Shelf Break Region Research Articles

Effects of Cold Air Outbreaks on Evaporation and Heat Loss from Three Regions in the Gulf of Mexico

Simultaneous hourly measurements of atmospheric pressure; wind speed; and air, sea-surface, and dew-point temperatures from three regions in the Gulf of Mexico in 1996 are incorporated in the analysis of sensible and latent heat fluxes and the evaporation rate from the Gulf to the atmosphere. The three regions included in the study are the deep western Gulf, the northern Gulf continental shelf break near De Soto Canyon, and the northern Gulf nearshore environment at Grand Isle, LA. After the case study of a severe cold air outbreak is presented, monthly variations of evaporation and heat fluxes are investigated. It is found that on an annual basis the sensible heat flux is nearly the same between the nearshore and the shelf break regions. For the latent heat flux, the northern shelf break and deep western Gulf are nearly equal and are higher than the northern nearshore region. Also, the evaporation rate and the rainfall amount are approximately in balance in the northern Gulf nearshore environment.

Metamorphosis of the final phyllosoma and secondary lecithotrophy in the puerulus of Panulirus cygnus George: a review

The final phyllosoma of Panulirus cygnus metamorphoses to a non-feeding puerulus that lives on energy reserves accumulated in the final larva, and the metamorphic moult occurs mainly in the slope region adjoining the shelf-break off Western Australia. A review of the literature on field studies, laboratory rearing and nutritional studies of phyllosomal and other decapod zoeal larvae provided no evidence that metamorphosis in P. cygnus (or other shallow-water palinurids) is triggered by a direct environmental cue. It did indicate that metamorphosis results from the culmination of sustained nutrition and reserve energy levels through the later larval phase. Therefore, since the puerulus is secondarily lecithotrophic, it is considered that metamorphosis occurs only after the final phyllosoma has reached some critical, specific, level of stored energy reserves. Appropriate food for later larval development and successful metamorphosis of P. cygnus is more abundant in the shelf-break region (than further offshore) because this is a region of high plankton and micronekton biomass dominated by the Leeuwin Current. It also explains why metamorphosis occurs mainly in the shelf-break region. This review indicates research necessary for evaluation of the present interpretation and of larval recruitment processes in this species.

Spatial Variations of Heavy Metals in the East China Sea Continental Shelf Surface Sediments

Abstract Surface sediments collected from continental shelf of the East China Sea were analyzed for heavy metals (iron, manganese, copper, zinc, lead, cadmium), carbonate, organic carbon contents, and grain sizes. the range of concentrations observed were iron: 0.3-1.3 wt%, manganese: 2.3-14 μmole/g, copper: 7.1-184 μmole/g, zinc: 0.16-0.77μmiole/g, lead: 15-98 μmole/g, cadmium: 0.17-3.9 μmole/g, carbonate: 3.6-87 wt%, sand: 10-100%, silt: 0-70% and clay: 0-50%. A zonal distribution pattern of the heavy metals was found in the East China Sea Continental shelf sediments. High concentrations of most heavy metals, organic carbon and fine-grained sediments were observed in the inner shelf zone, especially those near the discharge of the Yangtze River. Concentrations of these heavy metals decreased from the inner shelf to the shelf break region. High concentrations of metals were also found in sediments near Taiwan. Iron concentrations decreased north-east of the central shelf region. High concentrations of ca...

