Continental Shelf Research最新文献

The Regional Ocean Modeling System (ROMS) is integrated across the Arabian Sea (AS) from 1992 to 2021 and covers 33^°E to 80^°E and 5^°N to 32^°N at high horizontal resolutions of 1/4^°(∼25 km). The study demonstrates that the variability is controlled by a seasonal high-resolution setup, with superior performance for various seasons during the months of December-January-February (DJF), March-April-May (MAM), June-July-August-September (JJAS) and October–November (ON). The Sea Surface Temperature (SST) anomalies in the JJAS season werestudied. SST dominates the seasonal variability of the Indian summer monsoon (ISM). Our results show that the ROMS can simulate seasonal variability and its effects on the upper ocean properties over the AS. In addition, the impact of monthly variation is controlled. In recreating the spatio-temporal distribution of surface as well as subsurface hydrographic parameters such as surface and subsurface temperature, the simulation results are reasonable like observation and reanalysis.

Nitrogen, as the main bioactive element, plays an important role in biological productivity, ecosystem function, and biogeochemical processes in marine environment. In this study, seawater samples collected from China’s Leizhou Peninsula coastal water (LZPCW) during dry, normal, and wet seasons in 2018 were explored to reveal the spatiotemporal variation, composition, and transport flux of dissolved inorganic nitrogen (DIN) linked to hydrographic condition. DIN concentration and speciation showed significantly seasonal variation (P < 0.01), and the concentration of DIN species was significantly higher in dry season than other seasons. The annual mean DIN concentration was 8.01 ± 5.79 μmol L⁻¹. In addition, the N–NO₂⁻, N–NO₃⁻ and N–NH₄⁺ were significantly different in the DIN bulk in different seasons. The largest fraction of DIN was N–NO₃⁻, followed by N–NH₄⁺, the lowest was N–NO₂⁻, comprising up to 67.92 ± 23.20%, 23.90 ± 23.19% and 8.18 ± 8.19%, respectively. Besides, the Beibu Gulf was subjected to 7.28 × 10¹⁰ mol DIN annual flux through the Qiongzhou Strait. The annual transport fluxes of N–NO₂⁻, N–NO₃⁻, and N–NH₄⁺ accounted for 6.43%, 77.20% and 16.36%, respectively. The DIN concentration and coastal water flow led to the largest DIN flux transport in dry season. This study revealed that the coastal ocean currents, river plumes and human activities jointly drove the dynamic variations of N species in LZPCW. It provides a baseline data for studying the spatiotemporal effects of hydrographic condition on nitrogen distribution and transport flux in the LZPCW, which is implications for understanding nutrients dynamics and coastal water quality management in future.

Double diffusion (DD) structures, in two types of diffusive convection (DC) and salt-fingering (SF), occur due to vertical temperature and salinity gradients with different diffusion coefficients. Areas such as the Strait of Hormuz, which has a thermohaline exchange between the Persian Gulf and the Sea of Oman, are suitable places for the formation of DD structures. Based on the results, the formation of DC structure is significant compared to other processes in the west of the Strait of Hormuz in December. Fluctuations in temperature and the growth of DC structure cause mixing and changing the depth of the boundary layer of temperature and salinity in the water. So that in the presence of the DC structure, the decrease in the value of the sound speed (1552 m/s) extends to a depth of 40 m, but the sound speed increases to 1555 m/s at the place where the warm water rises. The results show that the sound transmission loss increases by 5-15 dB in the place of strong DC structure. When the sound frequency increases, the transmission loss caused by the DC structure decreases. But for rays with a small propagation angle, a significant expansion is created in the propagation steps and wavelength of the rays, which increases up to 2 times with the increase in the depth of the sound source.

The Slope Sea is the dynamic ocean region located between the United States and Canadian northeast continental shelves and the northeastward flowing Gulf Stream (GS) located farther offshore. Here we define it as located between the 200-m isobath and the monthly GS sea surface temperature (SST) front from −75° to −55° E. Monthly mean near-surface eddy kinetic energy (EKE) was computed for the Slope Sea using surface geostrophic current anomalies derived from gridded 1993–2016 Copernicus Marine Environment Monitoring Service (CMEMS) sea height anomalies. Long-term, monthly mean Slope Sea EKE anomalies show a robust seasonal cycle with a winter (February) minimum and summer (June) maximum. This agrees with both seasonally-varying density stratification and dissipation and also the seasonal variation in the formation of GS WCRs within the Slope Sea. The RMS of the Slope Sea EKE seasonal cycle generally increased after 2002 by a factor of up to ∼2 relative to prior years. The seasonal cycle of Slope Sea EKE displayed higher EKE in the vicinity of the New England Seamount Chain (NESC) that extends towards the shelf break front from approximately −67° E to −63° E. Interannual variability of annual mean near-surface EKE from individual digitized GS warm core ring (WCR) observations from a Bedford Institute of Oceanography (BIO) WCR database is highly correlated with Slope Sea EKE. However, interannual variability of annual mean near-surface EKE computed from a census of all newly formed WCRs displayed only a weak correlation. Many of the WCRs from both the BIO and WCR census displayed anomalously low EKE values and were observed within the northern Slope Sea away from the GS. Some were located inshore of the position of the climatological mean shelf break front. WCRs with higher EKE were located throughout the Slope Sea, with higher numbers in the vicinity of the NESC. The many observations of the less energetic features located close to or inshore of the mean shelf break front suggest they are important to cross-shelf fluxes of heat, salt, nutrients, shelf biota. They therefore likely impact the shelf ecosystem, similar to the more energetic and typical WCRs impacting the outer shelf as discussed by earlier workers.

