Journal of Geophysical Research-Oceans最新文献

Melt ponds are usually modeled for light transfer as horizontally infinite water layers on level ice, and the albedo of floe is determined by a linear combination (LC) of melt pond and bare ice albedos weighted by their areal coverages. However, this method does not reflect the actual conditions because ice floes have a limited size. In the present study, an idealized two-dimensional Monte Carlo (MC) model was employed to investigate the influence of melt ponds and floe size on the apparent optical properties (AOPs) of summer sea ice. The results showed that the albedo and vertical light transmittance of large floes mainly depend on the melt pond fraction and ice thickness, which is consistent to previous results. However, also the floe size plays an important role in the AOPs of small floes. Two parameters were proposed to present the accuracy of the LC method for small floes with lower sea ice concentration: the ratios of sea ice albedo and transmittance determined by the LC (α_line, T_line) to the values in the MC model (α, T), K_α = α_line/α, and K_T = T_line/T, respectively. Due to the lateral transmittance, K_α, K_T ≥ 1 and asymptotically approach 1 with floe size increasing to infinity. To reduce the biases in albedo and transmittance due to floe size, new parameterization formulas were provided for K_α and K_T with the distance into the marginal ice zone and in different melting stages. The results have potential to be implemented into future sea ice models to correct the AOPs of small sea ice floes obtained via the LC method.

In the past decades (1982–2012), the rapid increase in nitrogen and phosphorus inputs has led to a two-fold increase in nitrogen and phosphorus concentrations in the East China Sea. To investigate how the increasing nutrients in the East China Sea have been affecting the marine environment and growth of plankton, this paper establishes a modeling framework using the coupled ocean hydrodynamics-ecological model. Observational sea surface temperature, salinity, nutrients, and chlorophyll-a are compared with the predictions to validate the model performance. The study reveals that the concentrations of dissolved inorganic nitrogen (DIN) and phosphate (PO₄³⁻-P) are highest at the Yangtze River Estuary, slightly lower in the Zhejiang-Fujian nearshore, and much lower in the open sea, exhibiting significant seasonal variations closely related to the biomass of plankton. Over the past decades, DIN and PO₄³⁻-P concentrations at sea surface have increased by 294% and 253%, respectively, in the Yangtze River Estuary, and by over 291% and 76% in the Zhejiang-Fujian nearshore, both of which are characterized by high nitrogen-phosphorus ratios. As a result, the biomass of plankton at sea surface has increased by 26% averaged over the two regions. Sensitivity experiments indicate that the biomass enhancement in the Yangtze River Estuary is primarily attributed to the increase in phosphorus, whereas that over the Zhejiang-Fujian nearshore is explained by the combined effects of both nitrogen and phosphorus increases. This study contributes to the development of ecological conservation strategies for rivers and coastal seawaters in China in the future.

Compound floods are often thought of as large, infrequent floods during which extremes of coastal sea level and/or river flow combine with each other or additional factors (e.g., tides and rainfall) to induce major flooding. However, little is known about the potentially compound nature of more frequent, lower-level floods. Here, we introduce the term “compound minor floods” to define minor floods composed of two or more water-level sources. We use the Delaware River Estuary as a case study to investigate the prevalence and composition of these minor compound floods along the extent of a tidal river. We apply multiple linear regression to a 22-year time series of coastal water levels and river discharge to establish the contributions of tides, nontidal open-ocean effects, and river discharge to minor flood events at eight locations along the tidal Delaware River. We find that most minor flood events are compound in nature, requiring at least two components (e.g., tides and river discharge) to initiate flooding. We identify spatial structure in the relative importance of oceanographic and riverine contributions to minor flooding along the tidal reach of the estuary. These results suggest that incorporating fluvial components into minor flooding assessments is important to fully characterize flood risk along tidal rivers and estuaries.

Surface cold patches (SCPs) are important hydrological phenomena in the Yellow Sea (YS). We investigate the dynamics behind the appearance of the SCP off the Korean Peninsula Cape during summer. The numerical model results revealed that upwelling is the primary mechanism of SCP formation in this region. On one hand, the 3D vorticity balance of the dynamic processes of the SCP indicated that the combined effect of the tides and the baroclinic pressure gradient enhances dissipation in the bottom boundary layer, leading to seawater convergence in the bottom layer west of the tidal front and divergence east of the tidal front, forming a stable frontal secondary circulation and resulting in the formation of upwelling. On the other hand, the offshore transport, which is triggered by the Jeju Warm Current and persists throughout the year in the strait, leads to a divergence of seawater within the region. The joint effect of the above two processes ultimately contributes to the generation of upwelling. The presence of upwelling forces the thermocline to be changed. Strong tidal upwelling triggers the upward ventilating of the thermocline, which can bring cold water to the surface, and the dome-like upwarp of the thermocline isotherms caused by the current-induced upwelling, together lead to the formation of the SCP.

