Biological,physical and chemical properties at the Subtropical Shelf Front Zone in the SW Atlantic Continental Shelf

The physical aspects of the Subtropical Shelf Front (STSF) for the Southwest Atlantic Continental Shelf were previously described. However, only scarce data on the biology of the front is available in the literature. The main goal of this paper is to describe the physical, chemical and biological properties of the STSF found in winter 2003 and summer 2004. A cross-section was established at the historically determined location of the STSF. Nine stations were sampled in winter and seven in summer. Each section included a series of conductivity-temperature-depth (CTD) stations where water samples from selected depths were filtered for nutrient determination. Surface samples were taken for chlorophyll a (Chl-a) determination and plankton net tows carried out above and below the pycnocline. Results revealed that winter was marked by an inner-shelf salinity front and that the STSF was located on the mid-shelf. The low salinity waters in the inner-shelf indicated a strong influence of freshwater, with high silicate (72 μM), suspended matter (45 mg l⁻¹), phosphate (2.70 μM) and low nitrate (1.0 μM) levels. Total dissolved nitrogen was relatively high (22.98 μM), probably due to the elevated levels of organic compound contribution close to the continental margin. Surface Chl-a concentration decreased from coastal well-mixed waters, where values up to 8.0 mg m⁻³ were registered, to offshore waters. Towards the open ocean, high subsurface nutrients values were observed, probably associated to South Atlantic Central Waters (SACW). Zooplankton and ichthyoplankton abundance followed the same trend; three different groups associated to the inner-, mid- and outer-shelf region were identified. During summer, diluted waters extended over the shelf to join the STSF in the upper layer; the concentration of inorganic nutrients decreased in shallow waters; however, high values were observed between 40 and 60 m and in deep offshore waters. Surface Chl-a ranged 0.07–1.5 mg m⁻³; winter levels were higher. Three groups of zoo and ichthyoplankton, separated by the STSF, were also identified. Results of the study performed suggest that the influence of freshwater was stronger during winter and that abundance distribution of Chl-a, copepods and ichthyoplankton was related to the Plata Plume Waters (PPW), rather than to the presence of the STSF. During summer, when the presence of freshwater decreases, plankton interactions seem to take place in the STSF.     

Benthic foraminiferal assemblages of the South Brazil: Relationship to water masses and nutrient distributions

Using distributions of benthic Foraminifera and bottom-water variables (depth, salinity, temperature, oxygen, suspended matter, organic matter, phosphate, silicate, nitrite, and nitrate), we investigated movements of water masses on the South Brazilian Shelf (27–30°S) and assessed the seasonality of continental runoff on the distribution of shelf water masses. The data were obtained from water and sediment samples collected in the austral winter of 2003 and austral summer of 2004 in three transects. The terrestrial nutrient input was significantly reduced at stations away from the coast, but high values of nutrients were maintained in subsurface waters due the presence of South Atlantic Central Water (SACW) at greater depths. At shallow sampling stations the influence of freshwater runoff was related to (1) the dominance of calcareous benthic Foraminifera, such as lagoon-related Pseudononion atlanticum, Hanzawaia boueana, Bulimina marginata, Bolivina striatula, Elphidium poeyanum, together with several agglutinated species, including Arenoparrella mexicana, Gaudryina exilis, and Trochammina spp., common in coastal environments subject to wide salinity fluctuations. In contrast, smaller forms and higher species diversity characterized the assemblage at offshore stations. In winter, the presence of Buccella peruviana and Uvigerina peregrina at Santa Marta Cape suggest the possible transport of those species of Subantarctic Shelf Waters (SASW) origin. Foraminifera associated to Subtropical Shelf Water (STSW) were dominated by Globocassidulina subglobosa in both seasons. In summer, the occurrence of U. peregrina in the shallower stations suggested the influence of SACW nutrients brought up by upwelling of deeper waters.     

