Spatial variability of the barotropic M2 constituent tidal current over the Rías Baixas Galician shelf (NW Spain)

Using current meters and systematically repeated vessel mounted ADCP data legs, we describe the role of the semidiurnal and diurnal constituents in the tidal currents and investigate the mesoscale variability of the M₂ tidal currents field along three cross-shelf legs over the Galician shelf. We found that the estimated values of the semi-major axes of the main semidiurnal tidal constituents are closely related to those obtained in previous research and that the tidal currents are predominantly semidiurnal. Amplitudes of the M₂ and S₂ semi-major axes constituents are comparable, and important fortnightly variability should thus be expected in the amplitude of the semidiurnal tidal currents. Vertical profiles of the semi-major axes of the main tidal constituents reveal that, in the absence of stratification, and far from the bottom frictional effects, the tidal currents are mostly barotropic. Over the slope, the tidal ellipses tilt from a north–south orientation toward the northeast–southwest direction and, as we go further into the shelf, their amplitude is increased, a feature that is reasonably explained by the transition from the dynamics of a Poincaré wave offshore to the dynamics of a northward alongshore propagating Kelvin wave over the shelf. The Ría de Vigo exerts a notable influence over the tidal currents, acting as an extension of the shelf and introducing a meridional variation in the orientation of the ellipses from the alongshore direction in the southern straight coast toward the northeast–southwest direction at the latitude of the Rías. Finally, the spatial fitting methods we tried have proved to be successful in VMADCP data detiding in this region.     

The interaction of tidal advection, diffusion and mussel filtration in a tidal channel

Time series measurements of flow and pigment concentrations (Chl) in the Menai Strait have revealed that the strong residual flow in a tidal channel ( 500 m³ s^− 1) transports phytoplankton from the open sea into the channel where much of it is consumed by suspension feeders, mainly in commercial beds of Mytilus edulis. The progressive depletion of phytoplankton along the channel results in a strong horizontal gradient of plankton and hence Chl. Tidal displacement of this gradient causes large (± 50% of mean) oscillations of Chl in the vicinity of the mussel beds. Vertical mixing by the strong tidal flows is sufficiently vigorous for most of the tidal cycle to ensure that downward diffusion can resupply the near-bed layer although there are indications of some transient depletion around slack water.This paradigm of the interaction of advection, diffusion and filtration determining the distribution of plankton and its supply to mussels has been encapsulated in a series of simple models forced only by boundary values. In the first, a 1-D model of tidal flow in the channel reproduces the principal features of the observed currents including the unusually large spatial change in phase of the currents and the variation of the residual transport with tidal range. The flow field from this physical model is used to drive a second model based on the advection diffusion equation for Chl with a source at the Irish Sea boundary and a sink over the mussel bed. This model illustrates the formation of a strong Chl gradient along the channel and simulates the amplitude and phase of the M₂ oscillations of Chl and the development of the M₄ variation apparent in the observations. This second model has been extended to 2-D over the mussel beds to allow investigation of the effects of water column mixing. The model indicates that only for a short period ( 30 min), close to slack water, is mixing sufficiently reduced to permit the development of a depletion boundary layer and then only within  1 m from the bottom, a result which is consistent with the observations.     

The Gulf Stream, rings and North Atlantic eddy structures from remote sensing (Altimeter and SeaWiFS)

Seasonal SeaWiFS chlorophyll a concentrations cycles and annual changes of altimeter Sea Level Anomaly are derived for the subtropical North Atlantic near  35°N and along a Gulf Stream axis. Spatial structure of SeaWiFS, is defined in terms of deviations from a local seasonal cycle and examined in relation to altimeter eddy structure. In the subtropical region near 35°N, SeaWiFS structure is evident during the spring bloom period with a scale of  430 km, or about twice the eddy scale. A Gulf Stream axis has been selected as a region where the Sea Level Anomaly variance is a maximum. Eddy propagation speeds and scales are examined. Cold-core (cyclonic) rings correspond to areas of high SeaWiFS chlorophyll a. Warm-core (anticyclonic) rings relate to areas of low chlorophyll concentration. Both SeaWiFS structure and eddy structure have a spatial scale of  450 km or twice the ring scale along the Gulf Stream axis. SeaWiFS chlorophyll anomalies and Altimeter Sea Level Anomaly structure have an overall negative correlation coefficient of r = − 0.34. Swirl currents between eddies redistribute surface chlorophyll concentrations and can spatially bias maximum and minimum concentration levels off eddy centre.     

