northeast Walvis Ridge to obtain valuable insights about drivers of observed occurrences of benthic suspension feeders (cnidarians and sponges) in this data‐poor area. The spatial resolution in each model area was increased across two levels of nested grids from 1,500 m to 500 m resolution with 32 stretched terrain‐following (s‐) layers in the vertical with high resolution close to the bottom. The parent grids receive initial and boundary conditions from the basin‐scale model INALT20 and from solutions of the OTIS inverse tidal model. The model topography is based on GEBCO data with local refinements from multi‐beam data collected during different surveys in 2008, 2009, and 2010. Increasing model resolution is an important advancement for precisely evaluating the intrinsic dynamics within challenging rough terrain. We examined how near‐bottom currents vary over space and time and investigated potential links between observed Cnidarian and Porifera occurrences and ranges of physical variables and processes. We identified a close link between physical processes and species distributions and suggested that physical processes such as kinetic energy dissipation and internal wave dynamics may be considered in future research as proxies of food supply to benthic suspension feeders. Such mechanistic variables may also be used to supplement more traditional descriptors such as water mass and terrain properties in species distribution models, thus enhancing our ability to predict the occurrence of benthic communities characterized by cnidarians and sponges.

Open ocean polynyas, regions of open water surrounded by sea ice, frequently occur in the West Cosmonaut Sea, an Antarctic marginal sea in the southern Indian Ocean sector. These polynyas play a crucial role in regional energy exchange and influence Antarctic atmospheric processes. This study examines the spatial and temporal distribution of the West Cosmonaut Sea polynyas (WCP) from 1979 to 2023, using sea ice concentration (SIC) data collected from May to August. Our results reveal that a pronounced winter sea ice decline promotes the embayment shape formation, precursor to WCP with open water encircled on three sides by sea ice, mainly open on the northeast side. Statistical analysis identifies regions between 62.0–67°S and 28.0–50.0°E, centered near 65°S, 41°E, as hotspots of polynya occurrence. The annual mean WCP area ranges from 2.0 × 10³ to 0.7 × 10⁵ km², with maximum yearly extents between 3.6 × 10³ to 1.5 × 10⁵ km². The yearly accumulated lasting time spans 3–20 days, exhibiting interannual variability with periodicities of 2–3 years and 4–8 years, partially modulated by the Southern Annular Mode. Since 1987, the duration of WCP events has markedly increased, though a decline has been observed since 2012, likely linked to variations in SIC within the embayment. Enhanced wind stress curl supports WCP formation, increases precipitation, and contributes to polynya closure. WCP dynamics amplify evaporation, latent and sensible heat flux, further highlighting the complex interplay between the atmosphere and the ocean in the Antarctic.

We analyze the output of a regional ocean model that comprises the North Atlantic and the Arctic Ocean for the period 1975–2021. We focus on the flow through the cross sections closing the Nordic Sea basin. The simulated flow at Barents Sea Opening (BSO) shows a clear positive trend. To understand the origin of this trend, we reconstruct the BSO flow based on wind time series over the Nordic Seas using deep learning. To explore potential links between the results from this reconstruction and the major atmospheric modes, we perform a suite of idealized experiments where the ocean model is forced with wind field anomalies that refer to known changes in the leading modes of atmospheric circulation over the North Atlantic and Arctic Oceans. Known changes in the major atmospheric wind patterns over the North Atlantic have a weak impact on the simulated BSO flow, and the sign is not consistent with the overall trend of the full simulation. The latter holds as well for the known temporal changes in the intensity of the Arctic dipole mode. The weak temporal changes in the Arctic oscillation are consistent with the trend in the BSO flow but could not explain its amplitude. Ultimately, we could not establish a clear link between the BSO flow trend and changes in the major atmospheric modes. We conclude that the atmospheric pattern responsible for the BSO flow trend does not project directly on the leading modes of atmospheric variability over the North Atlantic and the Arctic.