Space–time variability of the Plata plume inferred from ocean color

Satellite ocean color and surface salinity data are used to characterize the space–time variability of the Río de la Plata plume. River outflow and satellite wind data are also used to assess their combined effect on the plume spreading over the Southwestern South Atlantic continental shelf. Over the continental shelf satellite-derived surface chlorophyll-a (CSAT) estimated by the OC4v4 SeaWiFS retrieval algorithm is a good indicator of surface salinity. The log (CSAT) distribution over the shelf presents three distinct modes, each associated to: Subantarctic Shelf Water, Subtropical Shelf Water and Plata Plume water. The log (CSAT) 0.4–0.8 range is associated with a sharp surface salinity transition across the offshore edge of the Plata plume from 28.5 to 32.5. Waters of surface salinity <31, derived from mixtures of Plata waters with continental shelf waters, are associated to log (CSAT)>0.5. In austral winter CSAT maxima extend northeastward from the Plata estuary beyond 30°S. In summer the high CSAT waters along the southern Brazil shelf retreat to 32°S and extend south of the estuary to about 37.5°S, only exceeding this latitude during extraordinary events. The seasonal CSAT variations northeast of the estuary are primarily controlled by reversals of the along-shore wind stress and surface currents. Along-shore wind stress and CSAT variations in the inner and mid-shelves are in phase north of the estuary and 180° out of phase south of the estuary. At interannual time scales northernmost Plata plume penetrations in winter (∼1200 km from the estuary) are associated with more intense and persistent northeastward wind stress, which in the period 2000–2003, prevailed over the shelf south of 26°S. In contrast, in winter 1999, 2004 and 2005, characterized by weaker northeastward wind stress, the plume only reached between 650 and 900 km. Intense southwestward plume extensions beyond 38°S are dominated by interannual time scales and appear to be related to the magnitude of the river outflow. The plume response to large river outflow fluctuations observed at interannual time scales is moderate, except offshore from the estuary mouth, where outflow variations lead CSAT variations by about 2 months.     

The effects of river discharge and seasonal winds on the shelf off southeastern South America

The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.     

Nutrient distributions over the Southwestern South Atlantic continental shelf from Mar del Plata (Argentina) to Itajaí (Brazil): Winter–summer aspects

Nutrient distributions observed at some depths along the continental shelf from 27°05′S (Brazil) to 39°31′S (Argentina) in winter, 2003 and summer, 2004 related to salinity and dissolved oxygen (mL L⁻¹) and saturation (%) data showed remarkable influences of fresh water discharge over the coastal region and in front of the La Plata estuary. In the southern portion of the study area different processes were verified. Upwelling processes caused by ocean dynamics typical of shelf break areas, eddies related to surface dynamics and regeneration processes confirmed by the increase of nutrients and the decrease of dissolved and saturation oxygen data were verified. High silicate concentrations in the surface waters were identified related to low salinities (minimum of 21.22 in winter and 21.96 in summer), confirming the importance of freshwater inputs in this region, especially in winter. Silicate concentration range showed values between 0.00 and 83.52 μM during winter and from 0.00 to 41.16 μM during summer. Phosphate concentrations worked as a secondary trace of terrestrial input and their values varied from 0.00 to 3.30 μM in winter and from 0.03 to 2.26 μM in summer; however, in shallow waters, phosphate indicated more clearly the fresh water influence. The most important information given by nitrate concentrations was the presence of water from SACW upwelling that represents a new source of nutrients for marine primary production. Nitrate maximum values reached 41.96 μM in winter and 33.10 μM in summer. At a depth ∼800 m, high nitrate, phosphate and silicate concentrations were related to Malvinas Current Waters, Subantarctic Shallow Waters and Antarctic Atlantic Intermediate Waters (AAIW). Dissolved oxygen varied from 3.41 to 7.06 mL L⁻¹ in winter and from 2.65 to 6.85 mL L⁻¹ in summer. The percentage of dissolved oxygen saturation in the waters showed values between 48% and 113% in winter and from 46% to135% in summer. The most important primary production was verified in the summer, and situations of undersaturation were mainly observed below 50 m depth and at some points near the coast. The anti-correlation between nutrients and dissolved oxygen which showed evident undersaturation also revealed important potential sites of remineralization processes. The nutrient behaviours showed some aspects of the processes that occur over the Southwestern South Atlantic continental shelf and in their land–sea interfaces between Mar del Plata and Itajaí.     

