Long edge waves over a continental shelf

Summary The existence of long edge waves over a continental shelf is examined for a representative collection of shelf profiles. The frequencies of both trapped and leaky modes are found, when they exist, in terms of the shelf parameters. Certain theoretical trapped waves over a semi-infinite shelf are shown not to exist in the real ocean.A typical shelf profile on the east coast of Australia is modelled in four different ways from the above collection. The amplification of the leaky mode at the shoreline is found in terms of frequency for each model. Comparison of the results illustrates the severe errors introduced by crude approximations to the profile both at the shoreline and at the shelf cut-off region.

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Lange Randwellen über einem Festlandsockel

Zusammenfassung Die Existenz von langen Randwellen über einem Festlandsockel wird untersucht für eine repräsentative Sammlung von Schelfprofilen. Die Frequenzen sowohl der trapped (geführten) als auch der leaky (energieoder impulsabgebenden) Eigenfunktionsformen werden — wenn sie existieren — ermittelt, ausgedrückt in Schelf-Parametern. Es wird gezeigt, daß gewisse, trapped waves (geführte Wellen) über einem semi-unendlichen Schelf im wirklichen Ozean nicht existieren.Mit Hilfe der obigen Sammlung hat man ein typisches Schelfprofil von der Ostküste Australiens in vier verschiedenen Arten modelliert. Die Verstärkung des leaky mode an der Küstenlinie wird für jedes Modell gefunden in Termen der Frequenz. Der Vergleich der Ergebnisse verdeutlicht die schweren Fehler, die durch grobe Näherungen an das Profil entstehen, sowohl an der Küstenlinie als auch in dem Gebiet des Übergangs vom Schelf in die Tiefsee.

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Ondes transversales à longues crêtes au-dessus d'un plateua continental

Résumé On examine l'existence d'ondes transversales à longues crêtes au-dessus d'un plateau continental, afin de constituer une collection représentative des profils du plateau. On a trouvé les fréquences du «trapped mode» (mode captif) et du «leaky mode» (mode de transfert d'énergie et du moment) — quand ils existent — et cela en fonction des paramètres du plateau. On montre qu'il n'y a pas, dans l'océan réel, certaines ondes captées théoriques qui existeraient au-dessus d'un plateau semi-infini.Grâce à la collection dont il est parlé ci-dessus, on a modelé de quatre façons différentes un profil typique de la côte Est de l'Australie. Pour chaque modèle on a trouvé l'amplification du «leaky mode» sur la ligne de côte en fonction de la fréquence. La comparaison des résultats montre les erreurs importantes introduites par des approximations sommaires du profil, à la fois près de la ligne de côte et dans la région de transition du plateau continental en mer abyssale.

     

Sediment transport event analysis on the western Adriatic continental shelf

The sediment-transport mechanisms that contribute to and redistribute the modern sediment deposits on the western Adriatic continental shelf were evaluated utilizing data collected from two instrumented benthic tripods deployed at 12-m water depth, one in the northern Adriatic basin on the Po River subaqueous delta, and the other in the central Adriatic basin on the Pescara River shelf. Sediment-resuspension events driven by cold, northeasterly Bora winds dominate the along-shelf transport climatology at both tripod locations, but at the Po delta site, the southwesterly Scirocco wind events also play a significant role. At the Pescara shelf site, interaction between Bora wind-driven currents and the Western Adriatic Coastal Current strongly contributes to the resuspension and advection of suspended sediment. Interannual variability of the forcing mechanisms (including strength, frequency, and relative mix of Bora and Scirocco wind events) is evident in the three winters of data collected on the Po River subaqueous delta. In both types of wind events, and throughout all years of data collection, the net along-shelf sediment transport is significantly larger than the net across-shelf transport at the 12-m sites. This may be characteristic of low-energy environments, where sediment resuspension and transport occurs in such shallow water that it is not subjected to strong downwelling features characteristic of higher-energy environments.     