This modeling study analyzes the circulation over the Agulhas Bank (AB). It is suggested that the time mean circulation over the bank is primarily driven by the inflow of shelf waters from the northeastern region, and not by local forcing as previously postulated. Seasonal variations of the circulation and temperature and salinity fields are highly correlated with the atmospheric forcing. Currents shift inshore during the winter, returning to its original position during summer. The equatorward flow in the western AB, which includes a deep, previously unreported, countercurrent, strengthens during spring and summer and wanes during fall and winter. Tracer diagnostics and Eulerian mass balances reveal very energetics mass exchanges between the eastern AB and the Agulhas Current (AC). The AB Bight is the preferential site for these exchanges. Lagrangian diagnostic show 0.45 Sv of deep open‐ocean waters entrained into the bottom layer of the shelf. Cross‐shelf exchanges produce significant water mass transformations. Tides play an unexpectedly significant role on the AB circulation. Preliminary considerations suggest that shelf/open‐ocean interactions could have a significant impact on water mass conversions within the AC.

Previous studies have highlighted the individual importance of diurnal warm layers (DWLs) and surface‐layer fronts within the surface boundary layer (SBL) in regulating energy, momentum, and gas exchange between the atmosphere and the ocean. This study investigates the interactions between DWLs and surface‐layer fronts using field observations and numerical turbulence models. Our study provides the real‐ocean relevance of the coexistence of DWLs and surface‐layer fronts in the SBL in an eddy‐rich tropical ocean subjected to intense solar heating and weak winds. We found that the presence of a DWL isolates the deeper layers of the SBL from diabatic and frictional surface forcing, causing these layers to quickly become non‐turbulent whereas remaining in a state of marginal stability. This condition suggests that small perturbations from local processes, such as internal tides and waves, can easily trigger instability and turbulence. Additionally, frontal dynamics were observed to deepen the SBL, allowing near‐surface diurnal shear associated with DWL dynamics to penetrate to greater depths during nighttime, compared to conditions without a front, thereby facilitating the vertical transport of heat and tracers. Our findings underscore the necessity of accurately representing the interactions between DWLs and surface‐layer fronts to enhance the precision of ocean circulation and climate models.

Understanding sediment transport processes within tidal flats is crucial for developing effective land‐ocean interaction management strategies. The cross‐shore sediment transport on tidal flats induced by episodic events, such as wind‐driven flow reversal (WDFR) and fluid mud (FM), is not sufficiently understood. This study focuses on the central Jiangsu tidal flat, where two field campaigns were conducted in the winter of 2021 and the summer of 2022. During the winter campaign, WDFR events were identified. During WDFR, the wind reversed the tide flow direction, resulting in significant cross‐shore sediment fluxes. In summer, FM occurred frequently during tidal slack periods when current‐induced bottom stress was low. The settling of sediment from the overlying fluid into the bottom layer plays a pivotal role in initiating FM events. These events resulted in substantial cross‐shore sediment fluxes, exceeding the long‐shore component. This study highlights the need to appropriately address the contributions of WDFR and FM to cross‐shore sediment transport in similar coastal environments.

Winter dissolved nickel (dNi) and particulate nickel (pNi) concentrations were measured in the Southern Ocean (GEOTRACES GIpr07 transect) to investigate biogeochemical cycling within the water column and over seasonal timescales. Concentrations of dNi ranged from 1.98 to 8.21 nmol kg⁻¹ with low surface concentrations and maxima in deepest sampled water masses. Combining our winter data with the GEOTRACES Intermediate Data Product (2021) shows insignificant seasonal dNi variation in surface waters north of the Antarctic Polar Front, indicating the dominance of year‐round mixing processes. However, lower summer concentrations than winter in the Antarctic Zone (∆0.23 nmol kg⁻¹) suggest a role for biological processes at high latitudes. For pNi, concentrations ranged from 5 to 49 pmol kg⁻¹ with higher values in surface/near‐surface water masses. Vertical attenuation factors (b values) for pNi (0.19 ± 0.06) and particulate phosphorus (pP; 0.43 ± 0.10) suggest a greater retention of Ni in particles than P, invoking scavenging processes or refractory Ni phases. Water mass analysis shows that remineralization of pNi contributes a maximum of 6% of the highest measured dNi. Instead, dNi distributions and macronutrient relationships were largely explained by phytoplankton uptake in surface waters, and mixing and advection of Atlantic and Antarctic origin water masses, each with different preformed nutrient compositions. Winter trace metal measurements provide new perspectives regarding the balance between biological and physical drivers in the Southern Ocean. For Ni, the biological component is small with respect to physical mixing processes and over the timescales in which water masses accumulate Ni during their transport.

