Journal of Geophysical Research-Oceans最新文献

The hydrodynamics of river mouths are the result of a complex interaction between river flow, tidal conditions, and outlet geometry. This complex interaction of factors shapes the jet that flows onto the continental shelf and influences the dynamics of these areas. This work analyses the influence of idealized but realistic outlet geometries under steady and unsteady hydrodynamic conditions on the jet structure. The analysis is carried out using a numerical model which is validated by comparison with the classical jet theory for highly simplified conditions where this theory applies. The results show that both the outlet geometry and the transient hydrodynamic conditions have a significant influence on the jet structure and evolution along the nearshore. For constant river discharge and water level conditions, the results indicate that the nearshore profile plays a key role in determining the expansion or contraction of the jet. The momentum balance shows that the jet behavior is related to the momentum transport and the barotropic terms. In cases where the river discharge and tidal conditions are transient, the jet alternates between a structure with two velocity maxima at the edges or a single peak in the center during the tidal cycle depending on the phase lag between the tidal conditions and the river hydrograph. These two different jet structures play an important role in the morphodynamic evolution of the river mouths and bar development, favoring in some cases the formation of lateral levees parallel to the channel walls.

As a unique phenomenon occurring in the subsurface ocean, submesoscale coherent vortices (SCVs) are believed to have a pivotal role in the long-distance ocean tracer transports. SCVs have been widely observed in the global oceans; however, most of them are captured accidentally, and their kinematic characteristics and water mass transports have only been studied in a limited number of regions. Here, we use 4-year observations of velocity, temperature, and salinity from five moorings in the northeastern South China Sea (NESCS) to examine dozens of newly discovered SCVs. A total of 34 SCVs were identified during the observational period, including 25 convex lens-like anticyclones and 9 concave lens-like cyclones. The maximum swirl velocity, mean radius, and vertical length scale of the anticyclones (cyclones) are 0.19 ± 0.07 m s⁻¹ (0.19 ± 0.07 m s⁻¹), 26.4 ± 13.9 km (17.0 ± 5.4 km), and 204 ± 62 m (188 ± 53 m), respectively. Vertically, the velocity structure of the observed SCVs conforms to a Gaussian function when the effect of stratification is removed. Water mass analysis suggests that 88% (30/34) of the SCVs carried Kuroshio water, which demonstrates the mechanism proposed by Zhang et al. (2022), https://doi.org/10.1029/2021jc018117 that they are formed by Kuroshio-islands interactions in the Luzon Strait. This category of SCVs is therefore named Luzon Strait island wake eddies (Liddies). We further estimate that Liddies can result in an equivalent mean volume transport of 0.20 Sv westward across the Luzon Strait, which suggests that they play a non-negligible role in the subsurface water transports between the NESCS and the northwestern Pacific.

Salt intrusion in estuaries can pose a threat to drinking water extraction, irrigation, industries, and ecology. Key factors influencing the salt intrusion length include freshwater river flushing and vertical salt mixing. This study investigates the potential of estuarine sand dunes, that is, large-scale rhythmic patterns, to serve as a nature-based solution to mitigate salt intrusion. To this end, we adopt a two-dimensional vertical (2DV) modeling approach, which numerically solves the flow and salt transport in an idealized single-channel estuary. By varying the tidal amplitude and river discharge, three types of estuaries are modeled; a salt wedge, a strongly stratified and a strain-induced periodic stratification estuary. Two types of dune configurations are considered: ’depth-neutral’ dunes (preserving mean water depth) and ’deepening’ dunes (crest level equal to original bed level, rest of dune profile entirely below). Model results demonstrate that the presence of dunes can effectively reduce the salt intrusion length by several kilometers, compared to the scenario without dunes. Systematically varying dune geometry demonstrates that steeper (higher and shorter) dunes yield a stronger reduction in the salt intrusion length. Deepening dunes have the potential to generate sufficient mixing to overcome the adverse effects of channel deepening and, as such, can mitigate salt intrusion while maintaining navigability. As a naturally existing bedform, estuarine sand dunes present a promising nature-based solution against salt intrusion without compromising accessibility to seaports.