蓬莱海域的潮汐潮流特征分析

基于2007年10月的水文测验资料,分析了蓬莱海域沿岸东西两侧在不同潮波系统控制下的潮汐潮流变化特性.分析结果表明:蓬莱东岸属正规半日潮,蓬莱西岸属不正规半日混合潮,潮流的性质与潮汐不同,东、西两岸的潮流都为不正规半日混合潮流.潮波自北黄海经渤海口门以接近前进波的形态向渤海内传播.     

白令海潮汐能通量和底边界能耗散

The spatial distribution of the energy flux, bottom boundary layer (BBL) energy dissipation, surface elevation amplitude and current magnitude of the major semidiurnal tidal constituents in the Bering Sea are examined in detail. These distributions are obtained from the results of a three-dimensional numerical simulation model (POM). Compared with observation data from seven stations, the root mean square errors of tidal height are 2.6 cm and 1.2 cm for M₂ and N₂ respectively, and those of phase-lag are 21.8° and 15.8° respectively. The majority of the tidal energy flux off the deep basin is along the shelf edge, although some of this flux crosses the shelf edge, especially in the southeast of the shelf break. The total M₂ energy dissipation in the Bering Sea is 30.43 GW, which is about 10 times of that of N₂ and S₂. The semidiurnal tidal energy enters mainly to the Bering Sea by Samalga Pass, Amukta Pass and Seguam Pass, accounting more than 60% of the total energy entering the Being Sea from the Pacific.     

Turbulent mixing and internal tides in Gaoping (Kaoping) Submarine Canyon, Taiwan

Turbulent overturning on scales greater than 10 m is observed near the bottom and in mid-depth layers within the Gaoping (formerly spelled Kaoping) Submarine Canyon (KPSC) in southern Taiwan. Bursts of strong turbulence coexist with bursts of strong sediment concentrations in mid-depth layers. The turbulence kinetic energy dissipation rate in some turbulence bursts exceeds 10^− 4 W kg^− 1, and the eddy diffusivity exceeds 10^− 1 m² s^− 1. Within the canyon, the depth averaged turbulence kinetic energy dissipation rate is ~ 7 × 10^− 6 W kg^− 1, and the depth averaged eddy diffusivity is ~ 10^− 2 m² s^− 1. These are more than two orders of magnitude greater than typical values in the open ocean, and are much larger than those found in the Monterey Canyon where the strong turbulent mixing has also been. The interaction of tidal currents with the complex topography in Gaoping Submarine Canyon is presumably responsible for the observed turbulent overturning via shear instability and the breaking of internal tides and internal waves at critical frequencies. Strong 1st-mode internal tides exist in KPSC. The depth averaged internal tidal energy near the canyon mouth is ~ 0.17 m² s^− 2. The depth integrated internal tidal energy flux at the mouth of the canyon is ~ 14 kW m^− 1, propagating along the axis of the canyon toward the canyon head. The internal tidal energy flux in the canyon is 3–7 times greater than that found in Monterey Canyon, presumably due to the more than 10 times larger barotropic tide in the canyon. Simple energy budget calculations conclude that internal tides alone may provide energy sufficient to explain the turbulent mixing estimated within the canyon. Further experiments are needed in order to quantify the seasonal and geographical distributions of internal tides in Gaoping Submarine Canyon and their effects on the sediment flux in the canyon.     

