Journal of Geophysical Research-Oceans最新文献

The formation and evolution of tidal jet vortices over complex bathymetry were investigated using numerical modeling and in situ observation. Delft3D FM simulation on an unstructured grid system, complemented by field measurements using recreational drones captured tidal dynamics in the Uldolmok Strait, known for its strong tidal currents up to 6.0 m/s and turbulent whirlpool formation. This study particularly focused on temporal changes in whirlpools near their narrowest points. Modeling across the entire strait showed current velocity fields consistent with field observations, revealing significant temporal and spatial variability influenced by strait geometry and bathymetry. Whirlpools induced by tides were identified by setting a swirl strength threshold, with their centroids and equivalent spherical diameters pinpointed. It was observed that larger whirlpools, upon reaching critical size, were entrained and shifted with the tidal jet at about half its maximum velocity, while smaller vortices separated from the nearshore boundary layer remained nearly stationary. Focusing on the initiation of whirlpools and their relations with the coastline and bathymetry, a targeted field survey using a recreational drone measured surface flow fields in detail near the strait's narrowing region. During the ebb phase, shallow regions exhibited numerous smaller eddies due to increased energy dissipation, showing a dual power-law scaling in the size distribution of eddies, contrasting with the single scaling exponent observed during the flood phase. The study underscored the role of coastline and bathymetry in developing whirlpools and shaping their dynamics, providing insights into complex tidal interactions in the natural coast.

In this study, we propose a scenario superposition method for real-time tsunami wave prediction. In the offline phase, prior to actual tsunami occurrence, hypothetical tsunami scenarios are created, and their wave data are decomposed into spatial modes and scenario-specific coefficients by the singular value decomposition. Then, once an actual tsunami event is observed, the proposed method executes an online phase, which is a novel contribution of this study. Specifically, the predicted waveform is represented by a linear combination of training scenarios consisting of precomputed tsunami simulation results. To make such a prediction, a set of weight parameters that allow for appropriate scenario superposition is identified by the Bayesian update process. At the same time, the probability distribution of the weight parameters is obtained as reference information regarding the reliability of the prediction. Then, the waveforms are predicted by superposition with the estimated weight parameters multiplied by the waveforms of the corresponding scenarios. To validate the performance and benefits of the proposed method, a series of synthetic experiments are performed for the Shikoku coastal region of Japan with the subduction zone of the Nankai Trough. All tsunami data are derived from numerical simulations and divided into a training data set used as scenario superposition components and a test data set for an unknown real event. The predicted waveforms at the synthetic gauges closest to the Shikoku Islands are compared to those obtained using our previous prediction method incorporating sequential Bayesian updating.

The supergyre in the Southern Hemisphere is thought to connect the Atlantic, Indian, and Pacific subtropical gyres together. The aim of the study is to investigate whether the supergyre is identifiable in the Coupled Model Intercomparison Project Phase 6 (CMIP6) models and in the Estimating the Circulation and Climate of the Ocean (ECCO) reanalysis and to evaluate the influence of the supergyre on the properties of Antarctic Intermediate Water (AAIW), the dominant water mass at intermediate depths in the Southern Hemisphere. CMIP6 models and ECCO are in agreement at the surface with supergyres connected across all basins but present some differences at depth in both position and strength. AAIW core properties (temperature and salinity) present a high degree of similarity across basins within the supergyre but not outside of it. By the end of the century, the supergyre reduces in size and intensifies at intermediate depths, and the AAIW core depth warms in all basins and freshens in the Pacific although no clear trend in salinity can be found in the Atlantic and Indian basins in the SSP5-8.5 scenario. The high degree of similarity across basins within the supergyre is maintained in the future scenario. The results suggest that by connecting the basins together at intermediate depth, the supergyre plays a key role in circulating and homogenizing the AAIW core properties. Our results emphasize the role of the supergyre in circulating water masses at the surface and intermediate depths in CMIP6 models and hence its importance to the global circulation.

