Journal of Geophysical Research-Oceans最新文献

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.

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 $��\sim �25�°�$) 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 $��\left(��\sim �10�°��\right)�$. 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 $��\sim �10�°�$. Results suggest that eddy-eddy interaction, the transformation of vorticity across the surf zone, and influence of bathymetry are fruitful topics for future work.

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.

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.

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.

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.

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.

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.

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.