Relationship between phytoplankton bloom and wind stress in the sub-polar frontal area of the Japan/East Sea

The seasonal dynamics of phytoplankton blooms in the central Japan/East Sea (JES) show pronounced year-to-year variability based on Sea-viewing Wide Field-of-view Sensor (SeaWiFS; 19972003) and Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra (20002003) observations. Wind seems to strongly influence this variability. To study the relationship between wind and bloom initiation, we analyzed daily, remotely sensed wind stress data (Active Microwave Instrument–wind [AMI–wind], NASA Scatterometer [NSCAT], and Quick Scatterometer [QuickSCAT]: 19972003) and daily chlorophyll concentrations based on ocean color data (SeaWiFS and MODIS). The results agreed well with the hypotheses; in spring, blooms began 615 days after wind stress weakened. Fall blooms started 39 days after wind strengthened. We also simulated seasonal changes using a simple light–nutrient model using two values for the respiration ratio: 10% and 20%. The use of 20% seemed to reproduce the timing of the spring bloom quite well but underestimated the absolute level of chlorophyll concentration. On the other hand, using 10% produced a better estimation of the chlorophyll concentration but failed to match the timing. Neither of the model runs reproduced the timing of the fall bloom well.     

Temporal variations of water flow between the Venetian lagoon and the open sea

Long-term measurements of the water flow at three Venice Lagoon inlets with the bottom-mounted ADCPs show that the main part of the variance (>90%) is associated with the tidal variability. Semi-diurnal constituents (mainly M2 and S2) are responsible for about 80% of the flow variance. Phase-lag between the axial current and sea-level is on the order of 2 h for M2 and 4 h for the K1, the maximum inflow leading the sea-level maximum. Phase-difference of tidal flows between inlets shows that Chioggia leads both Malamocco and Lido, suggesting that tidal signal progresses northward, thus in the opposite direction of both the semi-diurnal and diurnal tidal signals in the open Adriatic currents. The sea-level slope between the open sea and the lagoon interior controls the inlet flow, which is due to the time lag being constant for all tidal constituents. It was shown that tidal oscillations at Punta Salute (lagoon interior) lag those in Lido by about 45 min. The pressure gradient due to the sea-level slope generates the flow acceleration. Only for large current speeds (>0.5 m/s), the bottom friction term becomes equally important as the local acceleration and the horizontal pressure gradient terms. Wind effects manifest as a remote forcing through Adriatic seiches at semi-diurnal and diurnal scales, and as a local forcing at very long time scales on the order of a month. This latter mechanism is limited to a winter period (November–January). Seiches are present over the entire year, being however, more energetic and frequent during autumn and winter.     

大连太平湾海域潮位特征分析

根据太平湾海域实测潮位资料,对该海域的潮汐性质、潮位特征值、理论最低潮面、平均海平面、设计高水位、设计低水位等进行了分析计算。结果表明:太平湾海区的潮汐属不正规半日混合潮性质,日潮不等现象明显;平均海平面在国家85基面下0.05 m,理论最低潮面在国家85基面下1.70 m;设计高水位1.62 m,设计低水位-1.36 m(国家85基面)。     

Internal tidal currents in the Gaoping (Kaoping) Submarine Canyon

Data from five separate field experiments during 2000–2006 were used to study the internal tidal flow patterns in the Gaoping (formerly spelled Kaoping) Submarine Canyon. The internal tides are large with maximum interface displacements of about 200 m and maximum velocities of over 100cm/s. They are characterized by a first-mode velocity and density structure with zero crossing at about 100 m depth. In the lower layer, the currents increase with increasing depth. The density interface and the along-channel velocity are approximately 90° out-of-phase, suggesting a predominant standing wave pattern. However, partial reflection is indicated as there is a consistent phase advance between sea level and density interface along the canyon axis.     

Pollution history of a tropical estuary revealed by combined hydrodynamic modelling and sediment geochemistry

Hydrodynamic modelling of water movement in Hunts Bay, a protected part of Kingston Harbour, Jamaica, shows that depth averaged tidal flows are very low. In the northeast corner of Hunts Bay, water is essentially stagnant. Even under high flow conditions, much of the Bay bottom water is ‘bypassed' by buoyant, lower salinity surface flows. The muddy sediments of Hunts Bay reflect these sluggish to stagnant conditions; sediment cores from the northeast corner of the Bay contain progressively higher amounts of organic matter in their upper parts (last 15–20 years sedimentation). Combined C/N ratios and stable carbon isotope compositions of this organic matter imply a sewage origin. Both lead and chromium metal concentrations and enrichment factors relative to average crustal shales show geographically related patterns that reflect hydrodynamic circulation predicted by modelling. In particular, metal concentrations and enrichment factors are highest at the northern end of the bay, especially in the northeast corner. Modelling confirms that stagnant conditions would occur in the northeast part of the bay even without the presence of a major causeway. The causeway may contribute to low flow conditions, but is not the principal cause of organic contamination, which is simply an excessive input of sewage.     