A two-dimensional vorticity model of internal tides generated on the continental shelf/slope

A two-dimensional spectral-difference model (with vorticity and density equations) of internal tides is developed for studying the generation and propagation of internal tides generated at the continental shelf/slope. In general, internal tides propagate seaward in deep sea regions and shoreward on the shelf, and are dissipated rapidly. When the Vaisala frequency decreases vertically, waves may be mostly limited to the continental slope region. In deep sea region, motions may have either beam-like structure or modal structure, depending on the stratification strength and structure, whereas a modal structure may always exist on the shelf. Waves show strong bottom intensification on the slope when strong stratification exists on the bottom. The barotropic tidal advection may affect the temporal character of internal tides at the continental slope, shelf break and shelf regions, but may have little influence on the energy density and energy flux of internal tides. In the case of strong stratification, waveforms of internal tides on the continental slope, shelf break and shelf may be deformed by the barotropic tidal advection, and sometimes intermittency appears. Waves depend considerably on the relative slope δ=α/C (where α denotes topography slope andC is the characteristic slope) and the e-fold scaleb of the Vaisala frequency. As in Craig (1987a), the deep sea energy fluxF1 and on-shelf fluxF2 also vary exponentially, but our exponents are different from his in the subcritical case, and may have complicated dependence on δ in the supercritical case. For subcritical topography,F2 increases linearly withb butF1 remains nearly constant. For supercritical topography,F1 increases linearly withb andF2 varies complicatedly. For special values ofb, waves may have strong bottom intensification because of the self-closing of ray on the bottom of the slope.

Acoustic targets at South Georgia and the South Orkney Islands during a season of krill scarcity

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 138:51-61 (1996) - doi:10.3354/meps138051 Acoustic targets at South Georgia and the South Orkney Islands during a season of krill scarcity Brierley AS, Watkins JL Dual frequency (38 and 120 kHz) acoustic surveys of shelf-break regions in the vicinity of the Willis Islands, South Georgia, and Coronation Island, South Orkneys, both in the Atlantic sector of the Southern Ocean, were carried out from RRS 'James Clark Ross' during January 1994. The difference in echo signal strength between the 2 frequencies [deltaMVBS (mean volume backscattering strength) = MVBS 120 kHz - MVBS 38 kHz] was used to partition acoustic targets into 3 biological categories: Antarctic krill Euphausia superba, nekton (larger than krill) and zooplankton (smaller than krill). Krill density estimates were derived for both survey areas, and the relative contribution of each class of target to overall regional pelagic biomass determined. Krill distribution in both regions was extremely patchy, resulting in a highly skewed frequency distribution of density; for example one echo-integration interval contained 104 times the mean krill density. An approximate 6-fold difference in mean krill abundance was detected between the 2 areas but, in comparison with previously published density estimates, krill densities in both regions were extremely low (1.7 and 10.7 gm-2 for Willis and Coronation Islands respectively). Gross regional differences in pelagic faunal composition were apparent from inspection of echo charts, and were quantified by detailed analysis of acoustic signals. The water column in the vicinity of the Willis Islands was characterised by a series of diffuse but continuous scattering layers visible only at 120 kHz, and 73% by mass of acoustic targets in this region were classified as zooplankton. In marked contrast, the Coronation Island region was dominated by dense, discrete acoustic target patches which were classified as larger, nektonic, sound scatterers (fish or squid), and only 12% of targets there were classified as zooplankton. Elsewhere, data from predator diet analyses carried out at Bird Island, South Georgia, during the same season that our acoustic measurements were made, confirm the profoundly low levels of krill availability there and support the acoustic observation that the Willis Islands region contained large numbers of small zooplankton such as the amphipod Themisto gaudichaudii. Similarly, available data on analyses of stomach contents of Chinstrap penguins nesting in a neighbouring region within an oceanographic setting similar to that of Coronation Island reveal the presence of more fish in the diet than usual. Conclusions drawn from acoustic observations as to the relative regional composition of pelagic biomass are therefore clearly supported by independent evidence gathered from predators, and the value of integrating predator/prey and acoustic studies to characterise the content of localised marine ecosystems is demonstrated. Antarctic krill . Euphausia superba . Dual frequency acoustic survey . Predators . Penguin diet . Themisto gaudichaudii . Target discrimination . Southern Ocean Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 138. Publication date: July 25, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Seasonal variability of the M2 tide in the seas adjacent to Korea