A continental shelf scale examination of the Leeuwin Current off Western Australia during the austral autumn-winter

A continental shelf scale survey from 22°S to 34°S along the Western Australia coast provides the first detailed synoptic examination of the structure, circulation and modification of the southward flowing Leeuwin Current (LC) during the late austral autumn-early winter (May-June 2007). At lower latitudes (22°S-25°S), the LC was masked within a broad expanse of warm ambient surface water, which extended across the shelf and offshore before becoming constrained at the shelf break and attaining its maximum velocity of ∼1.0 m s⁻¹ at 28°S. The temperature and salinity signature of the LC experienced substantial modification as it flowed poleward; surface temperature of the LC decreased by ∼5.25 °C while surface salinity increased by ∼0.72, consistent with climatology estimates and smaller (larger) for temperature (salinity) than those found during summer. Subsequently, LC water was denser by ∼2σ_T in the south compared to the north, and the surface mixed layer of the LC revealed only a small deepening trend along its poleward trajectory. Modification of the LC resulted from a combination of mixing due to geostrophic inflow and entrainment of cooler, more saline surrounding subtropical waters, and convective mixing driven by large heat loss to the atmosphere. Air-sea heat fluxes accounted for 50% of the heat lost from the LC in the south, whilst only accounting for 25% in the north, where large geostrophic inflow occurred and the LC displayed its maximum flow. The onshore transport was characterised by distinct jet-like structures, enhanced in the upper 200 m of the water column, and the presence of eddies in the vicinity of the shelf break generated offshore transport.     

Variability of chlorophyll-a from ocean color images in the La Plata continental shelf region

Satellite-derived chlorophyll-a fields have been used to investigate temporal and spatial variability of chlorophyll-a concentration over the continental shelf zone (25–40°S and 60–45°W) close to the La Plata River estuary. Ocean color data used in this study were obtained by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and consisted of 368 weekly averaged Standard Mapped Images (SMI), from October 1997 to September 2005. Fourier harmonic and EOF analyses were used to study the variability of log-transformed chlorophyll-a concentration in the region. The harmonic analysis has shown that the annual cycle was the most dominant signal followed by the semi- and quadri-annual cycles, in certain areas. The strong annual cycle is mainly present in latitudes lower than 34°S where relatively high amplitudes (∼1.9 mg m⁻³) in pigment variation are seen over the southern Brazilian continental shelf. The semi-annual cycle is mainly associated with the Brazil–Malvinas frontal zone oscillation while the 4-year signal is related to positive La Plata discharge anomalies influenced by El Niño events. After removing the annual signal from the log-transformed chlorophyll anomalies, the EOF results showed that the first three modes captured 85.1% of the variability associated with the regional mean phytoplankton chlorophyll pattern in our smoothed data set. The first three modes explained, respectively, 63.4%, 14.1% and 7.6%. The EOF results showed that the long-term chlorophyll time/space patterns are associated with both La Plata discharge anomaly (mode 1) and alongshore wind stress (mode 2). A reconstruction of the chlorophyll anomaly fields has been made using the two leading EOF modes over two periods of high La Plata River discharge, during ENSO events. In the first event, the spatial patterns of high chlorophyll anomaly were confined to the southern portion of the region, associated with NE winds, which push the plume near the estuary mouth. The second period revealed an elongated tongue of positive chlorophyll anomalies over the Uruguayan and Brazilian middle continental shelves, associated with favorable SW winds. The analyses performed in this study allowed identification of the main modes of variability in SeaWiFS-derived chlorophyll in the region, which were consistent with modulations of important regional environmental forcing mechanisms.     

Wind-driven diversion of summer river runoff preconditions the Laptev Sea coastal polynya hydrography: Evidence from summer-to-winter hydrographic records of 2007–2009

This paper examines the role of atmospheric forcing in modifying the pathways of riverine water on the Laptev Sea shelf, using summer-to-winter hydrographic surveys from 2007 to 2009. Over the two consecutive winter seasons of 2007–2008 and 2008–2009 in the area of the winter coastal polynya, our data clearly link winter surface salinity fields to the previous summer conditions, with substantially different winter salinity patterns preconditioned by summer atmospheric forcing. In the summer of 2007, dominant along-shore westerly winds in the cyclonic regime force the Lena River runoff to flow eastward. In contrast, in the summer of 2008, dominant along-shore easterly winds over the East Siberian Sea and on-shore northerly winds over the Laptev Sea in the anticyclonic regime lock the riverine water in the vicinity of the Lena Delta. Over the coastal polynya area in the southeastern Laptev Sea these patterns precondition a surface salinity difference of 8–16 psu between the winters of 2008 and 2009. Overall, this indicates a residence time of at least half a year for riverine water on the Laptev Sea shelf. Future climate change associated with an enhanced summer cyclonicity over the eastern Arctic may turn more riverine water eastward along the eastern Siberian coast, resulting in weaker vertical density stratification over the Laptev Sea shelf, with possible impact on the efficiency of vertical mixing and polynya dense water production.     