On the residual advection of passive constituents

In this paper, the different approaches used to simulate the long-term advection of passive constituents on tidal shelves are discussed in the framework of large scale hydrodynamic modelling. The direct approach in which the unsteady mesoscale (tides and storm surges) and macroscale (monthly or seasonal mean processes) currents are used is very demanding in computer resources (CPU and memory). On the other hand, the use of residual velocities gives a larger understanding of the long-term transport processes and simplifies numerical treatments. However, the definition of such appropriate velocities is still an open question. In the context of large scale models, Lagrangian residuals are not applicable. Eulerian residual transport velocities fail to represent long-term motions when tidal non-linearities are important. The first order Lagrangian residual velocity introduced by Feng et al. (1986) and generalized here is shown to be a very good solution. The North-Western European Continental Shelf (NWECS) is used as an example to compare the numerical solutions obtained with the different approaches.     

石浦港下湾门航道工程潮流数值模拟研究

依据最新的水文实测资料,对石浦港的潮汐潮流特征进行了分析;并建立了基于不规则三角形网格的二维潮流数学模型,根据实测资料对模型进行了验证。在验证的基础上,对下湾门5万t级航道工程(开挖及炸礁通航)实施前后的流场进行了数值模拟计算,根据计算结果对工程前后的潮流特征及其变化进行了分析。研究结果表明:(1)工程海区潮汐属于正规半日潮类型,潮流性质属正规半日潮流型,潮流运动呈往复流形式;(2)岸线走向与涨落潮流走向基本一致;(3)航道总体挖深较小,流速变化很小,并未改变大范围海域潮流运动整体特征;开挖段航道流速呈减小的趋势,深槽水域流速呈增加趋势;(4)炸礁使得口门处航道轴线水域的流场趋于平顺,流向与航道走向基本一致,对周边流场基本没有影响;(5)方案实施后,对潮位变化无影响;下湾门口门处潮量增加,而其他口门潮量降低;(6)航道转弯位置、进港口门以及外海航道处横流较强,其他区段横流较弱。     

A trace metal (Pb, Cd, Zn, Cu) balance for surface waters in the eastern Gotland Basin, Baltic Sea

Trace metal inventories and transboundary fluxes were estimated for the surface waters of the eastern Gotland Basin, and a mass balance for this water body was derived.The study area was delineated by a box which bordered vertically by the sea surface and the halocline and horizontally by the coast of Gotland and the coast of Latvia/Lithuania. For the calculation of the trace metal inventories in the box, a high-resolution sampling was carried out during different seasons. Additional sediment trap studies were performed to calculate the vertical particulate trace metal fluxes through the halocline.The following transboundary fluxes have been estimated: (a) atmospheric input of trace metals; (b) lateral transport from adjacent bodies of surface water; (c) diapycnal fluxes of dissolved trace metals at the halocline; (d) sinking of trace metals, associated with particles, through the halocline, (e) vertical advection through the halocline.The lateral transport into and out of the surface box is important for the metals (Cd, Cu, Zn) with “nutrient-like” behaviour. For particle reactive elements like lead, vertical sedimentation and lateral transport out of the box as much as atmospheric input and lateral input into the box are in the same order of magnitude. Turbulent diapycnal mixing plays a minor role compared to the advective fluxes.The estimated range for the residence times for the region under investigation are  0.5–1.3 years for Pb;  2–11 years for Zn,  7–36 years for Cd and 42–89 years for Cu. This demonstrates that the system reacts very fast for particle reactive elements like Pb, while for copper sedimentation processes are not the preferential sink and can be neglected.     