Ecosystem history of Mississippi River-influenced continental shelf revealed through preserved phytoplankton pigments

Pigments determined by high performance liquid chromatography (HPLC) provide useful information concerning water column and epibenthic plant and microbial communities in both extant communities and accumulated sediments in lakes, estuaries and the ocean. Chlorophyll and its degradation products provide an estimate of overall biomass, and carotenoid pigments provide taxonomic biomarkers of phytoplankton. We examined the pigments preserved in sediment cores from the Louisiana continental shelf adjacent to the outflow of the Mississippi River system to document changes in phytoplankton community composition, phytoplankton abundance, and conditions of hypoxia over time. Carbon accumulated in sediments from water depths of 20-60 m is primarily derived from marine phytoplankton and represents the history of phytoplankton communities in the overlying water. There is a general increase in chlorophyll a, pheopigments, zeaxanthin, fucoxanthin and most carotenoids over time, with the change gradual from 1955 to 1970, followed by a fairly steady increase to 1997. The highest chloropigment concentrations are in cores from areas more likely to be exposed to seasonal hypoxia. These indicate an increase in eutrophication in the form of greater diatom and cyanobacterial production, or a worsening of hypoxia, or both. This trend expanded westward along the Louisiana shelf in the 1990s.     

Effect of upwelling on phytoplankton productivity of the outer southeastern United States continental shelf

Gulf Stream frontal disturbances cause nutrient-rich waters to frequently upwell and intrude onto the southeastern United States continental shelf between Cape Canaveral, Florida and Cape Hatteras, North Carolina. Phytoplankton response in upwelled waters was determined with three interdisciplinary studies conducted during April 1979 and 1980, and in summer 1978. The results show that when shelf waters are not stratified, upwelling causes productive phytoplankton (diatom) blooms on the outer shelf. Phytoplankton production averages about 2 g C m⁻² d⁻¹ during upwelling events, and ‘new’ production is 50% or more of the total. When shelf waters are stratified, upwelled waters penetrate well onto the shelf as a subsurface intrusion in which phytoplankton production averages about fives times higher than the nutrient-depleted overlying mixed layer. Phytoplankton within the intrusion deplete upwelled NO₃ in about 7 to 10 days, at which point no further net increase in phytoplankton biomass occurs.Current meter records show that upwelling occurs roughly 50% of the time on the outer shelf during November to April (shelf not stratified), and we estimate that seasonal primary production in upwelled waters is 175 g C m⁻² 6 months⁻¹ of which at least 50% is ‘new’ production. More than 90% of outer shelf primary and ‘new’ production occurs during upwelling and thus upwelling is the dominant process affecting primary productivity of the outer shelf. Our seasonal estimates of outer shelf primary and ‘new’ production are, respectively, three and ten times higher than previous estimates that did not account for upwelling.     

Flow convergence over a broad,flat continental shelf

The broad (～500 km) southeastern Bering Sea continental shelf contains three fronts; outer (shelf break, ～170-m depth), middle (～100-m depth), and inner (～50-m depth). The shelf break and inner fronts appear to be analogous to similar fronts reported from other mid-latitude continental shelves; extensively studied examples are from the mid-Atlantic bight, off Nova Scotia, and around the British Isles. The middle front may have counterparts on the broad North Sea and East China Sea shelves.One-month current and temperature records from either side of the middle front, ～150 km landward from the shelf break, showed convergence in the layers deeper than 30 m in both the cross-shelf flow field and heat flux. The convergence was ～3 cm s^?1, so an average upwelling at ～1 × 10^?3cm s^?1 and divergence in the surface layer were required to maintain continuity. Variations in the degree of convergence arose primarily from 1 to 5-day fluctuations in sub-tidal flow across the outer shelf domain seaward of the front.Diffusive landward heat flux was dominated by tidal scales. Horizontal eddy conductivities describing the flux were ～1 ? 10⁶ on the landward side and ～5 × 10⁶cm² s^?1 on the seaward side, and were less in the layers above the bottom layer. Advective flux by the mean flow was the same order as diffusive flux, but landward in the bottom layer and seaward in the mid-water column layers, in agreement with deductions from water mass analyses. Frontal effects reduced the net cross-shelf heat flux beneath ～30 m by about 50%. The observation of a flow convergence in the middle of a broad, flat continental shelf poses an important question of dynamics.     

