Variability of mesozooplankton spatial distribution in the North Aegean Sea, as influenced by the Black Sea waters outflow

The North Aegean Sea constitutes an important region of the Mediterranean Sea since in its eastern part the mesotrophic, low salinity and relatively cold water from the Black Sea (outflowing from the Dardanelles strait) meets the oligotrophic, warm and very saline water of Levantine origin, thus forming a thermohaline front. Mesozooplankton samples were collected at discrete layers according to the hydrology of the upper 100 m, during May 1997 and September 1998. In May highest biomass and abundance values (up to 66.82 mg m^− 3 and 14,157 ind m^− 3) were detected in the 10–20 m layer (within the halocline) of the stations positioned close to the Dardanelles strait. The front moved slightly southwards in September, characterized by high biomass and abundance values within the halocline layer. The areas moderately or non influenced by Black Sea water revealed lower standing stock values than the frontal area in both cruises and maxima were detected in the uppermost low salinity layer. Samples collected at the stations and/or layers more influenced by Black Sea water were distinguished from those collected at layers and/or stations more affected by Levantine waters in both periods. In May the former samples were characterized by the copepods Acartia clausi, Centropages typicus, Paracalanus parvus. The abundance of the above species decreased gradually with increasing salinity, in the horizontal and/or in the vertical dimension, with a parallel increase of the copepods Oithona plumifera, Oithona copepodites, Oncaea media, Ctenocalanus vanus, Farranula rostrata. During September the frontal area as well as that covered by the modified Black Sea water, were highly dominated by the cladoceran Penilia avirostris and doliolids. For both seasons, MDS plots, issued from the combination of mesozooplankton and water-type data, revealed the gradual differentiation of zooplankton composition from the frontal area towards the area covered by Levantine water, following the spreading and mixing of the Black sea water. The observed temporal and spatial variability in the distribution pattern of mesozooplankton standing stock and species composition seems to depend considerably on the variability of circulation and frontal flows.     

Numerical simulations of seasonal and interannual variability of the Black Sea thermohaline circulation

The Black Sea general circulation is simulated by a primitive equation model with active free surface. The forcing is seasonally variable and is based on available climatic data. The model reproduces the main features of the Black Sea circulation, including the river discharge effects on the mean sea level and the Bosphorus outflow. Model results show that the simulated sea surface elevation increases in spring over the whole sea, reaching a maximum in the Danube delta area. In the same region, a minimum is observed in winter. The amplitude of the seasonal oscillations (about 8–12 cm over the whole basin) is of the same order of magnitude as the maximum horizontal variations (about 15–18 cm between the coastal areas and the basin interior). This strong signal formed mostly by river discharges, along with the seasonal variability in the other forcing functions and the local dynamics creates a well-pronounced interannual variability. The performance of the model in simulating the seasonal and interannual variability is critically analyzed, with a special attention on the cold intermediate water formation and the circulation in the upper 150 m. The simulations demonstrate that the source of intermediate waters is on the shelf, and that the water mass in the core of cold intermediate layer changes with time as a response to the periodic forcing at sea surface. This type of variability is characterized by pronounced interannual changes, proving that important differences could exist between water mass structure in different years, even when using identical atmospheric forcings each year.     

Distribution of phytoplankton in the southern Black Sea in summer 1996, spring and autumn 1998

The species composition, abundance, and biomass of micro- (>15 μm) and nano- (<15 μm) phytoplankton were studied along the southern Black Sea during June–July 1996 and March–April and September 1998. A total of 150 species were identified, 50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77×10³ cells l⁻¹ in spring to 110×10³ cells l⁻¹ in autumn and biomass from 250 μg l⁻¹ in summer to 1370 μg l⁻¹ in spring. Based on the extensive sampling grid from June–July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin.     