The Antarctic Bottom Water flow through abyssal channels, fracture zones and troughs contributes to the lower limb of the global ocean meridional overturning circulation. These abyssal flows also control sedimentation processes and form channels, furrows, contourite drifts, and other erosional and depositional seafloor features. Here, we present new measurements that provide observational evidence of an abyssal flow along a more than 800-km-long erosional channel in the tropical Atlantic. Our measurements show that the discovered flow provides transport of the coldest abyssal waters into the Northwest Atlantic; the existence of the flow is also shown by a numerical model. The agreement between the observed and simulated flow structures provides a reliable estimate of the coldest water transport to the Northwest Atlantic, confirms the influence of erosional channels on bottom circulation, and increases our confidence in the ability of numerical models to simulate three-dimensional ocean circulation including its deepest layers.

The extreme winds of tropical cyclones generate high waves over the ocean, causing severe damage to offshore facilities and coastal communities. Disaster mitigation requires accurate prediction and forecasting of the highest potential waves. However, the physics of wave development is not fully understood, and the number of mid-ocean observations during extreme tropical cyclones is extremely insufficient. Therefore, physically and statistically evaluating the highest potential waves is difficult. However, ocean waves excite seismic noise (microseisms). Although source sites of microseisms under specific tropical cyclones have been identified by case studies, the extreme ocean wave magnitude under tropical cyclones have not been systematically analyzed using long-term historical records. Here, we, for the first time, utilize long-term microseisms observed by seismic observation networks to associate the historical maximum microseisms events with the highest tropical cyclone-generated wave events around Japan. We show that Typhoon Wipha in 2013, Typhoon Lan in 2017, and Typhoon Hagibis in 2019 were the maximum microseisms events in the past 20 years based on microseism energy within an 8–10 s period. We associate the events with the highest wave heights and the largest wave-induced sea surface pressures generated by tropical cyclones. Although ocean wave models have a large uncertainty of tropical cyclone-generated extreme waves, microseisms observations can endorse the results of an ocean wave model even if lack of direct ocean wave observation under tropical cyclones. Furthermore, rich information of microseisms on wave development and propagation has a potential to proceed understanding of the extreme wave physics.

Most oceanic lead (Pb) is from anthropogenic emissions into the atmosphere deposited into surface waters, mostly during the past two centuries. The space- and time-dependent emission patterns of anthropogenic Pb (and its isotope ratios) constitute a global geochemical experiment providing information on advective, mixing, chemical, and particle flux processes redistributing Pb within the ocean. Pb shares aspects of its behavior with other elements, for example, atmospheric input, dust solubilization, biological uptake, and reversible exchange between dissolved and adsorbed Pb on sinking particles. The evolving distributions allow us to see signals hidden in steady-state tracer distributions. The global anthropogenic Pb emission experiment serves as a tool to understand oceanic trace element dynamics. We obtained a high-resolution (5° station spacing) depth transect of dissolved Pb concentrations and Pb isotopes from Alaska (55°N) to just north of Tahiti (20°S) near 152°W longitude. The sections reveal distinct sources of Pb (American, Australian, and Chinese), transport of Australian style Pb to the water mass formation region of Sub-Antarctic Mode Water which is advected northward, columnar Pb isotope contours due to reversible particle exchange on sinking particles from high-productivity particle veils, and a gradient of high northern deep water [Pb] to low southern deep water [Pb] that is created by reversible exchange release of Pb from sinking particles carrying predominantly northern hemisphere Pb. ²⁰⁸Pb/²⁰⁶Pb versus ²⁰⁶Pb/²⁰⁷Pb isotope relationships show that most oceanic Pb in the North Pacific is from Chinese and American sources, whereas Pb in the South Pacific is from Australian and American sources.

The bathymetry of the northern Gulf of Mexico is strongly influenced by diapirism of subsurface salt. A competition between salt dynamics and the depositional mechanics of sediment laden density flows over geological timescales controls the scale of seafloor depressions, which are the dominant bathymetric features of the margin. Salt domes create topographic highs, and salt removal to the domes creates topographic lows, with sediment deposition driving the gravitational dynamics. The strength of bathymetric self-organization into depressions is inferred through analysis of a vast bathymetric data set made public by the U.S. Bureau of Ocean and Energy Management. Depression geometric scales follow Pareto distributions, and their tail indexes aid inference of the strength of bathymetric self-organization, with lower tail indexes linked to greater self-organization. A comparison is made of margin subregions defined by pseudo-flow drainage density maps, which inversely relates to the pre-deformation thickness of subsurface salt. Tail indexes of distributions decrease with the thickness of the underlying salt. This is linked to the merger of depressions, which is enhanced when depressions can grow wider and deeper, as occurs over thick salt fields, and the development of salt structure. The manner of self-organization results in most of the margin's ponded sediment accommodation residing in relatively few depressions that have reliefs exceeding 100 m. This relief is sufficient to induce sedimentation from even the thickest turbidity currents, which can further drive gravitational dynamics. The bathymetric complexity of depressions is also greatest over regions with the thickest salt, further supporting enhanced self-organization.