Interactions between freshwater input, light, and phytoplankton dynamics on the Louisiana continental shelf

We examined the effects of freshwater flow and light availability on phytoplankton biomass and production along the Louisiana continental shelf in the region characterized by persistent spring–summer stratification and widespread summer hypoxia. Data were collected on 7 cruises from 2005 to 2007, and spatially-averaged estimates of phytoplankton and light variables were calculated for the study area using Voronoi polygon normalization. Shelf-wide phytoplankton production ranged from 0.47 to 1.75 mg C m⁻² d⁻¹ across the 7 cruises. Shelf-wide average light attenuation (k_d) ranged from 0.19–1.01 m⁻¹ and strongly covaried with freshwater discharge from the Mississippi and Atchafalaya Rivers (R²=0.67). Interestingly, we observed that the euphotic zone (as defined by the 1% light depth) extended well below the pycnocline and to the bottom across much of the shelf. Shelf-wide average chlorophyll a (chl a) concentrations ranged from 1.4 to 5.9 mg m⁻³ and, similar to k_d, covaried with river discharge (R²=0.83). Also, chl a concentrations were significantly higher in plume versus non-plume regions of the shelf. When integrated through the water-column, shelf-wide average chl a ranged from 26.3 to 47.6 mg m⁻², but did not covary with river discharge, nor were plume versus non-plume averages statistically different. The high integrated chl a in the non-plume waters resulted from frequent sub-pycnocline chl a maxima. Phytoplankton production rates were highest in the vicinity of the Mississippi River bird's foot delta, but as with integrated chl a were not statistically different in plume versus non-plume waters across the rest of the shelf. Based on the vertical distribution of light and chl a, a substantial fraction of phytoplankton production occurred below the pycnocline, averaging from 25% to 50% among cruises. These results suggest that freshwater and nutrient inputs regulate shelf-wide k_d and, consequently, the vertical distribution of primary production. The substantial below-pycnocline primary production we observed has not been previously quantified for this region, but has important implications about the formation and persistence of hypoxia on the Louisiana continental shelf.     

Summer phytoplankton production and transport along the shelf break in the Bering Sea

A general model is presented for the production and fate of phytoplankton during summer in two regions over the continental shelf of the Bering Sea. We propose that both regions of productivity are supported by nutrients transported into the area with the Bering Slope Current and that the fate of the phytodetritus produced is significantly affected by advection. We hypothesize that one system of primary productivity is initiated at the Bering Sea shelf-break front and continues into the northern Bering Sea as part of the modified Bering Shelf water mass. Phytodetritus produced in this system is transported north through Anadyr and Shpanberg Straits and we estimate that in 1987 it supplied 26% of the daily carbon demand of the benthos in the Chirikov Basin. The second region of primary productivity is located in the northern Bering Sea. Nutrients from the Anadyr Current, the northern branch of the bifurcated Bering Slope Current, support a highly productive phytoplankton bloom throughout the summer. Phytodetritus produced in this surface bloom is probably advected into the southern Chukchi Sea and deposited in the sediments.     

Phytoplankton biomass and primary production responses to physico-chemical forcing across the northeastern New Zealand continental shelf