Response of the Black Sea mean level to atmospheric pressure and wind forcing

The response of the Black Sea mean level to atmospheric pressure (AP) and wind forcing is investigated using 5 years of TOPEX/POSEIDON (T/P hereafter) data. A coherence analysis is first applied to mean sea level and pressure to examine the validity of the inverse barometer (IB) approximation over this area. As expected, it reveals very significant deviations from an IB response attributed to the narrowness of the Bosphorus Strait and its limiting role in water exchanges. A comparison is drawn with the Mediterranean Sea case. A single basin version of the Candela analytical model [Candela, J., 1991. The Gibraltar Strait and its role in the dynamics of the Mediterranean Sea. Dyn. Atmos. Oceans 15, 267–300], which takes linear friction at the strait into account, is then used. The model explains a significant part of the T/P mean sea level variance (about 30%, while the IB correction only explains 5% of its variance) and provides a means to correct the altimeter data for the pressure effect much better than the standard IB effect. The response of the mean sea level to wind forcing is then analysed. Coherence analysis between sea level and along-strait wind stress (WS) reveals a significant coherence at periods ranging from 40 to 100 days, with an almost steady phase of 270°. This result is confirmed with a multiple coherence analysis (mean sea level vs. WS and AP). A plausible mechanism is a piling-up of water at the northern or southern end of the strait due to along-strait wind forcing. The associated along-strait pressure gradient would modify the barotropic flow in the strait and then the mean sea level. Using an extension of the Candela model, we show that this mechanism is consistent with T/P mean sea level observations.     

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.     

Pigments, size and distribution of Synechococcus spp. in the Black Sea

Pigments, size and distribution of Phycoerythrin-containing unicellular cyanobacteria Synechococcus spp. within the euphotic zone were studied for the first time in April–May 1994 in the western and southwestern Black Sea by epifluorescence microscopy and flow-cytometry. Synechococcus was present in varying quantities at every station and depth studied. Surface spatial distribution of Synechococcus revealed that cells were much more abundant in offshore waters than near coastal regions under the direct influence of the Danube river. Minimum and maximum cell concentrations ranged between 9×10² and 1.45×10⁵ cells/ml at the surface, between 2×10³ and 1.23×10⁵ cells/ml at the chlorophyll sub-maximum layer, and between 1.3×10² and 3.5×10² at the nitrite maximum layer. Cells at the chlorophyll sub-maximum layer (based on in-situ fluorometer readings) fluoresce brighter and longer than the ones at the surface and lower depths. Spectral properties of chromophore pigment types of total 64 clonal isolates from different depths down to the lower layer of the euphotic zone (60 m) in the southern Black Sea coast revealed that all have type 2 phycoerythrobilin in common, lacking in phycourobilin. In vivo fluorescence emission maxima for the phycoerythrobilin were about the same (578 nm) for all isolates. All isolates examined showed in vivo absorption maxima at between 435 and 442 nm and at about 681 nm due to chlorophyll-a. Based on the flow cytometer mean forward light scatter data for size distribution, it could be concluded that cells at the surface mixed layer (0–10 m) were larger in cell size than the cells at lower depths (20–60 m).     

Preoperational ocean forecasting in the southeastern Mediterranean Sea: Implementation and evaluation of the models and selection of the atmospheric forcing