Cross-front transport is a vital process in the offshore transport of terrestrial materials. The factors influencing cross-front transport are multifaceted and exhibit regional variability. As important factors regulating material transport patterns in the shelf seas of eastern China, the East Asian Winter Monsoon (EAWM) and Kuroshio current (including its branches) vary on multiple time scales, yet their role in controlling offshore transport is not fully understood. In this paper, cross-front offshore transport in the North Yellow Sea is chosen to investigate the aforementioned issue using numerical simulation. By excluding other potential processes for offshore transport, frontal instability is identified as the primary mechanism for offshore transport, which is triggered by winter storms (bursts of the EAWM); hence, the intensity of offshore transport is highly associated with the strength of winter storms on synoptic scales. However, on the inter-annual scale, the intensity of offshore transport is more likely regulated by the strength of the Kuroshio current, and a warming Kuroshio facilitates offshore transport. Although the role of each factor in controlling the intensity of offshore transport is delineated, the interactive effects of the EAWM and Kuroshio current on offshore transport are quite intricate, and how to quantitatively estimate their roles on multiple time scales remains a challenge using modelling. The results obtained from this study can be applied to analyse cross-front transport throughout the eastern China seas and bear significant implications for future studies on pollutant diffusion, nutrient distribution, and sediment transport in the coastal area.

At the northern tip of the Betic realm (SE Iberian Peninsula), some troughs (synclines) and elevations (anticlines) alternate, marking the present-day coastal lobed morphology of cape-bounded bays, where subsidence and uplift conditions prevailed, respectively. In this study, we were able to establish a clear coastal evolution. To this end, we considered the sedimentological and palaeoenvironmental conditions, the palaeogeographical reconstruction, and recent tectonics until Middle Pleistocene times (MIS 5) through the interpretation of onshore cores, raised marine deposits and geophysical profiles. In this regard, as reflected by both onshore and offshore information, there seems to be a stratigraphical gap from the end of the Pliocene to MIS 15 (Middle Pleistocene). In areas under uplift conditions, linked to tardive Alpine tectonics, the deposits of ancient shorelines and raised beaches were located at different post-depositional elevations, which were dated from odd MIS 15 to MIS 5 using amino acid racemization. Only deposits aged MIS 7 and MIS 5 are roughly at the present-day sea level or some meters above. In the troughs, which remain mostly as lagoons and salt marshes, subsidence did not allow the sedimentary record to be discerned. However, many borehole cores were recovered, attesting lagoonal, marsh, sabkha, or alluvial environmental conditions, which were usually unconnected from the sea. Micropaleontological and amino acid racemization dating revealed these cores to be of MIS15 to MIS5 age. Offshore seismic research revealed five erosive-bounded deposits that are stacked accretionary prisms corresponding to highstands between odd MIS 15 and MIS 5. In contrast, even MISs can be correlated to the erosive horizons that separated the seismic units, reflecting lowstands. In this regard, some bars, at a range of distances from the present-day coastline, protected wetlands from marine influence, allowing the development of diverse sub-environments under changing paleogeographical and paleoclimatological conditions.