Operational wave forecasts for the Great Lakes originate from the NOAA Great Lakes Waves Unstructured version 2 system. The model uses a simple ice blocking (IC0) parameterization for ice-wave damping, so ice-covered portions of the lakes are treated as land in the modeling system. Although simple and effective, the simple block can impede forecasting by eliminating wave forecast guidance from areas with thin or partial ice cover. We evaluate 12 ice-wave damping parameterizations within WAVEWATCH III (WW3, version 6.07.1) for Lake Erie, by comparing model results against wave observations made at several locations using moored acoustic wave and current profilers during the winters of 2010–2011 and 2012–2013. The comparisons show that the IC4M4 module performs the best among 12 ice modules with a root mean square error (RMSE) of 0.32–0.39 m and a root bias of −0.06 to −0.11 m, outperforming the existing IC0 parameterization (RMSE: 0.46–0.59 m; bias: −0.23 to −0.34 m) during the 2010–2011 analysis year. WW3 ice modules are mostly derived from measurements and studies of the Arctic and Antarctic Ocean. The dominant wave frequency is about 0.05∼0.10 Hz in the Arctic Ocean compared to 0.15∼0.2 Hz in the lake. Thus formulas built on frequency based on the studies from deep oceans may not be suitable for the shallow lakes because they cause too much damping. Although the IC4M4 ice module is from the study of the Antarctic Ocean , the wave attenuation formula based on incoming wave height is also suitable for Lake Erie.

A thermohaline staircase detection algorithm, applied to mooring and ice-tethered profiler data, systematically assessed the variability of fine-scale, diffusive-convective staircase abundance in the Arctic Ocean thermoclines in 2004–2023. Over that period, staircase occurrence statistically decreased in both the Amerasian and Eurasian basins, with thinner, shallower staircase layers preferentially decreasing over the Eurasian Basin's slope. In stark contrast to the Amerasian Basin, seasonality of detected staircase occurrence was pronounced in the Eurasian Basin and appeared to be increasing. Interannual and long-term variability of detectable staircase abundance and background thermocline density stratification were correlated, negatively so in the Amerasian Basin and positively in the Eurasian Basin, indicating reversed sensitivities of staircase constructive and destructive processes to stratification. Seasonal and long-term staircase variabilities in both basins were consistent with known environmental contrasts and tendencies, including upper freshening of the stronger, thicker Amerasian Basin halocline, the shift toward deeper winter ventilation of the weaker Eurasian Basin halocline, and more near-surface velocity shear over the Eurasian Basin's slope. There is no reason to believe that climate change will stop anytime soon, and we have good cause to believe that the observed tendencies in staircase structure will persist.

Wind stress deviates both in magnitude and direction under light wind conditions due to the modulation effect of swell waves. Eddy covariance flux measurements were conducted from an offshore wind tower in the northern South China Sea to investigate the influence of swell-induced perturbations on the evolution of wind velocity fluctuations and wind stress reductions. Direct evidence of the wind field being affected by dominant swell waves was observed at an observational height of 20 m, indicating that swell-induced perturbations can penetrate the wave boundary layer to at least this height. Time series analysis implies that a stable atmosphere helps preserve the observed prominent peak in wind velocity spectra, extending it to a higher range under large wave age. Therefore, swell-induced perturbations are more easily observed at night. Additionally, our results show that swell wave modulation on wind stress is not merely restricted to moments with pronounced peaks in velocity spectra, indicating that wind stress reduction is not an exception. The most significant reduction occurs at wind speeds around 2–4 m/s, leading to negative wind stress, which implies momentum transfer from the ocean to the atmosphere. Our conclusion illustrates that the deviation of wind stress magnitude increases under stable atmospheric conditions; however, with a simple correction model, the modified wind stress is comparable to the observed values.