Filament Off Research Articles

Productivity response in the North Canary Basin to climate changes during the last 250 000 yr: a multi-proxy approach

We present results from the investigation of the primary productivity record over the last 250 kyr in the North Canary Basin (30°N) off Northwest Africa. Two distinct productive systems interfere in this area: the oligotrophic open ocean and the upwelling filament off Cape Ghir, that occasionally carries offshore cool nutrient-rich waters. The following geochemical and micropaleontological paleoproductivity proxies have been used in our study: calcium carbonate, barium excess (Ba excess), total organic carbon (TOC) and diatoms. Time series analysis of these proxies indicates that paleoproductivity in the North Canary Basin underwent important changes following precession and eccentricity cycles. While the precessional signal appears to be mainly related to trade wind strength, superimposed peaks in Ba excess, TOC and diatom records point to large productivity events at Terminations I, II and III. Lowering of the North Atlantic sea surface temperatures by melt water discharges which in turn strengthened the Azores high-pressure center and increased trade wind velocities is postulated as the mechanism to explain the enhancement of the coastal upwelling and associated filaments at terminations. Additionally, the Canary Current may play a role in transmitting cold melt waters and nutrients from higher latitudes to the North Canary Basin.

Nonhydrostatic aspects of coastal upwelling meanders and filaments off eastern ocean boundaries

Coastal upwelling meanders and filaments are common features off eastern ocean boundaries.Their growth is reinvestigated herein using a nonhydrostatic three-dimensional model and areduced-gravity model, with the objective of assessing contributions from two mechanisms thatemerge in the nonhydrostatic regime. The first mechanism is caused by the vertical projectionof the Coriolis force in the momentum equation. It is found that the vertical Coriolis forceoften acts as a restoring force against numerical damping off eastern ocean boundaries andthus enhances the growth of meanders and filaments. The second mechanism arises fromunstable ocean stratification when the cold upwelled water intrudes seaward over the warmlayer. The unstable stratification, albeit transient, further enhances the growth of meanders andfilaments. It is concluded that although nonhydrostatic effects do not change our understandingof how meanders and filaments grow, the realism can be enhanced using a nonhydrostaticmodel insofar as meanders and filaments off eastern ocean boundaries are concerned.

Nonhydrostatic aspects of coastal upwelling meanders and filaments off eastern ocean boundaries

Nonhydrostatic aspects of coastal upwelling meanders and filaments off eastern ocean boundaries

Mesoscale distribution of fish larvae in relation to an upwelling filament off Northwest Africa

Fish eggs and larvae were studied in relation to an upwelling filament off Northwest Africa, near the Canary Islands, during August 1993. The filament was entrained around a quasi-permanent cyclonic eddy of diameter 100 km. The sampled ichthyoplanktonic community was dominated by neritic larvae, 94.2% of which were captured in the oceanic area. Horizontal distributions of neritic and oceanic larvae were strongly influenced by the oceanographic features characteristic of the coastal margin. Neritic larvae were associated with upwelling and filament structures, while oceanic larvae were strongly excluded from these features. The results suggest that coastal upwelling and filaments are mechanisms of transport for neritic ichthyoplankton into oceanic waters, that sardine larvae are good tracers of offshore movements of upwelled waters, and that the cyclonic eddy functions as a larval nursery ground for neritic fish species.

Seasonal and interannual pigment concentration in the Canary Islands region from CZCS data and comparison with observations from the ESTOC

We have compared historical data from the Nimbus-7 Coastal Zone Color Scanner (CZCS) with recent in situ measurements at the European Station for Time-Series in the Ocean, Canary Islands (ESTOC). CZCS data show an annual winter phytoplankton maximum in January but with a large interannual variation. In sediment traps moored at ESTOC we observed winter sedimentation peaks at 1000m and 3000m depth occurring about 1 month later. We also observed sedimentation peaks in other seasons which probably result from material advecting into the region. CZCS data suggest that one possible source is the Cape Ghir upwelling filament off the north-west African coast.

The physical structure of an upwelling filament off the North-West African coast during August 1993

Recent work in the Canary Current upwelling system highlights the role of previously uninvestigated filament structures. A filament located near 27°N in summer 1993 extended 150 km offshore with a width of ∼20 km and surface temperature anomaly up to 2°C. The cool temperature signal was restricted to a shallow surface layer, and the filament was entrained around a large cyclonic eddy. Offshore transport in the filament, in excess of 1 Sv, may be a major influence in dispersal of material originating in the region of active coastal upwelling. The location of the filament studied appears repeatable from year to year, suggestive of a strong relation with the topographically trapped eddy, which was situated downstream of a lateral ridge between the Canary Islands and the African coast.