Phytoplankton biomass and primary production were monitored in the Hauraki Gulf and on the northeastern continental shelf, New Zealand - using ship surveys, moored instruments and satellite observations (1998-2001) - capturing variability across a range of space and time scales. A depth-integrated primary production model (DIM) was used to predict integrated productivity from surface parameters, enabling regional-specific estimates from satellite data. The shelf site was dominated by pico-phytoplankton, with low chlorophyll-a (<1 mg m⁻³) and annual production (136 g C m⁻² yr⁻¹). In contrast, the gulf contained a micro/nano-phytoplankton-dominated community, with relatively high chlorophyll-a (>1 mg m⁻³) and annual production (178 g C m⁻² yr⁻¹). Biomass and productivity responded to physico-chemical factors; a combination of light, critical mixing depths and/or nutrient limitation—particularly new nitrate-N. Relatively low biomass and production was observed during 1999. This coincided with inter-annual variability in the timing and extent of upwelling- and downwelling-favourable along-shelf wind-stress, influencing the fluxes of new nitrate-N to the shelf and gulf. Relationships with the Southern Oscillation Index are also discussed. Our multi-scaled sampling highlighted details associated with stratification and de-stratification events, and deep sub-surface chlorophyll-a not visible to satellite sensors. This study demonstrates the importance of multi-scaled sampling in gaining estimates of regional production and its responses to physico-chemical forcing.     

Internal tides and waves near the continental shelf edge

Abstract

The subject is reviewed from the viewpoints of theory, internal tide and wave structure and their implications.

A wider theoretical context suggests scope for further investigation of natural or nearly-trapped forms above the inertial frequency.

Although internal tides in many locations are observed to have first-mode vertical structure, higher modes are seen offshore from shallow shelf-break forcing and for particular Froude numbers, and may be expected locally near generation. Bottom intensification is often observed where the sea floor matches the characteristic slope. Solitons form from internal tides of large amplitude or at large changes of depth.

Internal tides and solitons are observed also at many sills and in straits, and to intensify in canyons.

Non-linear effects of the waves, especially solitons, include the conveyance of water, nutrients, ‘‘mixing potential'’ etc. away from their source to other locations, and the generation of mean currents. The waves transfer energy and possibly heat between the ocean and shelf, may be a source of medium frequency waves on the shelf (periods of minutes) and can contribute to interior mixing and overturning, bottom stirring and sediment movement.     

Cyclone-induced net sediment transport pathway on the continental shelf of tropical Australia inferred from reef talus deposits

Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp. 107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on the leeward side of submerged coral reefs; they are up to 10 m in thickness, comprised of re-worked carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An explanation for the consistent along-coast cyclone transport pathway is presented based on previously published hydrodynamic modelling results. These models illustrate how currents generated by the passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over the entire length of northern Australia's coastline. This process provides an explanation for the observed sediment transport patterns on modern tropical continental shelves, as well as a basis for the interpretation of ancient tropical shelf deposits.     

Sediment transport on the Washington continental shelf: Estimates of dispersal rates from Mount St. Helens ash

Following the 18 May, 1980 eruption of Mount St. Helens, samples from 10 stations on the Washington continental shelf were collected during cruises in October 1980, January 1981, October 1981, and January 1982, in an attempt to trace the movement of ash which was introduced to the Pacific Ocean by the Columbia River. Prior to October 1980, marine ash dispersal was restricted to points west and southwest of the Columbia River mouth due to summer circulation patterns. Five winter storms were probably responsible for most of the north-northwesterly transport of ash from October 1980 to October 1981. The size of this ash ranged from 7 to 22 μm (equivalent spherical diameter; reference density = 2.65 Mg m⁻³) at distances as far as 84 km from the river mouth. Only fine ash (7–12 μm) was recovered 84–125 km from the river mouth.The calculated suspensate transport rate for 16–22 μm ash is 80 km y⁻¹ (16 km storm⁻¹). Finer ash (7–12 μm) was transported at estimated rates of 73–190 km y⁻¹ (14–38 km storm⁻¹). These calculated transport rates are similar to rates previously estimated by Smith and Hopkins (1972; 80 km y⁻¹) and Sternberg and McManus (1972; 220 km y⁻¹), but are less than those predicted by Kachel (1980; 25–330 km storm⁻¹), even though the ash was finer than the average sediment upon which these previous estimates were based.     

Observations of near-inertial oscillations along the Brazilian continental shelf break

Ocean Dynamics - Near-inertial oscillations (NIO) are intermittent motions with a frequency close to the inertial frequency and represent an important fraction of the energy to the currents in the...