Shelf-sea Fronts Research Articles

On the use of satellite-derived frontal metrics in time series analyses of shelf-sea fronts, a study of the Celtic Sea

Satellite-derived frontal metrics describe characteristics of oceanic thermal fronts, such as their strength or persistence. They are used in marine science to investigate spatio-temporal variability of thermal fronts or in ecological studies to assist in explaining animal distributions. Although these metrics represent highly processed data, which is based on sometimes complex algorithms, little guidance is available on their correct application in quantitative analyses, in particular for non-specialist users. This research aims to improve accurate use of frontal data. This case study investigates the inter--annual and seasonal variability of two tidal mixing fronts on the Celtic Sea shelf, based on monthly time series of daily frontal maps at ∼1km2 resolution from 1990 to 2010. Some metrics are almost identical and can be grouped, e.g. frontal probability, persistence and so-called “composites” (Pearson correlation: r = 0.8–1.0; p < 0.001), whereas the metric describing frontal strength is distinct from other ones. Consequently, strength and metrics of the frontal probability group showed pronounced differences in their inter-annual and seasonal variability: Strength displayed an oscillating pattern between 1990 and 2010 while there were no significant changes in probability over time. In addition, seasonal variability was affected by segments from adjacent fronts, not belonging to the fronts of interest, which could result in biased estimates. Most important, there was a doubling of available satellite imagery between 1990 and 2010 due to a greater number of operational satellites, which negatively affected frontal probability, positively frontal strength and consequently, changed the temporal pattern of both. When using frontal maps for temporal analyses, we should choose the metric carefully, be aware of biased estimates caused by variability from unwanted frontal segments in the data and account for the variable data quantity. This guide on the use of frontal metrics will be helpful to improve correct interpretations of statistical analyses.

Seabird diving behaviour reveals the functional significance of shelf-sea fronts as foraging hotspots.

Oceanic fronts are key habitats for a diverse range of marine predators, yet how they influence fine-scale foraging behaviour is poorly understood. Here, we investigated the dive behaviour of northern gannets Morus bassanus in relation to shelf-sea fronts. We GPS (global positioning system) tracked 53 breeding birds and examined the relationship between 1901 foraging dives (from time-depth recorders) and thermal fronts (identified via Earth Observation composite front mapping) in the Celtic Sea, Northeast Atlantic. We (i) used a habitat-use availability analysis to determine whether gannets preferentially dived at fronts, and (ii) compared dive characteristics in relation to fronts to investigate the functional significance of these oceanographic features. We found that relationships between gannet dive probabilities and fronts varied by frontal metric and sex. While both sexes were more likely to dive in the presence of seasonally persistent fronts, links to more ephemeral features were less clear. Here, males were positively correlated with distance to front and cross-front gradient strength, with the reverse for females. Both sexes performed two dive strategies: shallow V-shaped plunge dives with little or no active swim phase (92% of dives) and deeper U-shaped dives with an active pursuit phase of at least 3 s (8% of dives). When foraging around fronts, gannets were half as likely to engage in U-shaped dives compared with V-shaped dives, independent of sex. Moreover, V-shaped dive durations were significantly shortened around fronts. These behavioural responses support the assertion that fronts are important foraging habitats for marine predators, and suggest a possible mechanistic link between the two in terms of dive behaviour. This research also emphasizes the importance of cross-disciplinary research when attempting to understand marine ecosystems.

GRADHIST — A method for detection and analysis of oceanic fronts from remote sensing data

Abstract Oceanic shelf sea fronts have significant effects on local dynamics, ecology and climate. An assessment of the impact of climate change on frontal positions and frontal gradients requires reliable reference data and the possibility to monitor oceanic fronts. Therefore, the development of algorithms which automatically detect frontal positions from Earth Observation (EO) data is an important tool to analyse long EO time series, i.e. to process big data volumes. The development of GRADHIST was driven by the need to generate a climatology for North Sea fronts. GRADHIST is a new algorithm for the detection and mapping of oceanic fronts, which is based on a combination and refinement of the gradient algorithm of Canny (1986) and the histogram algorithm of Cayula and Cornillon (1992). GRADHIST preserves the main principles of both algorithms and can be applied to various ocean parameters as well as to different sensors with very little effort. GRADHIST was validated and tested using both synthetic and real data and applied to sea surface temperature and ocean colour parameters retrieved from satellite data; i.e. data from MODIS (Moderate Resolution Imaging Spectroradiometer), MERIS (MEdium Resolution Imaging Spectrometer), AVHRR (Advanced Very High Resolution Radiometer) and AATSR (Advanced Along-Track Scanning Radiometer). Selected results and statistical analysis of a new North Sea climatology for oceanic fronts are presented and discussed.