Laser photochemical ablation of CdWO4 studied with the time-of-flight mass spectrometric technique

Pulsed laser ablation of CdWO4 at 266 nm is studied with a quadrupole mass spectrometric (QMS) time-of-flight method. Ablation threshold, energy distribution, and angular distribution of the ablated species as well as nonlinearity of the ablated species mass intensity are elucidated as a function of laser fluence. Ablated species of O+2, Cd+, Cd2+, W+, and WO+ translate with energies strongly depending on the fragment mass, meaning that they are confined in a space with the same velocity distribution. Ablated species detected with the QMS filament off show a Gaussian distribution for their translation energy, which is interpreted by the Franck–Condon electron excitation mechanism. A simple model is proposed based on a photochemical bond breaking to explain the observed threshold and nonlinearity of the ablated species. Nonlinearity can be explained by photofragmentation of CdWO4 cluster ions and the successively occurring volume expansion. The latter will be the main cause for the desorption of ion species by ablation and supports the narrow angular spreading of the ablated species.

High-performance liquid chromatographic analysis of benzodiazepines using diode array, electrochemical and thermospray mass spectrometric detection

Twenty benzodiazepines were examined via a combination of high-performance liquid chromatography (HPLC)-diode array-dual electrochemical detection and HPLC-thermospray mass spectrometric detection. Although UV detection at 230 nm resulted in extensive overlap of the benzodiazepines, each compound gave unique UV spectra. Twenty percent of the benzodiazepines tested gave response ratios via oxidation detection, and the values obtained were unique. All compounds except chlorazepate gave MH + ions as the base peak via thermospray-mass spectrometric detection with filament off; additional ions were detected for many compounds. Using single ion chromatograms all benzodiazepines can be chromatographically resolved.

Drifter observations of a cold filament off Point Arena, California, in July 1988

We use the position fixes and temperature readings from 56 Tristar‐II mixed‐layer drifters, in conjunction with advanced very high resolution radiometer images, to provide a description of the mesoscale variability of the flows associated with cold‐water filaments in the California Current system off northern California in July 1988. We find that the northern, offshore temperature front of the filament is convergent and is closely associated with the core of a high‐speed (>100 cm/s) jet that has a broader spatial scale than the cold‐water filament. Mesoscale features are observed to be related to all developments of the cold‐water filament on a variety of time scales (1–30 days). In one well‐observed feature, estimates of vertical velocity (w) based on a vorticity budget are −20 m/d over an area of 150 km2 and are associated with a gain of vorticity. Estimates of w based on a local heat budget following a drifter are about 3 times larger over an area of 60 km2. These intense downwelling features are also places where horizontal stirring occurs.

A cool upwelling filament off Namibia, southwest Africa: preliminary measurements of physical and biological features

Mesoscale physical and biological measurements were made in a cool upwelling filament in the Benguela Current system, off Namibia in February 1988. The filament had been visible on a satellite infra-red image 2 weeks prior to the survey, and was found to be in a decaying state. Temperature, salinity, 0–50 m integratred nitrate, ammonium, oxygen and plankton data have enabled a consistent picture of the structure to be formed, involving four different water bodies. These are (1) typical continental shelf upwelled water inside the shelf break, (2) filament water characterized by salinity of 35.2, high surface integrated nitrate, small phytoplankton cells, and large numbers of salps, (3) warm, blue oligotrophic water to the north, and (4) a warm, high salinity anticyclonic eddy to the south. It is suggested that this southern eddy may have originated from the Agulhas Current near Cape Town. Anomalously low nitrate concentrations, that could not be explained, were found in a 100 m thick layer between 100 and 200 m depth on the southern edge of the structure.

Observations of phytoplankton and nutrients from a Lagrangian drifter off northern California

A Lagrangian drifter was deployed in a cold filament off northern California as part of the Coastal Transition Zone program. The drifter was equipped with an optical package (consisting of a spectroradiometer, a fluorometer, and a beam transmissometer) suspended at 8.5‐m depth and a water sampler suspended at 16.3‐m depth. The drifter was recovered after 8 days. Optical, chemical, and biological properties changed considerably as the drifter moved offshore in the cold filament. Concentrations of phytoplankton chlorophyll increased rapidly in the first 2 days, in parallel with the disappearance of nitrate and nitrite. After this initial period, chlorophyll decreased gradually over the next 6 days with prominent diurnal fluctuations present in the last 3 days. Water transparency also showed similar long‐term as well as diurnal fluctuations. The phytoplankton community became increasingly dominated by large centric diatoms throughout the deployment. Although total cell volume was higher towards the middle of the deployment, this increase occurred without a parallel increase in chlorophyll. In addition, total particulate concentrations were highest nearshore. Although the drifter slippage was approximately 1 cm/s, the biological, chemical, and physical characteristics of the water were affected by both in situ changes and vertical motions of the water. These results are generally consistent with results from other up welling studies.