Basal melting of ice shelves has become one of the main causes of mass loss from the Greenland ice sheet. However, most studies have focused on individual ice shelves, making it difficult to gain a more comprehensive understanding of basal melting across Greenland ice shelves. To address this issue, we utilized timestamped ArcticDEM strip data coregistered with ICESat-2 data to estimate the basal melt rates of the ice shelves in North Greenland at a resolution of 150 m from 2013 to 2022, employing a mass conservation approach within the Lagrangian framework. Additionally, to investigate the influence of temperature on basal melt rates, a basic analysis correlating the basal melt rates with temperatures was conducted. Overall, the mass loss caused by basal melting of the six ice shelves has amounted to 27.86 ± 35.63 Gt yr⁻¹, accounting for approximately 90% of the non-calving mass loss, equivalent to a sea level rise of 0.08 ± 0.10 mm yr⁻¹, far exceeding surface mass loss and glacier calving. The two larger ice shelves, Petermann and 79° North (79N), have contributed to 85% of the basal melt mass loss. Regarding the spatiotemporal distribution, the basal melt rates have gradually decreased from near the grounding line to the ice shelf front. Apart from the Ryder ice shelves, the basal melting of the other ice shelves is in a state of accelerated ablation. Moreover, compared to the skin temperature of the ice shelf, the sea water potential temperature has a greater impact on the basal melt rate.

Aerosol deposition is one of the major processes providing bioavailable Fe to the surface ocean. However, the quantification of aerosol Fe flux in the surface ocean is highly challenging operationally. In this study, we measured both Fe isotopic composition and specific elemental ratios in 5 size-fraction aerosols collected over the East China Sea (ECS) to quantify the relative contribution of lithogenic and anthropogenic aerosol Fe. Both the isotopic and elemental ratios indicate that anthropogenic aerosol Fe mainly originates from high-temperature combustion activities with the end member of the δ⁵⁶Fe to be −4.5‰. We found that the Cd/Ti ratio is a much more reliable proxy to quantify the contribution of anthropogenic aerosol Fe in coarse aerosols than δ⁵⁶Fe in the ECS. Attributed to extremely high deposition velocities and high total Fe concentrations for large size aerosols, lithogenic aerosols are still the dominant dissolved aerosol Fe source in the ECS.

On 12 August 2021 a large Mw 8.1 earthquake, detected by global seismic networks, occurred on the South Sandwich subduction zone in the southern Atlantic Ocean. Approximately 1.5 hr later, a tsunami was clearly recorded on King Edward Point coastal tide gauge (South Georgia Island), approximately 800 km north-west of the earthquake location. Subsequently it was recorded on other coastal stations both in the Atlantic Ocean, and also in the Indian and Pacific Oceans. A careful and systematic analysis of coastal and deepwater sea-level records highlights three points: (a) the tsunami propagated across four oceans following major submarine features; (b) despite its very low amplitude, it reached as far as the Canary Islands in the Atlantic Ocean, Hawaii and the US West coast as far as Alaska and the Aleutian Islands in the Pacific Ocean; (c) it was recorded twice on New Zealand DART system NZC, with one record of the tsunami from the East and one from the West. This event is an opportunity to highlight the lack of knowledge about the South Sandwich subduction region in terms of its tsunamigenic potential and the associated tsunami hazard in the Pacific ocean. It should lead to an improvement of national tsunami warning procedures, by including this region as a tsunami source zone, for neighboring regions but also for distant countries like New Zealand or French Polynesia.

Coastal Trapped Waves (CTWs) have the potential to transmit significant energy from atmospheric wind systems, causing dramatic coastal changes over vast areas, even far from the wind systems. This study investigated the impacts of CTWs generated by Typhoon In-fa (2021) on the northern South China Sea (NSCS) hydrodynamics using tide gauge observations, reanalysis data, and numerical model outputs. The analysis results indicate that the CTWs induced by Typhoon In-fa exhibit distinct characteristics in different phases of wave crest and trough. During the crest phase, coastal currents generated by CTWs flow opposite to the background circulation, while during the trough phase, they flow in the same direction. This process is accompanied by changes in cross-shore currents, such that during the crest phase of CTWs, the cross-shore currents are landward at the surface and seaward at the bottom, while during the trough phase of CTWs, the cross-shore currents become reversed. These changes further lead to downwelling in temperature, salinity, and density during the crest phase of CTWs, and upwelling during the trough phase of CTWs. Results from a linear CTW model demonstrated that the above characteristics agreed with the traditional CTW theory. Sensitivity experiments with the Regional Ocean Model System (ROMS) explored key factors influencing NSCS hydrodynamics, including local winds and CTWs from Typhoon In-fa. The local winds and CTWs have different effects: they compete during the crest phase and co-work during the trough phase, with local winds dominating the sea surface and CTWs dominating the seabed.