Influence of local and external processes on the annual nitrogen cycle and primary productivity on Georges Bank: A 3-D biological–physical modeling study

Georges Bank is one of the world's most highly productive marine areas, but the mechanisms of nutrient supply to support such high productivity remain poorly understood. Intrusions of nutrient-poor Labrador Slope Water (LSW) into the Gulf of Maine (NAO-dependent) potentially can reduce nutrient delivery to the bank, but this mechanism has not been quantitatively examined. In this paper, we present the first whole-year continuous model simulation results using a biological–physical model developed for the Gulf of Maine/Georges Bank region. This high-resolution three-dimensional coupled model consists of the Finite Volume Coastal Ocean Model (FVCOM) and a Nitrogen–Phytoplankton–Zooplankton–Detritus (NPZD) model, and was used to examine the influences of local and external processes on nitrogen and phytoplankton dynamics on Georges Bank. The model captured the general pattern of spatial-temporal distributions of nitrogen and phytoplankton and provided a diagnostic analysis of different processes that control nitrogen fluxes on Georges Bank. Specifically, numerical experiments were conducted to examine seasonal variation in nitrogen transport into the central bank (new nitrogen supply) versus nitrogen regenerated internally in this region. Compared with previous observation-based studies, the model provided a quantitative estimate of nitrogen flux by integrating the transport over a longer time period and a complete spatial domain. The results suggest that, during summer months, internal nitrogen regeneration is the major nitrogen source for primary production on the central bank, while nitrogen supply through physical transport (e.g. tidal pumping) contributes about 1/5 of the total nitrogen demand, with an estimated on-bank nitrogen transport at least 50% less than previous estimates. By comparing the model runs using different nitrogen concentrations in deep Slope Water, the potential influence of NAO-dependent intrusions of LSW was examined. The results suggest that the change of nitrogen concentration in the deep Slope Water may not have a significant impact on nitrogen and phytoplankton dynamics on the well-mixed central bank, largely due to limited nutrient exchange across the tidal mixing front and enhanced near-frontal nutrient uptake. However, relatively more significant impact was observed in the model simulations if both well-mixed and seasonally-stratified areas (inside 100 m isobath of the bank) were considered in flux calculations.     

Quasi-simultaneous observation of currents,salinity and nutrients in the Changjiang (Yangtze River) plume on the tidal timescale

During both the spring- and the neap-tide periods of November 2005, quasi-simultaneous observations were carried out by six boats over 26 h at 12 stations in the Changjiang (Yangtze River) plume. The simultaneous observations provided the actual distribution isopleths of salinity and nutrients that displayed considerable intra-tidal variations at surface, especially in the southeastern section of the study area. The lack of synopticity in sampling might lead to large discrepancies of the interpolated contours of salinity from the actual distribution isopleths. No clear flood-ebb asymmetry of salinity stratification was observed; whereas at inner stations, surface-to-bottom bulk velocity difference (current layering) always tended to be greater during the ebb fraction of a semidiurnal cycle. At a given station, the weaker neap tides commonly induced stronger salinity stratification, less intra-tidal variability of salinity and nutrients, and less intrusion of bottom saltwater. Nutrients (SiO₃^2?, NO₃^?, and PO₄^3?) showed more nonconservative behaviors during the neap tides, presumably as a result of the prolonged residence time of seawater and decreased suspended particulate matter levels than during the spring tides.     

Exploring relationships between recruitment of European hake (Merluccius merluccius L. 1758) and environmental factors in the Ligurian Sea and the Strait of Sicily (Central Mediterranean)

This paper explores the relationships between the spatial patterns of the distribution of the young hakes of the year (YOY) and the oceanographical features in two areas of the Central Mediterranean (the Ligurian Sea and the Strait of Sicily), characterised by the occurrence of straits and channels. Comparative and correlative approaches were used to investigate coupling between biological and physical patterns. Density indices of the YOY were derived from annual trawl surveys from 1994 to 2004 in spring and autumn. Mean patterns of the YOY distributions were compared with the mesoscale oceanographical features reported in literature. No evident trends in recruitment strength were found in either areas. Inter-annual variability in YOY abundance in the Ligurian Sea was higher than in the Strait of Sicily. The location of nursery grounds in the study areas coincides with zones of relatively higher production, where upwelling and other enrichment processes regularly occur. The presence of predictable eddies and the frontal systems play a major role in the localization of nursery areas in the Strait of Sicily, maintaining their stable position throughout the years. The strongest transport of southern waters from the Tyrrhenian to the Ligurian Sea, due to the East Corsica Current, which is negatively correlated to winter North Atlantic Oscillation, is associated with the highest abundance of hake recruits in the nurseries of the Northern Ligurian Sea.     