Estuarine dams are constructed in estuaries for reasons such as freshwater, flood control, and navigation. By changing tidal and river properties, estuarine dams can change the circulation and sediment transport in estuaries. Previous studies have investigated the along-channel changes in flow and sediment transport, however across-channel changes due to an estuarine dam are not well understood. To increase our understanding, this study analyzed an idealized estuary using the COAWST numerical modeling system. Models of strongly stratified, partially mixed, periodically stratified, and well-mixed estuaries were run for one year. Then, the models were subject to the construction of an estuarine dam and run for another year. For each estuarine type, scenarios with an estuarine dam at x = 20, 55, and 90 km from the mouth and freshwater discharge intervals of Δt = 0.5, 3, and 7 days were investigated, and the scenarios were compared. The results indicated that the river-dominated and tide-dominated estuaries behaved differently. In river-dominated estuaries, the residual circulation tended to be inflow in the channel and outflow over the shoals due to the estuarine exchange flow, and the secondary circulation was bottom divergent due to differential advection. The exchange flow and secondary circulation were found to weaken with longer discharge interval. The sediment fluxes in the channel were dominant, and, despite the weaker exchange flow, were found to be greater and directed seaward due to strong dam discharge for long discharge intervals. In tide-dominated estuaries, the residual circulation tended to be inflow over the shoals and outflow in the channel due to the tide-induced circulation, and the secondary flow was bottom convergent due to differential advection resulting from Stokes return flow. As the estuarine dam was located nearer to the mouth, the tide-induced circulation was replaced with exchange flow, and the secondary flow became bottom divergent. The sediment fluxes in the channel were dominant in this case as well, and were determined by tidal asymmetry from the interaction of the tides with the residual current, becoming directed landward as the seaward tide-induced circulation decreased with the estuarine dam near the mouth. With respect to bed level change, gradients in along-channel sediment fluxes were found to contribute most to bed level change near the mouth and estuarine dam, whereas gradients in across-channel sediment fluxes contributed most to deposition on the estuarine shoals. This study demonstrates that estuarine dam location and discharge interval can alter estuarine transverse variability and improves our understanding compared to previous analyses based solely on along-channel processes.

Information on the contribution of offshore oil and gas (O&G) platforms to fish productivity is required to contribute to the decision-making process to remove, partially remove, or retain these structures during decommissioning after petroleum production ceases. The present study assesses the biomass and fish production of one common and abundant fish species (Caesioperca lepidoptera – butterfly perch) and two commercially fished species (Helicolenus percoides – reef ocean perch; Nemadactylus macropterus – jackass morwong) on eight O&G platforms and in surrounding natural habitats in the Bass Strait, south-east Australia, where options for decommissioning are being assessed. High-definition stereo-video imagery was collected by remotely operated vehicle (ROV) from eight platform facilities, their immediate benthic surrounds, reference areas reflective of the likely pre-installation seabed state (sand-dominated) and a nearby natural ‘reef’ area referred to as south-east reef (some limestone foundation). The biomass of all three species was low in the benthic surrounds of platforms, at reference locations and at south-east reef where minimal cover by benthic organisms was recorded and, as such, there was little to no fish production for the three study species in these areas. We observed a total fish biomass of 2.85 tonnes across the eight platforms for the three fish species surveyed, with high variability across platforms. Total production (P) across all platforms was estimated at 1244 kg/year for the three species, with a mean fish production density of 82 g/m²/year. Approximately 79% of total production is considered ‘new’ production (984 kg/year i.e., the production attributed to the presence of the platforms; with a mean production density of 64 g/m²/year). The remaining 21% could be retained or redistributed into the surrounding area if platforms were removed. C. lepidoptera accounted for the majority (90%) of biomass and of total production for all three species across all locations surveyed. Despite only accounting for a small proportion of platform surface area, the bottom 5 m sections of platforms had 41% of the total biomass observed and accounted for 46% of total production of these three study species. Production measures for platforms surveyed here are relatively high compared to other artificial reefs and habitats around the world. Total removal of these platforms will likely result in a reduction of fish biomass and fish productivity (incl. several fishery species) in the immediate vicinity.

Variability in the Agulhas Current system is dominated by meanders, which constitute cyclonic eddies along the inshore edge of the Current on the southeast coast of South Africa. Few studies have investigated the influence of these meanders on hydrographic variability on the adjacent shelf and slope and to date only a handful have been sampled in situ. This study used available in situ data and GLORYS12v1 model output to investigate the impact of meanders on the distribution of Intermediate waters, namely Red Sea Water (RSW) and Antarctic Intermediate Water (AAIW), as well as mechanisms driving these variations. We focussed on four eddies, sampled in situ during July 1998, April 2010, January–February 2017, and July–August 2017. RSW dominated along the inshore edge of the Agulhas Current in the absence of meanders, but larger proportions of AAIW occurred in the presence of cyclonic eddies. During eddy events, the kinematic steering level was raised above the lower boundary of Intermediate waters, increasing cross-frontal mixing of waters at depths of 800–1800 m. Eddy-induced upwelling of Central and Intermediate waters onto the shelf appeared to be inhibited by bands of strong positive relative vorticity (>0.4 × 10⁻⁴ s⁻¹), which likely promoted downwelling conditions inshore of the July 1998, April 2010, and July–August 2017 eddies. Weak positive relative vorticity (<0.2 × 10⁻⁴ s⁻¹) inshore of the January–February 2017 eddy was associated with enhanced water mass exchange between the shelf and deeper (>1000 m) ocean. GLORYS12v1 was consistently comparable with satellite and in situ data, and simulated the overall distribution of water masses on the continental shelf and slope despite its inability to reflect the influence of river discharge in nearshore regions during austral summer. The model is thus suitable to investigate the influence of Agulhas Current meanders on the hydrography of South Africa's southeast coast.