Fundamentally, river plume dynamics are controlled by the buoyancy due to river effluent and mixing induced by local forcing such as winds and tides. Rarely the influence of far-field internal waves ...on the river plume dynamics is documented. Our 5-day fix-point measurements and underway acoustic profiling identified hydrodynamic processes on the dispersal pathway of the Pearl River plume. The river plume dispersal was driven by the SW monsoon winds that induced the intrusion of cold water near the bottom. The river effluent occupied the surface water, creating strong stratification and showing on-offshore variability due to tidal fluctuations. However, intermittent disruptions weakened stratification due to wind mixing and perturbations by nonlinear internal waves (NIWs) from the northern South China Sea (NSCS). During events of NIW encounter, significant drawdowns of the river plume up to 20 m occurred. The EOF deciphers and ranks the contributions of abovementioned processes: (1) the stratification/mixing coupled by wind-driven plume water and NIWs disruptions (81.7%); (2) the variation caused by tidal modulation (6.9%); and (3) the cold water intrusion induced by summer monsoon winds (5.1%). Our findings further improve the understanding of the Pearl River plume dynamics influenced by the NIWs from the NSCS.
Abstract
Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling ...moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP–CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s−1 and 300 m, respectively. Energy fluxes exceeding 60 kW m−1 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500–1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 × 10−6 W kg−1 (implying diapycnal diffusivity Kρ > 0.2 m2 s−1). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Temporal variation in seawater temperature plays a crucial role in coral reef ecology. Nanwan Bay, Southern Taiwan is home to well-developed coral reefs, which frequently experience cold-water ...intrusions caused by internal wave-induced upwelling, that manifest in distinct daily temperature minima. These temperature minima and their associated sources were studied by recording in situ bottom temperatures and sea levels observed at depths of 5 and 30 m from May 2007 to September 2008. These data were then compared to the East Asian Seas Nowcast/Forecast System, and it was found that daily temperature minima presented large variations with magnitudes of 2-3 °C over periods from days to months. It was further demonstrated that the cold-water intrusions may have originated from depths of ~100 m and were strongly affected by westward propagating mesoscale eddies from the Pacific basin. An impinging warm anticyclonic eddy in July 2007 may have combined with the El Niño, resulting in temperatures surpassing 29 °C and degree heating days >4.0 °C-days at both depths, which were coincidental with a mass coral bleaching event. This eddy's impact was additionally evident in high correlations between daily temperature minima and residual sea levels, suggesting that mesoscale eddies alter stratification, substantially influence temperature variation, and play important roles in understanding ecological processes on coral reefs.
Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and ...the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
The particulate organic matter (POM) of shallow-water hydrothermal fields has been studied in the context of food web reconstruction, but the processes governing its biogeochemistry and dynamics are ...poorly explored. Here, we investigate the POM in the Kueishantao hydrothermal field using chemical and hydrodynamic approaches. The depletion of total suspended matter, lower C/N ratios, and higher carbon isotopic values of particulate organic carbon (δ13CPOC) in the vertical plumes relative to values derived from Si-based models were attributed to the hydraulic sorting of the vented particles, which tend to have more N and 13C in the fine fraction. The particulate organic carbon (POC), unlike the total suspended matter, was enriched in the vertical plumes and explained by physicochemical processes rather than biological addition. The POC-enriched plume-top water was found to be a better endmember than the vent fluids to explain particle mixing in the lateral plume. Physical mixing played a steering role in shaping the particle chemistry of the lateral plumes, but markedly 13C-enriched POC was still observable in several near-vent, low-to-intermediate-Si plume waters, implying locally enhanced primary production of at least 0.1–0.4 mg C/m3/h. The presence of eddies, confirmed by flow field measurements, should have contributed to the detection of biogeochemical anomalies via extending the retention time of plume water to 1–2 h. The dominating mixing process resulted in decoupling between the δ13CPOC signatures and carbonate chemistry in this shallow-water hydrothermal plume.
•Mixing and sorting shape particle chemistry in the plumes.•Physical processes cause POC enrichment in the vertical plumes.•Eddies extend plume water retention time to 1–2 h in near-vent regions.•Primary production was intensified locally in plume waters near the vents.
Shoaling of large-amplitude (∼100m) nonlinear internal waves over a steep slope (∼3°) in water depths between 100m and 285m near Dongsha Atoll in the northern South China Sea is examined with an ...intensive array of thermistor moorings and a bottom mounted Acoustic Doppler Current Profiler. During the 44h study period in May 5–7, 2008, there were four groups of large internal waves with semidiurnal modulation. In each wave group a rapid transition occurred during the shoaling, such that the front face of the leading depression wave elongated and plunged to the bottom and the rear face steepened and transformed into a bottom-trapped elevation wave. The transitions occur in water depths of 200m and deeper, and represent the largest documented internal wave shoaling events. The observations repeatedly capture the detailed temperature and velocity structures of the incident plunging waves. Strong horizontal convergence and intense upward motion are found at the leading edge of transformed elevation waves, suggesting flow separation near the bottom. The observations are compared with the previous observations and model studies. The implication of the shoaling internal waves on coral reef ecology also is discussed.