Satellite observation of Brazil Current inshore thermal front in the SW South Atlantic: Space/time variability and sea surface temperatures

A data set of 199 sea surface temperature maps derived from the Advanced Very High Resolution Radiometer for the period 2000–2002 was processed to derive the position of the surface inshore thermal front of the Brazil Current (BCIF) in the SE Brazilian coastal and oceanic area. After the derivation of the position of the BC front for each image, the ensemble of digital frontal vectors was processed using the algorithm of frontal density (FD). For each 5′×5′ cell in the domain the calculated FD provided an index expressing the presence and persistence of the front in the area or the probability of finding the front in the region. In the paper we present the results of the FD analysis to get a better view of the space and time variability of the BC front in the region. The highest values of FD were in general observed close to or at the shelf break zone (between 200 and 1000 m isobaths). From 20°S to 23°S there is a tendency of BCIF to be positioned over the outer shelf, inshore of the 200 m isobaths. SE of Cape Sao Tome and S of Cape Frio it was observed a bimodal spatial distribution of highest FD caused by the presence of two semi-permanent frontal eddies. After moving offshore near Cape Frio, the BCIF tends to return to the shelf break zone south of 24°S probably due to a potential vorticity conservation mechanism. The position of the highest FD values calculated for different seasons confirms previous studies in that BCIF is closer to the coast during the summer and furthest offshore in the winter. Statistical analysis of the SST data gave for the BCIF an average SST gradient of 0.31°C km⁻¹ with a standard deviation of 0.15°C km⁻¹. A mean frontal width of 6 km was inferred from the average SST gradient and typical temperatures near the front at both sides, at outer shelf and in the BC itself. A Weibull probability density function can be fitted to describe the BCIF SST gradients with scale factor c=0.3460°C km⁻¹ and shape factor k=2.1737. The BCIF SST gradient showed a seasonal variability with the smallest gradients in summer (~0.24 °C km⁻¹) and the highest in autumn (~0.33 °C km⁻¹). Using a three harmonic Fourier fit for the SST field near the BCIF, at the outer shelf and at interior of BC, it was possible to derive an analytical model for the time variability of the SST gradient of BCIF.     

The seasonal variability of the northern Benguela undercurrent and its relation to the oxygen budget on the shelf

The seasonal variations in the advection and mixing of water masses in the northern Benguela were studied in relation to the oxygen minimum zone over the Namibian shelf. The used data set consists of hydrographic and current measurements from an oceanographic mooring 20 nm off Walvis Bay, monthly CTD transects from the Namibian 23°S monitoring line and recent large-scale hydrographic surveys. The current time series showed an intermittent southward continuation of the Angola Current (AC) through the Angola–Benguela frontal zone (ABFZ) into the northern Benguela, commonly known as poleward undercurrent. In austral summer hypoxic, nutrient rich South Atlantic central water (SACW) from the Angola Gyre is transported into the northern Benguela, whereas during the winter season the oxygen rich Eastern SACW (ESACW) spreads northward. The water mass analysis reveals a mixing between both water masses in the northern Benguela between the ABFZ and the Lüderitz upwelling cell (27°S). The oxygen balance over the Namibian shelf depends to a high extent on the water mass composition of the upper central water layer, controlled by the large scale and local circulation. The deviation of the measured oxygen concentration from its mixing concentration, calculated with the source water mass properties, was used to quantify the oxygen consumption. A new local definition SACW was derived to exclude biogeochemical processes, taking place in the Angola Gyre. The oxygen deficit in the northern Benguela central water amounted to about 60–80 μmol l⁻¹ at the shelf edge and increased up to 150 μmol l⁻¹ on the shelf, due to local oxygen consumption. In the austral summer anoxic bottom waters are observed at the central Namibian shelf, which correlate to an SACW fraction >55%. Periods with high SACW fraction in the water mass composition were congruent with hydrogen sulphide events detected by remote sensing.     