Coastal current encountering a protruding headland is a ubiquitous phenomenon. Previous studies indicated that the coastal current either moves well around headland or separates offshore, leaving the upstream region unaffected. Yet, these studies often assumed a deep vertical coastal wall, and the coastal current was either of barotropic character or surface‐advected, with weak interactions with the sloping topography. Here in this study, we conducted numerical experiments to investigate how a protruding headland regulates the “bottom‐trapped” buoyant coastal current over a sloping coastal topography. It was found that at the initial stage, the coastal current separates at the sharp headland tip due to local increased centrifugal force, forming a secondary bulge on the lee side of the headland. Upstream of the headland, a countercurrent is formed shoreward of the front, which fills the space between front and coast, thus pushing the front offshore. This process persists as long as the cross‐shelf scale of headland is larger than the baroclinic Rossby deformation radius. The final effect is that the front adapts its cross‐shelf location to minimize the form drag induced by the headland, and consequently the separation on the lee side of the headland was reduced. Downstream of the headland, the plume front weakens and the alongshore propagation is slowed down, because more freshwater is stranded upstream. Such dynamics are distinct from the surface‐advected buoyant coastal current, and may explain the fact that many buoyant coastal currents along zigzag coastline are wide and their alongshore extension distances are limited.

Salt intrusion poses a global threat to estuaries and deltas, exacerbated by climate change, drought, and sea level rise. This observational study investigates the impact of river discharge, wind, and tidal variations on salt intrusion in a branching river delta during drought. The complexity and spatial extent of deltas make comprehensive measurements challenging and rare. In this paper, we present a 17‐week data set of a historic drought in the Rhine‐Meuse Delta, capturing dynamics in a multiple‐channel system in a wide range of conditions. Key characteristics of this low‐lying delta are its branching channel network and complicated, human‐controlled discharge. Despite the system's complexity, we found that the subtidal salt intrusion length, defined by the 2 PSU isohaline L2 $\left({L}_{2}\right)$, follows a power law relationship with Rhine River discharge L2∝QR−0.35±0.03 $\left({L}_{2}\propto {Q}_{R}^{-0.35\pm 0.03}\right)$. Subtidal water level variations contribute to short‐term variations in intrusion length, shifting the limit of salt intrusion upstream and downstream with a distance similar to the tidal excursion length. This can be attributed to the up‐estuary transport of seawater, caused by the estuary adjusting to variations in water levels at its mouth. However, spring‐neap variation in the tidal range does not alter the subtidal salt intrusion length. Side branches exhibit distinct dynamics from the main river, and their most important control is the downstream salinity. We show that treating the side branches separately is crucial to incorporate the highly variable downstream boundary condition, and may apply in other deltas or complex estuaries.

Emerging high‐resolution global ocean climate models are expected to improve both hindcasts and forecasts of coastal sea level variability by better resolving ocean turbulence and other small‐scale phenomena. To examine this hypothesis, we compare annual to multidecadal coastal sea level variability over the 1993–2018 period, as observed by tide gauges and as simulated by two identically forced ocean models, at ∼1° ${\sim} 1{}^{\circ}$ (LR) and ∼0.1° ${\sim} 0.1{}^{\circ}$ (HR) horizontal resolution. Differences between HR and LR, and misfits with tide gauges, are spatially coherent at regional alongcoast scales. Resolution‐related improvements are largest in, and near, marginal seas. Near attached western boundary currents, sea level variance is several times greater in HR than LR, but correlations with observations may be reduced, due to intrinsic ocean variability. Globally, in HR simulations, intrinsic variability comprises from zero to over 80% of coastal sea level variance. Outside of eddy‐rich regions, simulated coastal sea level variability is generally damped relative to observations. We hypothesize that weak coastal variability is related to large‐scale, remotely forced, variability; in both HR and LR, tropical sea level variance is underestimated by ∼ ${\sim}$50% relative to satellite altimetric observations. Similar coastal dynamical regimes (e.g., attached western boundary currents) exhibit a consistent sensitivity to horizontal resolution, suggesting that these findings are generalizable to regions with limited coastal observations.