Wave-ice interactions are critical for correctly modeling air-sea exchanges and ocean surface processes in polar regions. While the role of sea ice in damping open-water swell waves has received considerable research interest, the impact of sea ice on locally generated wind-waves in partial ice cover remains uncertain. The current approach in spectral wave models is to scale the wind input term by the open-water fraction, $��1�-�\varphi �$, for $��\varphi �$ the sea ice concentration (SIC), but this neglects the impact of subgrid-scale patterns of sea ice coverage in limiting fetch for wind-wave growth. Here, we use the spectral wave model SWAN to simulate local waves in realistic, synthetic fields of explicitly resolved sea ice floes over a range of SICs and floe size distributions (FSDs). We consider cases with floe sizes much larger than the wavelengths, and absent of interstitial frazil or pancake ice. Through geometric arguments, we show that the fetch available for wind-wave growth, and thus the resulting wave statistics, depends on a combination of the SIC and the FSD. The combination of geometric scaling and empirical wave laws allows the prediction of bulk wave statistics as a function of SIC, a characteristic floe size, and wind speed. We show that due to the difference in spectral character from attenuated propagating open-ocean swell, these waves may have an outsized impact on ocean mixing regimes.

Intratidal variability in stratification, referred to as internal tidal asymmetry, affects the residual sediment flux of an estuary by altering sediment transport differently during ebb and flood. Although earlier studies suggest that flood-dominant mixing increases the residual landward sediment flux, the role of ebb-dominant mixing remains largely unknown. Based on field data, we investigate the mechanisms that cause ebb-dominant mixing and its effect on the residual sediment flux in a stratified estuarine channel. Observations based on two tidal cycles show that the pycnocline remains largely intact during flood. Vertical mixing during flood is inhibited by a strong fresh water outflow, confining landward transport of suspended sediment to the bottom layer. During ebb, the pycnocline height decreases until it interacts with the bottom boundary layer, resulting in enhanced vertical mixing and sediment transport extending further to the surface. Thus, ebb-dominant mixing increases the residual sediment flux in seaward direction. The long ebb period in combination with limited bed sediment availability further contributes to the residual ebb-flux. This is noteworthy since a long ebb duration corresponds to flood dominance, which is often associated with a landward residual sediment flux. Although our data represent average conditions and cannot readily be extrapolated to different forcing conditions, we conclude that asymmetries in vertical mixing considerably affect the residual sediment flux under average conditions.

Under the influence of buoyancy and the earth rotation, river outflows leaving an estuary typically have a two-part structure, including a recirculating bulge near the river mouth and a coastal current propagating downstream. A continuous river outflow causes the bulge to expand in size and to accumulate freshwater, with the bulge eventually becoming unstable due to baroclinic instability. We conducted a series of laboratory experiments on a rotating tank to simulate an idealized river plume under various Coriolis frequencies, density differences, and shelf slopes. The horizontal velocity structures of the river plume were qutantified using particle imaginary velocimetry (PIV). We designed a velocity-based vortex identification and tracking algorithm to capture anticyclonic eddies (ACEs) at the bulge center, cyclonic eddies (CEs) on the plume front, and coastal cyclonic return flow (CCRF) at the corner between estuary and coastal wall. Our results suggest a linear relationship between bulge instability parameter and bulge wavenumber, which can be used to predict the bulge instability patterns that are classified according to vortex stretching, splitting, and squeezing. Finally, we estimated the eddy kinetic energy contained within ACEs, CEs, and CCRF to explore the cross-scale energy transfer and dissipation. Our results show that the bugle is more unstable in gentle slope cases, and its instability decreases with the inflow Rossby number and increases with the Froude number. The generation of CEs on the plume front may extract the energy from the larger scale anticyclonic core, which plays an important role in mass transport and frontal mixing of river plumes.