Observation of the seasonal evolution of the Yellow Sea Cold Water Mass in 1996–1998

We use the hydrographic data obtained during the joint survey of the Yellow Sea by the First Institute of Oceanography, China and the Korea Ocean Research and Development Institute, Korea, to quantify the spatial structures and temporal evolution of the southern Yellow Sea Cold Water Mass (YSCWM). It is indicated that the southern YSCWM is a water mass that develops in summer and decays in fall. In winter, due to the intrusion of the Yellow Sea Warm Current (YSWC), the central area (approximately between 34°N and 35°N, 122°E and 124°E) of the Yellow Sea is mainly occupied by relatively high temperature water (T>10 °C). By contrast, from early summer to fall, under the seasonal thermocline, the central area of Yellow Sea is occupied by cold water (T<10 °C). In summer, the southern YSCWM has two cold cores. One is formed locally southeast of Shandong Peninsula, and the other one has a tongue-like feature occupying the area approximately between 34°N and 37°N, 123°E and 126°E. The bottom layer temperature anomalies from February to July in the cold tongue region, along with the trajectories of the bottom floaters, suggest that the cold water mass in the northeast region has a displacement from the north to the central area of the Yellow Sea during the summer.     

Patos Lagoon outflow within the Río de la Plata plume using an airborne salinity mapper: Observing an embedded plume

Major river systems discharging into continental shelf waters frequently form buoyant coastal currents that propagate along the continental shelf in the direction of coastal trapped wave propagation (with the coast on the right/left, in the northern/southern hemisphere). The combined flow of the Uruguay and Paraná Rivers, which discharges freshwater into the Río de la Plata estuary (Lat. ∼36°S), often gives rise to a buoyant coastal current (the ‘Plata plume’) that extends northward along the continental shelf off Uruguay and Southern Brazil. Depending upon the prevailing rainfall, wind and tidal conditions, the Patos/Mirim Lagoon complex (Lat. ∼32°S) may also produce a freshwater outflow plume that expands across the inner continental shelf. Under these circumstances the Patos outflow plume can be embedded in temperature, salinity and current fields that are strongly influenced by the larger Plata plume. The purpose of this paper is to present observations of such an embedded plume structure and to determine the dynamical characteristics of the ambient and embedded plumes.     

Nitrogen isotope ratios of nitrate as a clue to the origin of nitrogen on the Pacific coast of Japan

To clarify the sources and transformation of NO₃⁻ on the Pacific coast of Japan, observations over the continental shelf were conducted during the summer in 2005 and 2006 when the Kuroshio flowed close to and away from the coastal area, respectively. Below the halocline, there are two prominent salinity peaks that originated in two different waters. In the subsurface layer, the salinity maximum (S_max) was indicative of the Kuroshio Water (KW), while the salinity minimum (S_min) in the middle layer at ∼400 m depth was indicative of the North Pacific Intermediate Water (NPIW). δ¹⁵N_NO3 ranged from 4.1‰ to 5.1‰ with a mean of 4.8±0.4‰ in the deeper water around S_min. Below 50 m depth over the shelf break, δ¹⁵N_NO3 values (3.1±0.8‰ in 2005 and 4.6±0.3‰ in 2006) clearly increased as contribution of NPIW increased in 2006. On the contrary, subsurface δ¹⁵N of NO₃⁻ values (−1.1±0.1‰) remained unchanged in both years, although the contribution of the KW to the subsurface water changed significantly. This suggests that the source of NO₃⁻ has little effect on the δ¹⁵N of NO₃⁻ in this layer. The negative δ¹⁵N values also coincided with the base of the chlorophyll maximum layer suggesting that these isotopic signals must be evidence of active nitrification in the upper layer.     

Thermal and haline variability over the central Bering Sea shelf: Seasonal and interannual perspectives

We examine multi-year conductivity-temperature-depth (CTD) data to better understand temperature and salinity variability over the central Bering Sea shelf. Particular consideration is given to observations made annually from 2002 to 2007 between August and October, although other seasons and years are also considered. Vertical and horizontal correlation maps show that near-surface and near-bottom salinity anomalies tend to fluctuate in phase across the central shelf, but that temperature anomalies are vertically coherent only in the weakly or unstratified inner-shelf waters. We formulate heat content (HC) and freshwater content (FWC) budgets based on the CTD observations, direct estimates of external fluxes (surface heat fluxes, ice melt, precipitation (P), evaporation (E) and river discharge), and indirect estimates of advective contributions. Ice melt, P−E, river discharge, and along-isobath advection are sufficient to account for the mean spring-to-fall increase in FWC, while summer surface heat fluxes are primarily responsible for the mean seasonal increase in HC, although interannual variability in the HC at the end of summer appears related to variability in the along-isobath advection during the summer months. On the other hand, FWC anomalies at the end of summer are significantly correlated with the mean wind direction and cross-isobath Ekman transport averaged over the previous winter. Consistent with the latter finding, salinities exhibit a weak but significant inverse correlation between the coastal and mid-shelf waters. The cross-shelf transport likely has significant effect on nutrient fluxes and other processes important to the functioning of the shelf ecosystem. Both the summer and winter advection fields appear to result from the seasonal mean position and strength of the Aleutian Low. We find that interannual thermal and haline variability over the central Bering Sea shelf are largely uncoupled.     