Ocean circulations and sea level are undergoing long-term changes as part of anthropogenic global warming. Understanding these changes through the water column is important for comprehending the ocean's response to climate change. This study employs regional dynamic height (RDH) referenced to a no-motion layer (2,000 m) as a proxy to diagnose ocean circulations and sea levels using four observation-based datasets and 11 Ocean Model Intercomparison Project Phase 2 model simulations spanning from 1960 to 2018. Ocean Model Intercomparison Project Phase 2 simulations reproduce the major changes in ocean circulation indicated by observations, although regional differences are present, with certain regions exhibiting higher or lower RDH trends. North Pacific subtropical gyre shows positive RDH trends in the upper 400 m without a clear poleward shift but negative trends (spins down) below ∼400 m, while the South Pacific subtropical gyre shows positive RDH trends from surface to the no-motion layer and undergoes a poleward shift. These asymmetrical RDH trends between the North and South Pacific, are dominated by thermosteric components. The wind stress curl trends drive most changes of ocean circulations, resulting in asymmetric dynamic topography rise. Continued global warming in the 21st century is anticipated to intensify the asymmetric development of subtropical gyre circulations and associated sea levels in the Pacific Ocean.

The 2021 shallow plate-boundary thrust-faulting and 2023 outer rise normal-faulting M_W 7.7 earthquakes southeast of the Loyalty Islands produced significant, well-recorded tsunamis around the North and South Fiji Basins. The two earthquakes occurred in close proximity on opposing sides of the Southern Vanuatu Trench with similar seismic moments and east-west rupture lengths but different faulting mechanisms. This provides a basis to examine tsunami sensitivity to source geometry and location for paths in the complex southwest Pacific region. Finite-fault models of the source processes for both events were inverted from teleseismic body wave data with constraints from forward, nonhydrostatic modeling of regional tide gauge and seafloor pressure sensor recordings. The wave motions are reversed in sign, with a leading crest generated by 1.31 m uplift on the upper plate slope for the 2021 tsunami and a leading trough from 2.37 m subsidence on the subducting plate near the trench for the 2023 tsunami. The more recent outer rise normal faulting produces narrower seafloor deformation beneath deeper water resulting in shorter period tsunami waves that shoal and refract more effectively along seamounts and island chains to produce a more elaborate radiation pattern. The source location relative to seamounts and small islands in the near field influences the energy lobes and directionality of the far-field tsunami to the north. In contrast, both events have very similar radiation patterns to the south due to absence of major bathymetric features immediately southward of the sources.

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.

Estimates of vertical land motion (VLM) from an updated Canadian crustal velocity model (NAD83v70VG) are used to evaluate the budget of contributions to trends of relative sea level (RSL) at tide gauge sites along the east and west coasts of Canada and northern USA during 1958–2015. The RSL trends result from a combination of the processes of sterodynamics (SD), glacial isostatic adjustment (GIA), and changes in earth Gravity, earth Rotation, and viscoelastic solid earth Deformation (GRD). For these contributions, we used data from a recent study on global RSL trends (Wang et al., 2021, https://doi.org/10.1029/2021gl094502). Incorporating the NAD83v70VG VLM in the derivation of the trends results in an improvement of the RSL budget. Estimates of SD contributions to the observed tide gauge water levels are obtained using this updated VLM, and are compared with the SD-component from the global ocean models used in Wang et al. (2021, https://doi.org/10.5281/zenodo.5554494). The comparison demonstrates the need to include interannual variations of river runoff for modeling the sea level changes in the St. Lawrence River and its estuary, and to improve the model's spatial resolution in the Gulf of St. Lawrence and on the open shelf of the Northwest Atlantic. Along the coast of the Northeast Pacific, the SD sea level changes are well simulated by the global models despite their coarse spatial resolutions and the use of river runoff climatology.

In recent years, as the ocean absorbs increasing amounts of atmospheric CO₂, ocean acidification has intensified. Simultaneously, global warming and enhanced ocean stratification have led to the continuous expansion of the oceanic oxygen minimum zone (OMZ). Under the combined effects of these processes, a key question arises: How will the transport of particulate organic carbon (POC) to the deep ocean (>1,000 m) be affected? Analysis of POC flux data from 547 stations, collected via global sediment traps since the 1980s, reveals that POC flux has increased only in the shallow ocean (<300 m) but has significantly decreased in the deep ocean. These findings suggest that the expansion of the OMZ has not led to more carbon being transported to the deep ocean. Instead, more POC is being retained in the mid-upper ocean (<1,000 m), where its degradation results in significant consumption of dissolved oxygen, contributing to the expansion of the OMZ.