The influence of shelf-sea fronts on winter monsoon over East China Sea

Strong sea surface temperature fronts in open seas are known to affect the atmosphere. Shelf-sea fronts in winter have comparable strengths, yet their impacts on winds have not been studied. In January of 2012, a persistent, narrow band of cloud stretching 600–1,000 km was observed along the front of East China Sea (ECS). Numerical and analytical models show that the cloud was formed atop a recirculating cell induced by the front and, more generally, that β-plumes of low and high pressures emanate and spread far from fronts. Consistent with the theory, observations show that in ECS at inter-annual time scales, strong fronts co-vary with on-shelf convergent wind, strong northeasterly monsoon, and alongshelf alignment of clouds with low clouds near the coast and higher clouds offshore. Our results suggest that shelf-sea fronts are potentially an important dynamic determinant of climate variability of East Asia.

Seasonal shelf-sea front mapping using satellite ocean colour and temperature to support development of a marine protected area network

Front detection and aggregation techniques were applied to 300-m resolution MERIS satellite ocean colour data for the first time, to describe frequently occurring shelf-sea fronts near to the Scottish coast. Medium resolution (1-km) thermal and colour data have previously been used to analyse the distribution of surface fronts, though these cannot capture smaller frontal zones or those in close proximity to the coast, particularly where the coastline is convoluted. Seasonal frequent front maps, derived from both chlorophyll and SST data, revealed a number of key frontal zones, a subset of which were based on new insights into the sediment and plankton dynamics provided exclusively by the higher-resolution chlorophyll fronts. The methodology is described for applying colour and thermal front data to the task of identifying zones of ecological importance that could assist the process of defining marine protected areas. Each key frontal zone is analysed to describe its spatial and temporal extent and variability, and possible mechanisms. It is hoped that these tools can provide guidance on the dynamic habitats of marine fauna towards aspects of marine spatial planning and conservation.

Scales and structure of frontal adjustment and freshwater export in a region of freshwater influence

Sea surface temperature satellite imagery and a regional hydrodynamic model are used to investigate the variability and structure of the Liverpool Bay thermohaline front. A statistically based water mass classification technique is used to locate the front in both data sets. The front moves between 5 and 35 km in response to spring–neap changes in tidal mixing, an adjustment that is much greater than at other shelf-sea fronts. Superimposed on top of this fortnightly cycle are semi-diurnal movements of 5–10 km driven by flood and ebb tidal currents. Seasonal variability in the freshwater discharge and the density difference between buoyant inflow and more saline Irish Sea water give rise to two different dynamical regimes. During winter, when cold inflow reduces the buoyancy of the plume, a bottom-advected front develops. Over the summer, when warm river water provides additional buoyancy, a surface-advected plume detaches from the bottom and propagates much larger distances across the bay. Decoupled from near-bed processes, the position of the surface front is more variable. Fortnightly stratification and re-mixing over large areas of Liverpool Bay is a potentially important mechanism by which freshwater, and its nutrient and pollutant loads, are exported from the coastal plume system. Based on length scales estimated from model and satellite data, the erosion of post-neap stratification is estimated to be responsible for exporting approximately 19% of the fresh estuarine discharge annually entering the system. Although the baroclinic residual circulation makes a more significant contribution to freshwater fluxes, the episodic nature of the spring–neap cycle may have important implications for biogeochemical cycles within the bay.