Seasonal variability of the M 2 tidal harmonic constants is revealed through analyses of monthly tidal data at 12 representative tidal stations in the seas adjacent to the Korean peninsula. The variability remain systematic over the 9 years (1965–1973) of data analysis with a range comparable to that of the 18.6 year nodal modulation. Spatial inhomogeneity of the seasonal variability in the observed harmonic constants is found to exist. The largest seasonal variability in M 2 appears in the stations located along the Korea Strait. This variability is not explained by the equilibrium theory of tides, and such a variability or irregularities in the harmonic constants are considered as either a noise as done by Cartwright and Amin (1986), Deutsch Hydrography Zeitschrift, 39, 235–253, or a manifestation of frictional interaction as done by Godin and Gutierrez (1986) Continental Shelf Research, 5, 379–402 for the Bay of Fundy. Considering the opposite relation between monthly mean sea level differences in Izuhara-Pusan section and tidal characteristics in the Korea Strait, it is hypothesized that the interaction between the predominant tidal currents and oceanic currents varying with the seasons might be the main cause of the observed temporal variability in the M 2 tide. The nonlinear effect of the Kuroshio is investigated along the shelf break region through scale analyses, which show that the presence of a mean current increases the non-linear terms in the momentum balance by about one order of magnitude. The seasonally different damping effect of the Tsushima Current to the M 2 tide is also discussed to explain the process of dominant seasonal variability along the Korea Strait based on the actual current data, but further thorough investigation, considering the advection effect of the mean current, is required to investigate the associated dynamics more completely.

Sedimentary organic matter preservation: an assessment and speculative synthesis

The pool of particulate organic matter driven by primary production in western boundary continental margins is known to be smaller than the highly eutrophic upwelling systems of eastern margins. Its production and export, however, can be high compared to productive ocean environments due to inherent specific physical dynamics such as mesoscale instabilities driven by the boundary current. To investigate how such physical process influence nutrients, chlorophyll-a and particulate matter on the outer shelf of the South Brazil Bight, two oceanographic cruises were conducted three months apart (November 2012 and January 2013), both under wind conditions that favor intrusions of South Atlantic Central Water (SACW). Our results showed two distinct hydrographic scenarios derived by the relative positions of oceanographic fronts in the outer shelf and shelf break region, associated with the Brazil Current (BC). In November 2012, the oligotrophic Tropical Water occupied a larger area of the continental shelf. Low nutrients and chlorophyll-a concentrations were found, typical of a regenerated production system. In contrast, maximum concentration of particulate matter and particulate inorganic carbon occurred at deeper layers in this period, together with low POC:PON ratio — probably from sediment resuspension. In January 2013, a strong shelf break upwelling was detected that enhanced SACW intrusion, notably increasing nutrient availability in the euphotic zone and the chlorophyll-a concentration at deeper layers. These deep chlorophyll maximum layers contribute to the production and accumulation of particulate organic carbon and nitrogen under these conditions. BC mesoscale instabilities appear to act as a particulate matter transport and export agent to the deep waters throughout the South Brazil Bight outer shelf.

Abyssal water mass formation off the eastern Wilkes land cost of Antarctica

The area off the eastern Wilkes Land coast between 147 and 162°C has been investigated as a site for the formation of bottom and deep water using seven hydrographic sections obtained in the austral summer 1985. The sections showed that there was high salinity water near the freezing point upon the continental shelf in this region. The deeper shelf water was potentially more dense than the water off the shelf in this region, but a reversely sloping shelf and the Coriolis effect constrain this water from flowing off continuously. Since the sections are very similar to those found in the southern Weddell Sea, it is proposed that the same mechanisms that are probably responsible for shelf-break mixing and bottom water formation in the Weddell Sea also take place here. Most of the water on the shelf above the shelf-break sill depth, while not dense enough to form true bottom water, potentially could mix with the Warm Deep Water and interleave with water off the shelf at intermediate and near-bottom depths. Thus this region may contribute to what is commonly denoted as Antarctic Bottom Water as effectively on an areal basis as the shelf-break region of the Weddell Sea.