Assimilation of sea surface temperature in the POL Coastal Ocean Modelling System

A one-dimensional scheme is used to assimilate satellite Sea Surface Temperature data into the Proudman Oceanographic Laboratory Coastal Ocean Modelling System, set up in the Irish Sea with a fine resolution ( 1.8 km). The capabilities of the assimilation scheme are investigated using two different sets of satellite data, of lower and similar resolution to that of the model respectively. Comparison of results with independent data show that assimilation improves the modelled Sea Surface Temperature, but does not address model representation of the temperature vertical structure. It is concluded that for the Irish Sea and at the scales resolved by the model, the assimilation problem cannot be approached in a one-dimensional framework. It is also pointed out that forecast error needs to account explicitly for errors in the representation of the vertical structure of the thermal field.Three-dimensional methods that are suited for coastal systems are then suggested.     

A 3D, finite element model for baroclinic circulation on the Vancouver Island continental shelf

This paper describes the development and application of a 3-dimensional model of the barotropic and baroclinic circulation on the continental shelf west of Vancouver Island, Canada. A previous study with a 2D barotropic model and field data revealed that several tidal constituents have a significant baroclinic component (the K1 in particular). Thus we embarked on another study with a 3D model to study the baroclinic effects on the residual and several selected tidal constituents.The 3D model uses a harmonic expansion in time and a finite element discretization in space. All nonlinear terms are retained, including quadratic bottom stress, advection and wave transport (continuity nonlinearity). The equations are solved as a global and a local problem, where the global problem is the solution of the wave equation formulation of the shallow water equations, and the local problem is the solution of the momentum equation for the vertical velocity profile. These equations are coupled to the advection-diffusion equation for density so that density gradient forcing is included in the momentum equations. However, the study presented here describes diagnostic calculations for the baroclinic residual circulation only.The model is sufficiently efficient that it encourages sensitivity testing with a large number of model runs. In this sense, the model is akin to an extension of analytical solutions to the domain of irregular geometry and bottom topography where this parameter space can be explored in some detail.In particular, the consequences of the sigma coordinate system used by the model are explored. Test cases using an idealized representation of the continental shelf, shelf break and shelf slope, lead to an estimation of the velocity errors caused by interpolation errors inherent in the sigma coordinate system. On the basis of these estimates, the computational grid used in the 2D model is found to have inadequate resolution. Thus a new grid is generated with increased accuracy in the region of the shelf break. However, even with increased resolution, spurious baroclinic circulation seaward of the shelf break and in the vicinity of Juan de Fuca canyon remained a significant problem when the pressure gradient terms were evaluated using the σ coordinate system and using a realistic density profile.With the new grid, diagnostic calculations of the barotropic and baroclinic residual circulation are performed using forcing from the observed σ_t (density) field and from the gradient of this field.     

Circulation, hydrographic and nutrient characteristics of the Cilician Basin, Northeastern Mediterranean Sea

The water mass, circulation and chemical properties of the Cilician Basin, the northeastern Levantine Sea, are described on the basis of three hydrographic cruises performed during May 1997 (spring), July 1998 (summer) and October 2003 (autumn). The hydrographic data reveal the presence of Levantine Surface Water (LSW) and Modified Atlantic Water (MAW) within the upper 90 m layer, Levantine Intermediate Water (LIW) between 90 and 250 m, and Transitional Mediterranean Water (TMW) further below. The temporal variability of the circulation system is manifested by a change in shape, size and intensity of eddies as well as the pathways of the Lattakia Basin coastal current system. The nutrient concentrations varied between nitrate + nitrite = 0.16–0.31 μM, phosphate = 0.02–0.03 μM and silicate = 0.95–1.2 μM for the surface layer during sampling periods. Dissolved nutrient concentrations in the Transitional Mediterranean Water were: 2.1–5.3 μM for NO₃ + NO₂, 0.10–0.21 μM for PO₄ and 5.7–10 μM for Si. The molar ratios of nitrate to phosphate in the water column range between 5 and 20 in the surface layer and reach up to a value of 45 at the top of the nutricline at the depths of 29.05 kg/m³ isopycnal surface for most of the year. Below the nutricline the N / P ratios retain the values around 24–28.