Physical controls on phytoplankton physiology and production at a shelf sea front: a fast repetition-rate fluorometer based field study

Observations of phytoplankton physiology collected using a fast repetition-rate fluoro- meter (FRRF) in the vicinity of a shelf-sea tidal-mixing front are presented. These data are combined with more traditional 14 C-based measurements and observations of environmental parameters, including estimates of turbulent dissipation rates, in order to investigate the influence of physical forcing on the productivity of the system. Low nutrient concentrations on the stratified side of the front result in a reduction of photosynthetic efficiency. Conversely, the high degree of vertical mixing on the mixed side of the front constrains the ability of phytoplankton to adjust their photosynthetic apparatus to the ambient irradiance field. Redistribution of phytoplankton biomass and variations in physiological parameters also result from the spring-neap tidal cycle. FRRF- and 14 C-derived physio- logical measurements are compared in the context of environmental gradients in the region. A strong correlation was found between independently measured functional absorption cross-sections (σ PSII) and maximal photosynthetic rates (P*max). Such a relationship was unlikely to have been causative and may have resulted from shifts in the balance between light-harvesting and carbon fixation across the front. The association of changes in P*max with variations in σ PSII provided the basis for the devel- opment of an empirical model, specific to the system and time of study, which utilised FRRF data to extrapolate between primary productivity rates measured at fixed sites. When applied to high- resolution cross-frontal data, the model suggested small-scale variations in productivity related to both spatial and temporal physical forcing including the spring-neap cycle.

Pursuit plunging by northern gannets (Sula bassana) feeding on capelin (Mallotus villosus).

Northern gannets (Sula bassana) are considered to obtain prey usually by rapid, vertical, shallow plunge dives. In order to test this contention and investigate underwater foraging behaviour, we attached two types of data-logging systems to 11 parental northern gannets at Funk Island in the North-Wiest Atlantic. We documented, for the first time to the authors' knowledge, gannets performing long, flat-bottomed, U-shaped dives that involved underwater wing propulsion as well as rapid, shallow, V-shaped dives. The median and maximum dive depths and durations were 4.6 and 22.0 m and 8 and 38 s, respectively. Short, shallow dives were usually V-shaped and dives deeper than 8 m and longer than 10 s were usually U-shaped, including a period at constant depth (varying between 4 and 28s with median 8s). Diving occurred throughout the daylight period and deepest dives were performed during late morning. On the basis of motion sensors in the loggers and food collections from telemetered birds, we concluded that extended, deep dives were directed at deep schools of capelin, a small pelagic fish, and we hypothesized that V-shaped dives were aimed at larger, pelagic fishes and squids. Furthermore, these V-shaped dives allowed the birds to surprise their pelagic prey and this may be critical because the maximum swimming speeds of the prey species may exceed the maximum dive speeds of the birds.

Effects of along-shore wind stress and surface cooling on the formation of shelf sea fronts in a simple air-sea interaction model

This paper presents numerical simulation results from a set of control and sensitivity experiments on the effects of winter-time air-sea interaction on the variability of sea surface temperature (SST) on a two-dimensional continental shelf which is uniform in the along-shore direction and is bounded to the west by a straight coast and to the east by a prescribed Gulf Stream front. In the control experiment, the model ocean circulation is driven by a time-dependent wind forcing which is parameterically coupled to the cross-shelf SST gradient. In the sensitivity experiments, wind stress, diabatic cooling and air-sea coupling are turned on separately to estimate the individual contribution of each effect to the cross-shelf SST variation. Experiments have also been carried out for different coupling strengths and diabatic cooling rates to examine the model sensitivity to these paramenters. The model results indicate that air-sea interaction could induce a secondary SST front on the shelf. Comparisons of model results with observations obtained during the Genesis of Atlantic Lows Experiment conducted off the east coasts of the Carolinas during January and February 1986 qualitatively confirm our finding.