Mesoscale eddies, jets, and fronts off Point Arena, California, July 1986

A broad (about 40 km wide) cold filament observed off Point Arena in advanced very high resolution radiometer images at the end of June 1986 evolved into a narrow (10–20 km wide) cold filament embedded in a large‐scale (about 200 km alongshore) cool anomaly by mid July. The cool anomaly was part of a much larger‐scale anomaly, stretching from north of Cape Mendocino to south of San Francisco, in which several cold filaments were embedded. The filament off Point Arena, surveyed by ship from July 7 to 19, 1986, extended offshore around the edge of a cyclonic center. The horizontal temperature gradient was sharpest (up to 1.5°C/2.2 km) across the southern edge of the filament, and a salinity front was located within the cold filament; temperature and salinity fronts were not coincident. The surface salinity was high (33.2 psu (practical salinity unit)) in the cyclonic center and decreased across the cold filament to about 32.6 psu. The entire cool anomaly was denser than surrounding water, and there was a sharp density front (1 sigma‐t unit/10 km) at its northern edge. At the inner (cyclonic) edge of the filament the temperature and salinity variations were density compensating. The cold filament was advected offshore by a jet which was symmetric about its axis, located about 10 km north of the cold filament core. The surface manifestation of the cold filament was highly variable in space and time: the location of the filament core changed by about 5 km in 14 hours along one transect; the velocity field was less variable. Several narrow (10 km wide) water mass anomalies were evident, including a salinity subduction zone in the northern segment of the cold filament. Thermohaline intrusions 50–100 m thick may have been caused by complex frontal interactions upstream of the anomaly and offshore in a region of jet confluence. From a second survey (July 27 to August 5, 1986) the offshore jet had reoriented to alongshore, possibly related to an observed increase in the poleward flow over the continental slope. Prior to the second survey there were changes in the nearshore wind stress and the large‐scale wind stress curl.

Genetic tracers of zooplankton transport in coastal filaments off northern California

Genetic markers were used to describe the transport of the calanoid copepod Metridia pacifica in a cool filament off Point Arena, California, in July 1986. The markers used were genetic variants of enzymes (allozymes); individual copepods were assayed for allozymic variability at nine enzyme‐encoding genes (loci) by polyacrylamide gel electrophoresis. Zooplankton samples were collected in a 300 km by 170 km domain. Maps of surface dynamic height in this domain showed an offshore jet, with strong cross‐shore transport, in the presence of a cool anomaly comprising multiple fronts and filaments. The genetic data sorted the samples into two heterogeneous groups (according to maximum likelihood analysis): one comprising samples near the coast and at the head of the filament, and the other comprising offshore samples. The groups themselves were heterogeneous, revealing smaller‐scale patterns of transport and mixing. The genetic spatial structure of the calanoid copepod, Metridia pacifica, collected in the flow field was used to infer patterns of transport in the filament flow field. Patterns of transport inferred from genetic differences between zooplankton samples were compared with expected particle transport based on models which consider physical (geostrophic advection, Ekman drift, and mixing) transport plus vertical migration behavior. According to both the flow models and the genetic data, transport of zooplankton occurred primarily along paths of geostrophic flow. There was little cross‐jet exchange and almost no exchange between samples in the filament and samples in a cyclonic eddy to the south of the jet. The genetic data further indicated that copepod populations at the coastal source of the filament were genetically heterogeneous. Advection in the filament thus resulted in the juxtaposition of genetically distinct zooplankton populations in the jet axis. The genetic structure of zooplankton in the filament resulted from advection of previous distant populations, not (because of the time scales of the processes) from changes in the genetic character of samples during transport. The presence of multiple fronts and filaments within the cool anomaly prevented homogenization of the plankton assemblages during transport. Thus samples collected less than 10 km apart across the jet axis were genetically different. These findings indicated that plankton populations in complex flow fields may show patchiness in biological, biochemical and/or genetic character at small time/space scales.

Structural changes during elongation of polymer solutions

During elongational flow experiments on polymer solutions certain anomalous examples of flow behaviour have been noted. These are, 1. (1) a non uniform velocity profile in the radial direction in strongly non-Newtonian fluids being elongated at high rates. 2. (2) the loss of tack in the filament as rate of elongation increased, 3. (3) the bouncing of the filament off a rotating drum at high elongation rates, and 4. (4) the development of sustained rubber-like lateral vibrations in the fluids elongated at high rate. In addition, droplet formation occurred on the surface of the solutions which were elongated rapidly then suddenly stopped. Photography showed that the filaments continued through these droplets. When captured and dried, it was observed that the filament contained the bulk of the solute. The droplets appeared to have only a low concentration of polymer. On the basis of the photographic evidence and the thermodynamics of complex formation between elongated polymer chains, it would appear that complexation of the chains is occurring at high elongation rates. This could partly account for the strain hardening encountered during elongation of many polymeric fluids. Where solubility parameter difference between polymer and solvent was large, the complex formed could come out of solution forming a gel and a solvent rich phase.