Hydrography and chlorophyll a in a transect from the coast to the shelf-break in the Argentinian Sea

On the basis of data obtained during eight research cruises from a section of the Buenos Aires province shelf, three systems are characterized: (1) coastal system; (2) subantarctic shelf waters system; and (3) Malvinas system. These domains are frequently separated by two quasipermanent fronts: (1) the coastal front; and (2) the shelf-break front. The patterns of seasonal variation of phytoplankton biomass in these systems are strongly related to changes in vertical stability, that controls the availability of nutrients and light energy. In the coastal system, the mixing produced by winds and tides gives homogenenous waters all year round, having the lowest nitrate concentration and the lowest N:P ratio. The amplitude of the seasonal variation of chlorophyll was relatively small, although the highest concetrations were detected in spring and autumn. The subantarctic shelf waters system is characterized by the typical development and breakdown cycle of the seasonal thermocline. Two well defined chlorophyll a maxima are observed: the main peak during spring and the secondary one during autumn. Geographical differences occur in the timing of the development and breakdown of the thermocline. Another factor of variability is the advection of low salinity waters from the coastal region of the Rio de la Plata during spring. During winter, when nutrient concentration is the highest, an increasing nitrate gradient from the coastal front to the shelf-break region is observed. During summer, surface nitrate concentration is low over the whole continental shelf, and the highest chlorophyll a concentrations are associated with the depth of the pycnocline. However, associated with the surface nitrate maximum, chlorophyll a values higher than 2 mg m −3 are recorded at the shelf-break front. The observed surface maxima are thought to be related to physical processes associated with the slope, where the enhanced mixing results in nutrient renewal and subsequent phytoplankton growth in the shelf-break region.

Temporal and spatial patterns of spawning in jack mackerel, Trachurus declivis (Pisces:Carangidae), during 1988-91 in eastern Tasmanian waters

Ichthyoplankton surveys conducted during the summer and autumn of 1988-89, 1989-90 and 1990-91 along the eastern coast of Tasmania were used to examine the spatial and temporal patterns of eggs and larvae of jack mackerel, Trachurus declivis. Results indicate that the species spawns along the entire eastern coast during summer. Trachurus declivis eggs were most abundant at shelf-break stations, indicating that spawning is concentrated in this region, although high concentrations of eggs were present on the inner shelf in 1988-89, which can be attributed to rapid onshore transport in that year. Larvae were evenly dispersed over the shelf, with the distribution of larval ages showing no indication of inshore recruitment. Considerable interannual differences in sea surface temperatures and vertical thermal structure were apparent, with the warmer waters and strong thermal stratification in the summer of 1988-89 resulting from the influx of subtropical East Australian Current (EAC) water onto the shelf, which corresponded with a major La Nina 'cold event' at that time. It is suggested that the distribution of spawning is unaffected by the interannual variations in oceanography as the mature population spawns in deeper water in the shelf-break region that is unaffected by the warming in surface waters.

Observations of Internal Tide Propagation on the Australian North West Shelf

Abstract The propagation of internal tides (12-h period internal waves) is observed over a continental shelf break region on the Australian North West Shelf. The observed phase speeds of the waves, calculated from thermistor chain and current meter data, are shown to agree closely with simple linear theory for phase velocity Of freely propagating waves. Even over the relatively steep bathymetry of a continental shelf break region (ratio of bathymetric slope to slope of internal wave characteristics reaching 0.27), a simple modal model for phase speed is found to be accurate, providing that slowly varying depth is taken into account. The direction of wave propagation is determined from the orientation of baroclinic current ellipses derived from current meter data; a two-dimensional, horizontal field of phase information; and visual observations of surface slicks. 0bservations also demonstrate that substantial temporal variations in phase are a result of variations in the thermal stratification of the water...