Tidal dissipation and stratification in the Gulf of California

The effect of tidal mixing upon the distribution of stratification and sea surface temperature (SST) in the Gulf of California is studied by direct observation and by numerical modeling. Data from conductivity‐temperature‐depth surveys made in September and December of 1986 are used to describe the distribution of stratification and SST. A vertically integrated finite differences numerical model was used to obtain the geographical distribution of available turbulent kinetic energy (TKE) from the M2 tide. The following three areas were found to be strongly affected by tidal mixing: the shallow coastal area at the head of the gulf, the shelf south of Tiburón island, and the area of the midgulf archipelago. The first two are vertically well mixed and are therefore similar to other shelf sea frontal areas. Dissipation occurs all around the islands of the archipelago, and the sills are foci of the most intense tidal dissipation. Stratification in this area was reduced with respect to the background, but complete vertical mixing does not occur. Therefore the strong SST fronts that occur in this area do not mark the boundary between well‐mixed and stratified water. The Simpson‐Hujiter criterion (Simpson and Hunter, 1974) for stratification and frontal position produced 2.75 ≤ λcrit ≤ 3.0 for the frontal position in the shallow northern gulf. This value of λcrit is larger than in other shelf sea fronts, which may be due to horizontal advection, which is not included in the criterion. We found that the effect of advection may be of the same order of magnitude as that of buoyancy flux. In the area over the sills, λ &gt; λcriti the available TKE is not enough to produce complete mixing over the ∼ 400 m of water over the sills.

A barotropic model of the currents off SE South America

A depth‐averaged numerical model with 13‐km grid resolution was applied to the region defined by the 55°W meridian, the 56°S parallel and the southeast coast of South America. This region is characterized by high biological productivity, and by an exceptional tidal regime that interacts with the subpolar Falkland/Malvinas (FM) current. The results presented contribute to the development of an ecohydrodynamic system intended for the study of the implications of increasing exploitation of natural resources in the region. Detailed tidal charts show the propagation on the continental shelf of tidal waves related to clockwise amphidromes in the South Atlantic, indicating the occurrence of resonance. The averaged velocity field is dominated by the FM current, although tidal residuals around headlands are significant on the Patagonian coast. In contrast, “snapshots” describe the interaction between the tides and the FM current, where the flow patterns displayed are expected to influence the population dynamics of biological species. The mean wind stress induces strong currents and a large anticlockwise circulation cell northeast of Tierra del Fuego. The strong tidal and FM currents generate shelf sea fronts whose locations, revealed by contours of the Simpson‐Hunter parameter, are related to major fishing grounds. Because of the lack of velocity measurements in the region, the FM and wind‐driven currents simulated by the model should be taken as experimental results until proper comparisons with observations can be made.

Palaeotidal modelling of continental shelves: marine implications of a land-bridge in the Strait of Dover during the Holocene and Middle Pleistocene

Abstract Numerical models of the tide on the NW European continental shelf are useful in predicting basic tidal dynamics (amplitude, current, bed stress, mixing) from bathymetry, coastline configuration and a known ocean tide at the shelf edge. Using the M 2 constituent of the ocean tide, a numerical model has been used to investigate the tidal regime of the English Channel/southern North Sea at successive stages during the Holocene transgression, and also during the maximal sea-levels of Middle Pleistocene temperate stages, by incorporating palaeogeographical interpretations of geological data from these time-slices. Prior to the breaching of the Strait of Dover in the early Holocene, the Southern Bight of the North Sea was a quiet, shallow embayment of low M 2 tidal amplitude (&lt;0.5 m); following breaching it attained the present state of vigorous tidal action with mean amplitudes of around 2 m. Changes in sediment transport paths and shelf-sea fronts are also indicated. Models of the Middle Pleistocene interglacial English Channel incorporate modified palaeobathymetries to compensate for tectonic uplift in the central Channel area over the past 400 000 years. These models indicate increased seasonal stratification in the Channel compared with the present mixed condition. This is in agreement with independent evidence from coccolith assemblages contained within sediments of the same age from the central Channel area.

The Oceanographic and Biological Tidal Cycle Succession in Shallow Sea Fronts in the North Sea and the English Channel

Two major discrepancies between existing theories concerning the positioning and development of shallow shelf sea fronts and available data are that these models (1) do not adequately describe the neap-spring adjustment of frontal position and (2) these theories fail to explain the relatively small changes in the location of the front despite the rapid seasonal decay of the stabilizing surface heat flux.In the model presented here, these weaknesses in earlier attempts to describe frontal position are overcome. This is achieved partly by incorporating wind stirring in the heat and mixing balance for the frontal area and partly by allowing for a cross-frontal circulation. This new model defines two types of shallow sea fronts: tidal 'surface' and tidal 'bottom' fronts. Surface fronts are dominated by tidal and bottom fronts by wind stirring. The front type determines the circulation pattern on the stratified side of the front. The circulation pattern, in turn, governs the biological activity. The model describes the persistent but temporally varying redistribution of water from the bottom and surface layers to the thermocline layer which supplies nutrients from the deep (light limited) to the shallow regions where phytoplankton activity is stimulated. This new interpretation of the spring-neap adjustment also implies that, during the heating season, an increasing part of the potential energy to be overcome during a tidal cycle stems from the mixing of nutrient-rich dense bottom water and light surface water imported from offshore. The existence of this mechanism is supported by field observations.