Storm-driven shelf-to-canyon suspended sediment transport at the southwestern Gulf of Lions

Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300 m depth from November 2003 to March 2004. Two major storm events producing waves H_s>6 m coming from the E–SE sector took place, the first on 3–4 December 2003 (max H_s: 8.4 m) and the second on 20–22 February 2004 (max H_s: 7 m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events.     

Why does the Kuroshio northeast of Taiwan shift shelfward in winter?

Observations indicate that off the northeastern coast of Taiwan a branch of the Kuroshio intrudes farther northward in winter onto the shelf of the East China Sea. We demonstrate that this seasonal shift can be explained solely by winter cooling. Cooling produces downslope flux of dense shelf water that is compensated by shelfward intrusion. Parabathic isopycnals steepen eastward in winter and couple with the cross-shelf topographic slope (the “JEBAR” effect) to balance the enhanced intrusion. The downslope flow also increases vortex stretching and decreases the thickness of the inertial boundary layer, resulting in a Kuroshio that shifts closer to the shelf break.     

High salinity winter outflow from a mega inverse-estuary—the Great Australian Bight

Conductivity-temperature-depth (CTD) observations taken in the Great Australian Bight (GAB) during ORV Franklin cruise Fr 07/94 in July 1994 indicated the presence of a dense bottom layer at the head of the GAB, which flowed along the sea floor towards the shelf-break as a gravity current The north central region of the GAB was stratified with a maximum salinity difference of between 0.4 and 0.5. The outflow was confined to the shelf and was directed in a south-easterly direction with little evidence of cross-shelf transport. The flow exhibited a well-defined bottom interface evident from the head of the GAB to near the mouth of Spencer Gulf (SG), where the surface-bottom salinity difference was about 0.3. The mean thickness of the outflow was about 15 m. An estimate of the speed of the outflow at the discharge over the shelf-break was made using the zero entrainment assumption. This yielded a speed of <16 cm s⁻¹, which remarkably was consistent with near bottom current meter measurements (16 cm s⁻¹) on the continental shelf edge, reported south of the Eyre Peninsula. A mass budget analysis indicated that the outflow, which probably is partially maintained by the gravity current and partly by a wind-driven circulation would exist over the period, July–December, with a peak transport of about 10⁶ m³s⁻¹ (1 Sverdrup) which is approximately twenty times that of the bottom outflow from the adjoining Spencer Gulf.     

Estimation of water clarity in Taihu Lake and surrounding rivers using Landsat imagery

In China, the increase in exogenous-source pollutants from rivers is one of the most important causes of lake eutrophication. The application of remote sensing technology to water quality monitoring of rivers connected to these lakes has special significance for lake management at regional scales. Many research studies have estimated water clarity using Landsat imagery. However, most of this work focused on lakes or reservoirs, for which abundant water-only pixels (i.e., pure pixels of water, PPW) were available. Few of these studies have addressed rivers, especially rivers with an average width less than 100 m. In our study, we sought to determine whether water clarity in the rivers connected to Taihu Lake could be estimated using Landsat imagery. We obtained 18 Enhanced Thematic Mapper Plus (ETM+) images from 2009 for 13 rivers ranging from an average of 37.3 to 173.6 m wide. Three field campaigns conducted in May 2009, September 2009, and January 2010 were used to obtain field measurements of Secchi disk depth (SDD). Our results suggested that the widely used model, a(TM1/TM3) + b(TM1) + c, was suitable for the estimation of SDD for Taihu Lake. The brightness of the panchromatic band of ETM+ showed significant correlations with TM1, TM3 and TM1/TM3 (p < 0.001). As a result, SDD in the lake could also be estimated using the Landsat panchromatic band. The multispectral image of ETM+ did not provide adequate PPW for estimation of water clarity in rivers. However, PPW derived from the panchromatic image captured about 93% of the variation in SDD, on average, for the every worst-case scenario in the 13 rivers. Using the PPW in rivers, a significant correlation was found between the brightness of the panchromatic image and SDD (R² = 0.64, p < 0.001). Our results demonstrate that the panchromatic image of Landsat, but not the multispectral image, can be used to estimate water clarity in rivers with an average width greater than 40 m in the Taihu basin.     