► Shoaling of large-amplitude solitons over a steep slope is examined with 5 thermistor moorings. ► Transitions of depression to elevation waves were observed in water depths of 200m and deeper. ► There are horizontal convergence and upward motion at the leading edge of the elevation waves. ► Details of temperature and velocity structure of a collapsed depression wave were observed.
CO2 released from subduction zones plays a vital role in the tectonic carbon cycle. However, the contribution of submarine backarc components to carbon emissions remains poorly understood compared to ...subaerial arc volcanos. This study utilized a combination of geochemical and hydrodynamic approaches to investigate the sources and flux of dissolved inorganic carbon (DIC) in the deep basin of the southwestern Okinawa Trough. Most deep‐water (depth ≥1,000 m) samples, even those retrieved from sites distant from active venting, exhibited mild geochemical anomalies of elevated 3He/4He ratios and higher DIC content than reference sites. Hydrodynamic observations using bottom‐mounted acoustic instruments in the deep basin revealed strong tidal currents and enhanced turbulent mixing. These findings suggest that active solute dispersal and mixing are responsible for the prevalence of mild geochemical anomalies in the basin. Mixing models indicated that hydrothermal vent fluids are the primary source of excess 3He and DIC above background levels in the deep basin, whereas both CO2 and mineral acid in hydrothermal fluids cause excess acidity. Based on hydrodynamic and geochemical data, a box model estimated a hydrothermal DIC flux of 0.62 ± 1.40 × 1010 mol y−1, equivalent to 40% of CO2 emissions from persistently degassing volcanos in the Ryukyu Arc. Our results suggest that submarine carbon emissions are quantitatively important in subduction systems with subaqueous backarc components.
Plain Language Summary
The CO2 released from subduction zones, where tectonic plates collide, is important for the Earth's carbon cycle. However, we know little about CO2 released from submarine backarc basins compared to volcanoes on land. This study examined dissolved inorganic carbon (DIC) sources and quantities released into the deep basin of the southwestern Okinawa Trough. Even far from active hydrothermal vents, deep‐water samples had higher DIC levels than expected. Using instruments placed on the seabed, we discovered strong tidal currents and increased water mixing in the basin. These findings suggest that water movement and mixing contribute to the widespread occurrence of elevated DIC levels. Our calculations show that the primary source of the extra DIC in the deep basin is fluids released from hydrothermal vents, and both CO2 and mineral acid cause excess acidity in the water. Based on our data, we estimate that hydrothermal vents release approximately 6 billion moles of DIC per year, equivalent to 40% of the CO2 emissions from continuously active volcanoes in the Ryukyu Arc. These results demonstrate the potential significance of underwater carbon emissions in submerged backarc basins within subduction zones.
Key Points
Mild geochemical anomalies in 3He and dissolved inorganic carbon were widespread in the southwestern Okinawa Trough deep water
Mixing models confirmed that hot vent fluids are the primary source of excess dissolved inorganic carbon in the deep water
The carbon outgassing flux of the Ryukyu subduction system increases by at least 27% when the backarc contribution is considered
The deformation of nonlinear internal waves (NLIW) from depression to elevation waves during the shoaling process has been attributed to three key factors: wave amplitude, mixed-layer depth and water ...depth. This study examined the critical conditions for wave deformation by means of five strings of thermistors deployed at the Dongsha Atoll in the northern South China Sea (SCS). During a two-day experiment, four wave trains consisting of 78 solitone at five sites were identified. The parameters related to soliton transformation were formulated by applying the extended Korteweg-de Vries (eKdV) theory. The results indicated that the soliton energy dispersed dramatically along the sharp bottom slope, likely as a result of bottom friction and turbulence mixing. Large-amplitude depression waves were observed to transform into elevation waves before the thickness of the lower layer became equal to that of the upper layer. The ratio of the wave amplitude to the lower-layer thickness was found to be a good indicator of wave deformation. The critical conditions for the transition of the internal waves (IWs) occurred at the ratio approximately equal to 0.66 ± 0.2. This means that a bottom-trapped elevation wave could form before passing through the theoretical critical point. The solitons felt the bottom and deformed when their amplitudes approached half of the lower-layer thickness. The solitons existed in the form of elevation waves when the waves were in contact with the bottom.
Large-amplitude internal solitary waves and internal tides are common oceanographic features in the South China Sea, which are generated in the Luzon Strait and propagate westward cross the basin ...area to the continental shelf and hit the east wage of DongSha Atoll. These waves shoal near Dongsha plateau and force nutrients and plankton and potentially entraining fishes, squids, and nutrients into the upper ocean, potentially foraged by cetaceans. Several studies have indicated that dolphins and whales may exploit prey aggregations associated with predictable but ephemeral ocean features, such as upwellings, fronts, and eddies. Prey availability for dolphins in the South China Sea influenced by the aggregative properties of internal waves is speculated and once was reported by one photo in 2005 in a benign sea state. This study presents evidence that internal waves attract and aggregate dolphins by using integrative analysis of current data, pressure data, and passive acoustic monitoring data.
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