Wind, wave, and acoustic observations are used to test a scaling for ambient sound levels in the ocean that is based on wind speed and the degree of surface wave development (at a given wind speed). The focus of this study is acoustic frequencies in the range 1–20 kHz, for which sound is generated by the bubbles injected during surface wave breaking. Traditionally, ambient sound spectra in this frequency range are scaled by wind speed alone. In this study, we investigate a secondary dependence on surface wave development. For any given wind‐speed, ambient sound levels are separated into conditions in which waves are 1) actively developing or 2) fully developed. Wave development is quantified using the non‐dimensional wave height, a metric commonly used to analyze fetch or duration limitations in wave growth. This simple metric is applicable in both coastal and open ocean environments. Use of the wave development metric to scale sound spectra is first motivated with observations from a brief case study near the island of Jan Mayen (Norwegian Sea), then robustly tested with long time‐series observations of winds and waves at Ocean Station Papa (North Pacific Ocean). When waves are actively developing, ambient sound levels are elevated 2–3 dB across the 1–20 kHz frequency range. This result is discussed in the context of sound generation during wave breaking and sound attenuation by persistent bubble layers.

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.

Suspended matter (SPM) was sampled in a grid off the Pearl River mouth in the northern South China Sea (SCS). SPM concentrations and the content of total organic carbon (TOC), total nitrogen (TN), amino acids (AA) and hexosamines (HA) and derived biogeochemical indicators were used to study organic matter sources on the shelf and slope. A surface SPM maximum curtailed the water mass of mixed riverine and marine origin off the Pearl River mouth with salinities <30. SPM in this river plume was rich in organic matter of fresh planktic origin. In areas outside the river plume chlorophyll (Chl‐a) maxima were found at the subsurface nutricline. The AA composition shows that the degradation state of organic matter is very similar in all samples except in bottom water samples. Rather than degradation indicators, an indicator of SPM residence time in the ocean shows differences between samples from the upper 200 m and the deeper SPM samples. On the shelf and the shelf break a distinct SPM maximum was found above the sea floor. AA and HA spectra revealed that its organic matter was more degraded than the other SPM samples and that part of the organic matter in the bottom water turbidity maximum originated from the fine fraction of sediments. The state of organic matter implies that degradation of this resuspended material possibly adds to bottom water hypoxia; furthermore, contaminants originally deposited in shelf sediments can be redistributed into distal areas of the South China Sea.

Plain Language Summary In this study, we analyze tritium and helium‐3 (³He) data from 21 oceanographic expeditions conducted over 27 years to track the movement and speed of the Atlantic Layer—waters originating from the North Atlantic, found in the Arctic Ocean. The combined measurement of tritium and ³He acts as a “natural clock,” allowing us to estimate the water age and understand its movement through various oceanic pathways. Our findings confirm that the Atlantic Layer circulates in a consistent counterclockwise pattern along the continental slope, functioning as a boundary current. This circulation is influenced by underwater mountain ridges, which generate northbound flows where they intersect with the continental slope. We noted that the speed of the Atlantic Layer varies horizontally depending on the route it follows, but there is no significant speed difference between the upper and lower parts of the layer. Our research supports the hypothesis originally proposed by Rudels et al. (1994), https://doi.org/10.1029/gm085p0033, suggesting that the pathways of Atlantic water are stable and shaped by the ocean's underwater topography. This study provides new insights into the intricate patterns of ocean circulation in the Arctic.

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.

Understanding spatiotemporal variations in phytoplankton biomass is crucial to the health of marine ecosystems. Therefore, the Finite Volume Community Ocean Model (FVCOM)‐based Ecological Model (Integrated Compartment Model) was implemented to assess nutrient and phytoplankton dynamics in the Bohai Sea from 2010 to 2019. From 2010 to 2013, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) levels in the Bohai Sea were higher (∼0.400 and 0.030 mg/L, respectively) in nearshore areas compared to other years. During the summer and fall, the spatial distribution of the DIN/DIP ratio in the Bohai Sea exhibited a double‐core structure. Higher values (>100) were primarily concentrated in the central region of the Liaodong Bay and south of the Central Basin near the Yellow River Estuary. Statistical analyses and numerical experiments revealed that increasing DIP loading from rivers had a greater effect on phytoplankton biomass in the Laizhou and Bohai Bays than increasing DIN loading. However, the phytoplankton biomass in the Liaodong Bay was strongly influenced by both increasing DIN and DIP loading from rivers. Notably, the increase in phytoplankton biomass resulting from increasing DIN loading exceeded that from increasing DIP loading by 17% in the Liaodong Bay. The reduction in river discharge weakened circulation at the river mouths, thereby partially retaining surface phytoplankton. This was more predominant in the nearshore areas of the Yellow River owing to the higher river discharge in August 2019. This study provides valuable insights for the management and conservation of marine ecosystems.