Does the Earth's rotation influence the location of the shelf sea fronts?

Two alternative hypotheses which attempt to explain the position of the shelf sea fronts are (1) control by the energetics of tidal and wind mixing and (2) limitation of the penetration of tidal stresses by the effects of rotation. A turbulence‐closure (T/C) point model has been used to investigate the interaction of these two constraints and predict the seasonal cycle of stratification and frontal position for varying current polarization and water depths of h = 70 m and h = 100 m. For clockwise‐rotating tidal currents the frontal positions are in close accord with the previous prescriptive model, which is based only on the energy constraint, and with the observed positions of fronts at critical values of χ = log (h/〈u3〉) = 2.7 ± 0.4 (where h = depth and u = depth mean tidal current amplitude for M2, and angle brackets denote a time mean). For rectilinear tidal currents (equal clockwise and anticlockwise components), there is a small displacement (∼0.25) toward lower values of χ. When the currents are predominantly anticlockwise, the frontal positions predicted by the T/C model deviate significantly from the constant χ condition with a maximum discrepancy, when polarization P = +1, of Δχ = 0.86 and 1.56 at h = 70 and 100 m, respectively. This displacement is in the same sense as, but considerably less than, that predicted by the combined boundary layer theory. It may be detectable in the few areas of the shelf where P may reach values of up to 0.7 in depths of ∼100 m. For most regions of the European shelf, where −0.3 &lt; P &lt; 0.3, the model suggests that the influence of rotation will be small and control of frontal boundaries will rest primarily with the energy constraint.

The variability and forcing of currents within a frontal region off the northeast coast of England

During the summer of 1988 a collaborative experiment between the Ministry of Agriculture, Fisheries and Food (MAFF), the Proudman Oceanographic Laboratory (POL) and University College North Wales (UCNW) was undertaken to study the dynamics of a near-shore frontal region off the northeast coast of England. The experiment is one of the most intensive studies of the current structure of a shelf sea front undertaken. Currents were measured using an ocean surface current RADAR (OSCR), ship-borne ADCP, Lagrangian drifters and moored current meters. The current meter moorings held conventional meters at mid-depth and near the bed and S4 electromagnetic current meters near the surface. The mean, low frequency (<0.04cph) and tidal currents measured by the moored current meters will be discussed in this paper. A persistent southeastward along-shore mean flow was observed at the surface and mid-depth throughout the array. The results from simple correlation models and a multivariate spectral model suggest that the wind plays a secondary rôle to the cross-frontal density gradients in forcing this mean flow. The percentage of current variance coherent with the local wind stress varied from 50 to 60% at the surface to approximately 40% near the bed, except at a shallow inshore site where three-quarters of the variance was wind related. Inshore surface currents displayed a directionally dependent response to wind forcing while offshore surface currents demonstrated a directionally independent response. The offshore surface response resembles that predicted by classical Ekman theory but the insignificant change in angle between wind and current with depth, above the thermocline, suggests a secondary barotropic response is dominant. Below the thermocline a strong baroclinic response, forced by along-shore winds, is evident. The vertical structure of diurnal and semi-diurnal tides are qualitatively in agreement with the theory of Prandle (1982, Continental Shelf Research, 1, 191–207) with some deviations due to nearshore effects.

Dynamics of tidal mixing fronts in the North Sea

Twenty years since the discovery of tidal mixing fronts there are still few convincing observations of the velocity field associated with these structures. Simple models of shelf sea fronts predict strong along-front jets, weaker convergent circulations and instabilities. During the North Sea Project a series of studies of the Flamborough frontal system has used a new approach based upon novel combinations of modern instrumentation (HF radar, acoustic Doppler current profiler, Decca-Argos drifting buoys and towed undulating CTD) and have provided one of the first directly observed pictures of shelf sea frontal circulation. Observational confirmation of jetlike along-front flow has been found together with evidence of cross-frontal convergence. A new generation of eddy-resolving models will help to focus the next phase of frontal circulation studies in relation to questions concerning baroclinic instability and eddy generation.

