Continental Shelf Research最新文献

The near-ubiquitous presence of freshwater over the inner to mid-continental shelf off of the Otago Peninsula in southeast Aotearoa/New Zealand has been previously identified in long-term cross-shelf transects. Occasional influxes of this silicate-rich neritic water past the shelf break and beyond the Subtropical Front have been observed, potentially supporting elevated chlorophyll-a concentrations in SubAntarctic waters. Nearshore salinity variability has been connected to flow rates of the Clutha River, Aotearoa’s largest river by discharge, however, other physical mechanisms influencing the structure of freshwater along this coast have not been explicitly explored. A numerical model of Aotearoa’s southeast continental shelf forced with realistic discharge from the Clutha River is used to assess the drivers of river plume variability and identify conditions associated with the transport of neritic water beyond the shelf break. Event scale, lagged correlation, EOF, and composite analysis of a model passive dye tracer revealed three generalised plume structures that are dependent on river discharge, along-front wind stress, and shelf current velocities. Downfront wind stress suppresses bulge formation at the river mouth and constrains the river plume against the coast, whereas upfront wind stress causes the river plume to move offshore over the shelf and spread across the upper water column. Rarely, upfront wind stress generates a nearshore current reversal over the shelf, causing the plume to deflect straight off-shelf and across the shelf edge before meeting the frontal current and advecting to the northeast. Future observational work is required to validate the mechanisms described and address the biological implications of these findings.

The paper deals with the morphodynamics of the artificially engineered outlet of the Vistula River, Poland. The delta is a coastal form typical of semi-closed micro-tidal or non-tidal seas, such as the Baltic Sea. This accumulative sandy form in front of the Vistula mouth constitutes a serious obstacle to the flow of water and sediment at the river-sea interface. Therefore, regulation works need to be carried out periodically. The present mathematical modeling applies the representative flushing discharge accompanied by marine hydrodynamic processes to particular variants of regulation of the Vistula mouth. The envelope method of analyzing the modeled bottom changes is used to formulate practical conclusions. The major findings concern the optimal extension of the breakwaters at the river mouth, sufficient to ensure the outlet patency.

Benthic foraminifera are valuable proxies for oceanographic and environmental research due to their high degree of species-specific habitat use. The present study describes the distribution of benthic foraminifera across the continental carbonate shelf of northeastern Brazil, and relates their distribution patterns to environmental parameters associated with the texture of the sediment and the depth of the water. Sediment samples were collected at depths between 0.5 and 57 m. In the laboratory, the samples were analyzed following the standard protocol established for Quaternary foraminifera research. Sediments dominated by carbonate sand, which were approximately symmetric in the first depth interval (0.5–20 m), and asymmetric in the second (20–40 m) and third (40–57 m) intervals. The foraminiferal assemblages composed of 96 species, although they were dominated by the five species – Archaias angulatus, Textularia agglutinans, Amphistegina lessonii, Quinqueloculina lamarckiana, and Peneroplis carinatus. Most tests were intact and the species composition varied across the continental shelf, with Pseudotriloculina laevigata and Globobulimina? sp. Only occurring from 0.5 to 20 m depth in shelf, while Pyrgo elongata, Quinqueloculina sp. A, Quinqueloculina sp. C, Fissurina sp. A, Fissurina sp. B, Elphidium morenoi, Glabratella brasilensis, and Siphogenerina sp. Were restricted from 20 to 40 m, and Sigmoilina sp., Fursenkoina sp., and Textularia sp. A was only recorded in depth >40 m. The diversity of the foraminiferal assemblages is favored by the attenuation of hydrodynamic conditions at deeper intervals (20–57 m). Therefore, it can be concluded that the benthic foraminiferal species respond to the depth and variation in sediment characteristics through changes in their population and assemblage structures.

In recent years, dredging reef sand and aggregate as building materials for infrastructures has become a common practice in many atoll islands, which can significantly reshape the wave hydrodynamic environment at the fringing reef. Hence, it becomes necessary to evaluate the hydrodynamic impacts of excavation pits at fringing reef. However, previous research adopted a simplified two-dimensional (2D) excavation pit model, which ignores the influences of 3D geometric characteristics of excavation pit. This study performs a three-dimensional (3D) numerical study on the hydrodynamic impacts of 3D excavation pit on tsunami-like wave at fringing reef by using a nonhydrostatic wave model (NHWAVE). Impacts of several main factors, such as water depth, wave height, pit location, geometric dimensions of pit, the spacing between two serially connected pits and the spacing between two parallel connected pits are carefully discussed. Research results reveal that 3D excavation pit can significantly reshape the wave hydrodynamic environment at the fringing reef, especially the local wave height at the pit. In addition to wave breaking at the reef edge, the 3D excavation pit can further damp out a portion of energy of the breaking surge bore, which can reduce the maximum value of wave runup height at centerline of computational domain. However, maximum value of wave runup height near the two sides of pit are slightly increased to some extent.