Storm effects on the baroclinic tidal field in the Bay of Biscay

The effect of eastward travelling storms over the baroclinic tidal field of the Bay of Biscay is studied. A three-layer bidimensional model of generation and propagation of internal tides is constructed over the whole area of the Bay of Biscay. This model is forced by the M 2 and S 2 barotropic tidal components. Mixed layer physics and thermocline mixing are incorporated in the model. Numerical experiments show the development of a temperature field in the surface layer. The most important feature of this temperature distribution is the creation of patches of cold water near the shelf-break, in the north of the studied domain. These patches which are geographically localized in areas of large baroclinic tidal intensity, are conspicuous in shelf-break regions. This is consistent with satellite infra-red images taken from April–May to September–October which often show persistent ribbons of cool water along the European continental shelf, from west Ireland to south-west Brittany. The geographical location of these cool surface ribbons is well predicted by the model. Such results suggest that interactions between internal tides and surface fluxes can play an important role in the shelf-break cooling process.

On the use of the σ-coordinate system in regions of large bathymetric variations

The σ-transformation is a widely used coordinate change that maps the actual depth-varying sea onto a computational domain, the depth of which is constant. The advantages of this technique are numerous. It permits an efficient use of computer resources, a simple treatment of the surface and bottom boundary conditions, and an accurate representation of the bathymetry. However, if the range of the depth is too large, or when the depth varies too rapidly, as in the shelf break region, it may be shown that the σ-transformation leads to severe numerical errors. In the application of GHER's three-dimensional model to the Western Mediterranean, the occurrence of those numerical errors is avoided by the introduction of a two-fold σ-coordinate system in the deep sea.

Coupling of coastal zone color scanner data to a physical‐biological model of the southeastern U.S. continental shelf ecosystem: 1. CZCS data description and Lagrangian particle tracing experiments

Nine Coastal Zone Color Scanner (CZCS) images from the southeastern U.S. continental shelf area during April 1980 were processed, and the resulting chlorophyll distributions were analyzed in conjunction with concurrent flow and temperature fields which were obtained from an optimal interpolation of current meter measurements from 17m. The chlorophyll distributions obtained from the CZCS showed high chlorophyll concentrations over the shelf and low chlorophyll concentrations over the offshore shelf region. The variance of the CZCS‐derived chlorophyll distributions was highest at the shelf break region. The chlorophyll patterns at the shelf break varied with time and space and showed shapes characteristic of typical Gulf Stream frontal eddies, high‐chlorophyll bands that were oriented along‐shelf, isolated high‐chlorophyll patches, and simple waveforms. Each event observed in the chlorophyll patterns corresponded with a Gulf Stream frontal eddy event that could be identified in the optimally interpolated flow and temperature fields. Lagrangian particle tracing experiments were also conducted to track the movement of the features observed in the CZCS images. The particles traced in these experiments showed movement that was consistent with the evolution of the patterns observed in the CZCS chlorophyll distributions. Results of the Lagrangian experiments indicate that the optimal interpolation of the current meter measurements reproduced the flow between successive CZCS images. These experiments provide evidence that the high‐chlorophyll bands seen in the CZCS chlorophyll distributions were produced by the passing of Gulf Stream frontal eddy events.

Mixing and bottom water formation in the shelf break region of the southern Weddell Sea : Foster, T.D., Arne Foldvik and J.H. Middleton, 1987. Deep-Sea Res., 34(11A):1771–1794

Mixing and bottom water formation in the shelf break region of the southern Weddell Sea : Foster, T.D., Arne Foldvik and J.H. Middleton, 1987. Deep-Sea Res., 34(11A):1771–1794

Shoreward Intrusion of Upper Gulf Stream Water onto the U.S. Southeastern Continental Shelf

Abstract In winter, cooling of the South Atlantic Bight continental shelf water results in higher density in the middle shelf region relative to the shelf-break region where the western flank of the Gulf Stream flows. Shoreward, estuarine-like intrusion of the upper Gulf Stream water in the presence of such a positive onshore density gradient is then possible through advective processes triggered either by the meander of the Stream or onshore Ekman transports by southward wind stresses. Repeated cross-shelf hydrographic transects were conducted from 10 January through 30 January 1986 to more closely study this intrusion process. These observations show many features predicted by a previous numerical model study. A semi-empirical model is proposed here wherein the state of stratification of water on the outer continental shelf region just inshore of the shelf break is used as an indicator of the intrusion process. Model analysis suggests correlating the observed time rate of change of potential energy of t...