Temperature distribution related to shelf-sea fronts on the Patagonian Shelf

Hydrographic data taken from the fishery research ship Dr. Holmberg and an infrared image from the NOAA-6 satellite show the location of shelf-sea fronts over the Patagonian Shelf off the coast of Argentina. High tidal dissipation rates in this area [ Simpson and Bowers (1981) Deep-Sea Research, 28, 727–738], intensified by headlands such as the Peninsula of Valdes, generate zones of enhanced vertical mixing in the near-shore waters, which lowers the sea-surface temperature. Shelf fronts separate this water from the seasonally stratified water offshore. The irregular sea-surface temperature pattern suggests the transfer of some of the cold mixed water into the upper layer of stratified water farther offshore, leading to the inference of a northeastward mean flow.

The Influence of Wind Stress and River Runoff on a Shelf-Sea Front

Runoff from land gives rise to the formation of a front-jet system separating fresher coastal waters from more saline waters offshore, over some continental shelves, notably in the Mid-Atlantic Bight in winter. Three first-order characteristics of such fronts are the anchor depth H, where the front intersects the bottom, the stretch l, i.e., the distance between its surface and bottom intersections, and the buoyancy contrast b across the front. The three characteristics may be determined from the cross-front property transfer rate, the along-front momentum balance, and the turbulence-energy dissipation rate. Key independent variables are the horizontal buoyancy transport Bx, the friction velocity u*, and direction θ of the wind, and the Coriolis parameters f. Simple approximate formulae connecting dependent to independent variables arewhere m, λ and μ are constants of order 300, 1 and 0.1 respectively. The entrainment rate across the front is μu2*f−1 cosθ, which is small compared to Ekman transport on some continental shelves, but possibly not on all.

A three-dimensional numerical shelf-sea front model with variable eddy viscosity and diffusivity

A three-dimensional numerical model of circulation and eddy development in shelf-sea fronts is applied to three frontal structures, with two parameterization schemes for vertical eddy viscosity and diffusivity. The three fronts resemble those in the German Bight (a front between relatively fresh coastal water and saltier water offshore, with an interface extending from surface to bottom), the Norwegian Coastal Current (also formed by fresh coastal water but with a thermocline on one side), and the Celtic Sea (a front between water which is stratified in summer and water which is well mixed throughout the year). The two mixing assumptions, modelling the reduction of turbulence in stratified zones, are based on the Munk-Anderson scheme and the turbulent energy equation. Many features of frontal dynamics are common to all the results: strong surface currents along the front, cross-frontal circulation cells, a considerable enhancement of vertical velocities when eddies are formed, and development of eddies into cyclonic-anticyclonic vortex pairs. Cross-frontal circulation and frontal sharpening are the variables most sensitive to the different mixing assumptions. The German Bight front is the one most affected by changing these assumptions. The comparisons suggest that realistic results may be obtained from models despite the present uncertainty about vertical mixing in stratified shelf seas.

The shelf-sea fronts: implications of their existence and behaviour

This paper reviews the results of recent observations of the shelf-sea fronts and discusses the relation of these features to w hat is known of the mean circulation. Evidence from satellite infrared imagery and ship data are presented to illustrate the movements and structure of the fronts. Many of the data can be rationalized in terms of models of tidal mixing based on vertical exchange only, with a feedback element, which is needed to account for the small observed adjustment to the springs-neaps stirring cycle. The density gradients within the front imply strong geostrophic currents along the isobars but the available evidence does not support the idea of a frontal jet. The complicated velocity field observed may be the result of large-scale (20—40 km) eddies that develop from instability of the flow. The relative success of local energy models implies that horizontal advection normal to the fronts is weak. A simple model of depth-uniform advection indicates that frontal positions and gradients should be sensitive to flows as small as 1 cm s -1 , and two examples of such influence are tentatively identified.