The assessment of morphological mapping capacity of X-band radars is performed by comparing radar-derived shoreline estimates with in-situ data measured from DGPS-RTK surveys in Cala Millor beach (October 22–24, 2018). Radar estimates are obtained by applying an automatic method based on digital image processing, threshold segmentation and first-order derivative filters on X-Band radar variance images. The study site is covered by the endemic Posidonia oceanica seagrass meadow at the seabed. It represents a challenge on the estimation of local shoreline. Results reveal that X-band radar estimate the shoreline location with a mean bias of 4.66 m, a relative error of 2.95% and a RMSE of 14.07 m. Also, the beach morphology is assessed by the identification of surrounding rocky outcrops and a submerged sandbar. Since the obtained radar estimation errors are comparable with the reported by previous studies but using video-cameras, results reveal a potential complementary use of both marine radars and optical cameras to provide higher-resolution and more accurate shoreline measurements in a broader area.

The energy transfer between near-inertial internal waves (NIWs) and super-inertial internal waves (SIWs) in the South China Sea (SCS) after the passage of Typhoon Son-tinh was investigated on the basis of a three-dimensional numerical model. Our model nicely reproduces the spectral peak (6.6 × 10⁻² m² s⁻² cpd⁻¹) within the near-inertial frequency band, which agrees well with that (6.5 × 10⁻² m² s⁻² cpd⁻¹) in the in-situ observations at the Xisha mooring. Model results demonstrate that the energy transfer rate from NIWs (0.8f∼1.8f) to SIWs (>1.8f) within the mixed layer in the wake of typhoon Son-tinh is an order of magnitude larger than that during the typhoon-free period. Analogously, the super-inertial shear variance increases by nearly an order of magnitude as well. The increase in the energy of SIWs is mainly due to the energy cascade of NIWs through the nonlinear wave-wave interaction. The interaction between NIWs and SIWs was also revealed by the bicoherence spectrum. Compared with that on the left side of the typhoon track, the NIW kinetic energy on the right side is stronger, where the interaction between NIWs and SIWs is more intense. Several sensitivity experiments were designed to further investigate the effects of three typhoon parameters, namely the radius of maximum typhoon wind speed ($R_{\max }$), the maximum typhoon wind speed ($V_{\max }$) and the moving speed of typhoon ($U_m$), on the energy transfer rate (ETR) from NIWs to SIWs. It is shown that, the ETR increases linearly with $R_{\max }$. Among three parameters, $V_{\max }$ has the strongest effects on the ETR which grows in power with increasing $V_{\max }$. The ETR increases and then decreases with the enhancement of $U_m$, which gets maximum when $U_m$ is about 6∼6.5 m/s. Overall, our results highlight the energy transfer between near-inertial internal waves and super-inertial internal waves under the influence of a typhoon.

Exact tidal information is essential for multifarious human activities in the global ocean. Satellite altimeters especially Topex/Poseidon (T/P) and Jason series play fundamental roles in improving our understanding of global tides. Compared to T/P-Jason series, other satellite altimeters such as Geosat Follow-On (GFO) have been rarely used in tidal studies mainly due to their unfavorable aliasing features for tidal estimates. Classical harmonic analysis requires 12-year GFO records to fully resolve eight major tidal constituents while the longest GFO data is only 8-year. As a result, it is challenging for classical harmonic analysis to derive reliable tidal estimates from short GFO records. In this study, a modified tidal harmonic analysis algorithm based on the credo of smoothness (i.e. MHACS) is introduced to extract ocean tides from GFO records in the Bohai Sea. Via utilizing the inherent natural links between main constituents, MHACS breaks the constraint of the Rayleigh criterion and effectively solves tidal aliasing problems in GFO records. Practical experiments indicate that GFO-derived tidal constants via MHACS have high consistencies with the tidal theory and satellite-based EOT20 tidal model. Spatially-averaged vector difference between the EOT20 model and GFO results in the Bohai Sea is only 2.12 cm (eight major constituents averaged). Moreover, except for GFO, MHACS also has the potential to be applied to sun-synchronous satellites such as Envisat, Sentinel series, and Haiyang series.