Mixing and bottom water formation in the shelf break region of the southern Weddell Sea

The shelf break region of the southern Weddell Sea was investigated during the period 1977–1980 with sections of closely spaced CTD stations and long-term moored current meters. An intrusion of warmer water onto the shelf from intermediate depths offshore was found in all cross-shelf sections. Evidence that this warmer water intrudes onto the shelf throughout the year was found in the current meter temperature records. Continental shelf waves appear to be the most likely mechanism for bringing this warmer water onto the shelf. Mixing of very cold water on the shelf with the warmer intruded water appears to be promoted by stirring induced by the semidiurnal tides. The mixing also may be directly caused by shear instabilities. Bottom water that forms near the shelf break and flows down the continental slope in the southern Weddell Sea appears to represent a mixture of the cold shelf water with the warmer intruded water and not cold, dense shelf water alone.

Diurnal Shelf Waves in the Southern Weddell Sea

Strong evidence is presented for the existence of barotropic shelf waves at diurnal frequencies in the shelf-break region of the southern Weddell Sea. Rotary spectra from long-term current meter records on the slope, at the shelf break and on the shelf show the same rotational sense as that predicted by a barotropic model. Phase speeds of the zero-mode shelf waves calculated from tidal analysis and rotary coherencies of current meter pairs oriented in an along-shelf direction are in the same range as that predicted by barotropic theory except for the O1 constituents. The weak stratisfication of the Weddell Sea in the region of the slope results in nearly barotropic motion at diurnal frequencies. The anomalous behavior of the O1 constituent could not be explained.

Internal hydraulic jumps and solitons at a shelf break region on the Australian North West Shelf

A large‐amplitude, shoreward propagating, semidiurnal internal tide has previously been observed at a shelf break region on the Australian North West Shelf. This paper discusses detailed observations of the wave shape as the internal tide shoals over the shelf slope, near the shelf break. The leading face of the internal tide is seen to steepen and form an internal hydraulic jump. The properties of the jump are well described by a two‐layer hydraulic jump model in which the nearly uniform background temperature (density) profile is strongly distorted by the internal tide to an approximately two‐layer stratification just before a jump. The height of the jumps reaches 60 m in 123 m water depth, and the jumps contribute significantly to the strong dissipation of the internal tide that has previously been observed in this region. Following some of the jumps, packets of ∼ 12 internal waves of 20 min period are observed. These are described as solitons, and it is found that a second‐order theory (in wave amplitude) is necessary to describe the wavelength and phase speed. These waves also play an important role in the energy dissipation of the internal tide.

Internal tides and sediment transport at the shelf break in the Celtic Sea

Measurements of current and temperature fluctuations at a site on the European continental shelf have revealed the presence of large M 2 tidal phase current surges produced by the internal tide generated at the shelf break. These have been found to extend with little attenuation throughout the water column and to have a significant effect on the flow near the seabed. A numerical model of the internal tide has been used to investigate the surge current generation mechanism and the way in which the surges propagate across the shelf. Good agreement has been obtained between the model predictions and the observations for surges propagating onto the shelf. The model is also used to examine sediment transport at the shelf break. In particular it is shown that for realistic shelf edge topography, the shelf break is likely to be an area of bed load parting. The sedimentological and geological implications of this result are discussed in the light of what is known concerning the present day relief of the shelf-break region and likely processes of sediment transfer from the continental margins to the deep ocean.