Shelf water entrainment by Gulf Stream warm-core rings between 75°W and 50°W during 1978–1999

Previous work concerning Gulf Stream warm-core rings (WCRs) and their associated shelf water entrainments have been based upon single surveys or time series from individual WCRs. To date, estimates of annual shelf water volume entrained into the Slope Sea by WCRs and its interannual variability have not been made. Using a long time series of satellite-derived sea surface temperature (SST) observations of Slope Sea WCRs, we have completed an analysis of 22 years of WCR data (1978–1999) between 75°W and 50°W to understand the interannual variability of WCRs and their role in entraining shelf water. Satellite-derived SST data digitized at Bedford Institute of Oceanography are analyzed using an ellipse-fitting feature model to determine key WCR characteristics including WCR center position, radius and orientation. Key characteristics are then used to compute WCR swirl velocity by finite-differencing WCR orientations (θ) obtained from the feature model time series. Global mean WCR-edge swirl velocity calculated from all observations is 105.72±10.7 km day⁻¹ (122.36±12.4 cm s⁻¹), and global mean WCR radius is 64.8±6.2 km. Primary and derived WCR data are incorporated into a two-dimensional ring entrainment model (RM) using the quasi-geostrophic approximation of the potential vorticity equation. The RM defines ambient water as entrained by a WCR only if the gradient of relative vorticity term (horizontal shear) dominates the potential vorticity. Proximity of a WCR to the position of the shelf-slope front (SSF) is then used to determine whether the ambient water is entrained from the outer continental shelf. WCR-induced shelf entrainment derived from the RM displays considerable spatial variability, with maximum entrainment occurring offshore of Georges Bank, advecting a mean total annual shelf water volume of 7500 km³ year⁻¹ from the region. Estimates of shelf water fluxes display significant interannual variability, which may be in part due to the observed covariance between WCR occurrences and the state of the North Atlantic Oscillation (NAO). Increased (decreased) occurrences of WCRs are evidenced during positive (negative) phases of the NAO. The total mean annual shelf-wide WCR-induced shelf water transport is estimated to be 23,700 km³ year⁻¹ (0.75 Sv), accounting for nearly 25% of the total transport in the Slope Sea region neighboring the outer continental shelf.     

Modification of halocline source waters during freezing on the Beaufort Sea shelf: evidence from oxygen isotopes and dissolved nutrients

During some, but not all winters, waters on the Mackenzie shelf of the Beaufort Sea become sufficiently saline to ventilate the halocline of the adjacent Canada Basin. This occurred in March 1988, at which time a survey of the temperature, salinity, dissolved nutrient and ¹⁸O properties of the ventilating waters was completed. The regional hydrography of 1988 was very similar to that of 1981, when ventilation also occurred in this area. The δ¹⁸O-salinity properties of the cold, saline shelf waters revealed that in the winter of 1987–1988, ice was grown from water initially more saline by about 1.5 [psu] than is typical for the area. The higher initial salinity appears to have been a consequence of a two-stage conditioning of shelf waters by storms in the autumn of 1987. Since the amount of ice growth, and consequent salt rejection, over the winter of 1987–1988 was abnormally low, this conditioning played a crucial role in the formation of the ventilating water mass. Nutrient concentrations in ventilating waters were the same as those of waters unaffected by freezing. Thus significant regeneration of nutrients within the cold saline shelf waters did not occur during their 6-month period of formation. In consequence, the nutrient signatures carried into the arctic halocline by winter shelf waters from this area tended to erode, rather than to reinforce the nutrient maxima. For this reason they are not the dominant source of supply to the arctic halocline. Waters in the Chukchi and northern Bering Seas during the same period had δ¹⁸O values intermediate between those on the Mackenzie shelf and those in the arctic halocline. Thus winter shelf waters are supplied to the arctic halocline with a range of nutrient, temperature, salinity and δ¹⁸O properties. On average, the southern Canada Basin is an impressive net producer of sea ice. The net rate of production from waters in the upper 350 m in this area is about 2 m y⁻¹, approximately twice the net rate of production in the central Arctic Ocean.