The upper ocean mixed layer plays a crucial role in regulating the exchange between the ocean and the atmosphere. The Mixed Layer Depth (MLD) is a key parameter affecting the air-sea exchanges of momentum and heat and determining the upper ocean temperature. Numerous previous studies have investigated MLD variability in the global ocean or regional seas but no such studies were carried in the Baltic Sea. In this study, we present the first observational assessment of the MLD and its properties in the Southern Baltic Sea including quantification of its seasonal and long-term changes and identification of the multi-year winter shoaling. We calculated monthly maps of MLD in the southern Baltic Sea using a large number of historical CTD profiles collected in 1995–2021 from a combination of different data sets. To test the robustness of the results we compared the MLDs calculated using different threshold methods. Throughout the southern Baltic Sea, across its three basins, a distinct seasonality is evident in the MLD, with values varying from 12 m in July to 60 m in December–March. During winter the water column is well mixed down to the upper halocline depth and the MLD reaches about 45 m in the Bornholm Basin, 50 m in the Slupsk Furrow, and 60 m in the Gdansk Basin. The observed global warming and decadal changes in the salty inflows from the North Sea to the Baltic have had an impact on stratification by increasing water densities in the intermediate and deep layers. Consequently, density gradients have strengthened with depth while the upper ocean mixing has weakened during the winter season. The results reveal a significant winter shoaling of the mixed layer by 4 m per decade, driven by the increased stratification due to rising temperatures and salinity. These changes could have significant impacts on the dynamics and productivity of marine ecosystems.

Based on the measurement data obtained during the 121th cruise of the RV Professor Vodyanitsky, regularities were established between spatial distribution of quantitative characteristics and phytoplankton species composition, chlorophyll a concentration, and hydrometeorological parameters of the environment on the southern Crimean coast in the spring 2022. Chlorophyll a concentration was compared with satellite observation data of high spatial resolution. In selected areas with different shelf depths, the features of the distribution of phytoplankton indicators under conditions of changing hydrometeorological parameters were studied. The mechanisms of vertical water exchange were described. In coastal waters of the Crimea (Foros–Novy Svet), with intense western wind contributing to Ekman upwelling and low water temperature is a consequence of the upwelling, rather high values of phytoplankton biomass and abundance and chlorophyll a concentration were recorded, which were accompanied by the highest values of oxygen content. In the area of Sudak–Feodosiya, with intense northeastern wind and waves and with quite high water temperature, low dissolved oxygen, low values of phytoplankton biomass and abundance and chlorophyll a concentration were registered. Apparently, in Sevastopol area, Ekman upwelling arises under the effect of northern and northwestern winds. In three regions, dinoflagellate microalgae prevailed in spring 2022 in phytoplankton biomass. Those accounted for 52, 66, 81 % of the total number of species identified in the regions: Foros–Novy Svet, Sudak–Feodosiya, near Sevastopol, respectively. Thus, the main mechanisms were analyzed, which drive spatial distribution of quantitative characteristics and species composition of the phytoplankton community in the Crimean coastal water area with different shelf depths in spring.

Coral bleaching is a major concern for the health and survival of coral reefs worldwide, as it can significantly reduce coral growth and increase coral susceptibility to disease. The coral reefs at Sodwana Bay, located on the northeastern coast of South Africa, have shown resilience to coral bleaching which has been attributed to cold water temperature anomalies observed at Sodwana. This study investigates the regional hydrodynamic processes associated with 63 temperature anomalies identified at Sodwana between 1994 and 2015. The study found that 65% of the anomalies are associated with remote upwelling of cold water near the Delagoa Peninsula, followed by advection from the Delagoa Bight towards the Sodwana region. The cold upwelled water advects directly from the peninsula along the shelf towards Sodwana or recirculates within the Delagoa Bight before advecting along the coastline to Sodwana. The remaining 35% of the anomalies were associated with local upwelling within the KwaZulu-Natal Bight south of Sodwana.

The study also found that the separation of the strong intermittent southward stream from the Delagoa Peninsula is strongly linked to the upwelling at the Delagoa Peninsula. The cold upwelled water is then advected to the Sodwana region by regional flow patterns, resulting in a temperature anomaly at Sodwana. The local upwelling is similarly linked to an increase in the Agulhas Current velocity magnitude around Sodwana and flow separation of the current from the coastline at the northern extent of the KwaZulu-Natal Bight.

The cross-correlations between the conditionally averaged flow fields associated with remote and local upwelling near the Delagoa Peninsula and the instantaneous flow fields at the time of the upwelling showed a positive correlation for all the anomalies over the 21 years. The anomalies associated with remote upwelling have an average correlation coefficient of 0.67 with a maximum correlation of 0.96. The anomalies associated with local upwelling have an average correlation coefficient of 0.74 with a maximum correlation of 0.98.

This study shows that the unique geometry of the southeast African coastline is a key factor associated with the cold water temperature anomalies at Sodwana. The interaction between the regional hydrodynamics and coastline features such as the Delagoa and Natal Bights, are crucial for the upwelling that eventually leads to the cold water temperature anomalies at Sodwana. Without these coastline features and the intermittent strengthening of the southward streams along the coastline due to the high eddy kinetic energy associated with the region, it is likely that these temperature anomalies would not occur at Sodwana. Without these temperature anomalies, the Sodwana coral reefs would become more vulnerable to climate change and coral bleaching.