Within the framework of several local and international programs, a quasi-operational ocean-forecasting system for the Southeastern Mediterranean Sea has been established and evaluated through a series of preoperational tests. The Princeton Ocean Model (POM) is used for simulating and predicting the hydrodynamics while the Wave Model (WAM) is used for predicting surface waves. Both models were set up to allow varying resolution and multiple nesting. In addition, POM was set up to be easily relocatable to allow rapid deployment of the model for any region of interest within the Mediterranean Sea. A common requirement for both models is the need for atmospheric forcing. Both models require time varying wind or wind stress. In addition, the hydrodynamic model requires initial conditions as well as time dependent surface heat fluxes, fresh water flux, and lateral boundary conditions at the open boundaries. Several sources of atmospheric forcing have been assessed based on their availability and their impact on the quality of the ocean models' forecasts. The various sources include operational forecast centers, other research centers, as well as running an in-house regional atmospheric model. For surface waves, higher spatial and temporal resolution of the winds plays a central role in improving the forecasts in terms of significant wave height and the timing of various high wave events. For the hydrodynamics, using the predicted wind stress and heat fluxes directly from an atmospheric model can potentially produce short range ocean forecasts that are nearly as good as hindcasts forced with gridded atmospheric analyses. Finally, a high-resolution, nested version of the model has shown to be stable under a variety of forcing conditions and time scales, thus indicating the robustness of the selected nesting strategy. For the southeastern corner of the Mediterranean, at forecast lead times of up to 4 days the high-resolution model shows improved skill over the coarser resolution driving model when compared to satellite derived sea surface temperatures. Most of the error appears to be due to the analysis error inherent in the initial conditions.     

Seasonal variability of the Black Sea chlorophyll-a concentration

The optimal spectral decomposition (OSD) method is used to reconstruct seasonal variability of the Black Sea horizontally averaged chlorophyll-a concentration from data collected during the NATO SfP-971818 Black Sea Project in 1980–1995. During the reconstruction, quality control is conducted to reduce errors caused by measurement accuracy, sampling strategy, and irregular data distribution in space and time. A bi-modal structure with winter/spring (February–March) and fall (September–October) blooms is uniquely detected and accurately documented. The chlorophyll-a enriched zone rises to 15 m depth in winter and June, and deepens to 40 m in April and 35 m in August. The June rise of the chlorophyll-a enriched zone is accompanying by near-continuous reduction of upper layer maximum chlorophyll-a concentration.     

弹性钢索中纵波与弯折波的耦合作用研究

针对目前研究弹性钢索中应力波传播规律时忽略纵波与弯折波相互作用影响的问题,文章从理论研究的角度介绍了纵波与弯折波相互耦合作用的机理和计算方法,并采用该方法进行了弹性钢索四次冲击应力的分析计算。结果显示,计算结果与文献中试验观察现象一致,从而验证了该计算方法的有效性。     

Spectral signature of wave breaking in surface wave components of intermediate-length scale

This paper investigates the length scale of ocean surface breaking waves in the spectral range of intermediate wavelength components a few centimeters to a few meters long. The spectral properties of wave breaking are examined first with the dissipation function of the wave action density conservation equation. The analysis reveals a strong breaking signature in wave components between 0.15 and 1.5 m long in the form of a quasi-singular behavior of the dissipation function using the present formulation of the wind-generation and breaking dissipation functions. Independent studies of more-direct breaking observations of radar tracking of sea spikes in the past have shown close correlation between sea spikes and scatterers traveling at the speed of surface waves a few meters long and much shorter than the dominant wavelength. This feature of sea-spike properties is consistent with the breaking signature of the dissipation function in similar wavelengths. The intermediate-scale waves are the primary contributor of the ocean surface mean-square slope. The close correlation between the gas transfer rate and the mean-square slope has been demonstrated repeatedly. A better understanding of the wave dynamics of intermediate-scale waves is important for clarification of various gas transfer mechanisms.     

波流联合作用下振荡浮子式波浪发电机动力学分析

参考某型振荡浮子式波浪发电装置的具体参数,结合该装置工作过程,使用水动力分析软件OrcaFlex建立了在不同海况下装置工作过程中的动力学分析简化模型,分析了流、波以及波、流联合作用对振荡浮子式波浪发电装置张力、弯矩沿缆长方向的变化规律以及装置能量转换效率的影响。结论对海洋可再生能源研究有一定的参考价值。     

Second-order wave run-up on a vertical cylinder adjacent to a plane wall based on the application of quadratic transfer function in bi-directional waves

A numerical model was developed in this study to simulate the wave diffraction caused by an arbitrarily shaped structure in the presence of bi-directional incident waves based on a higher-order boundary element method (HOBEM). Based on the developed model, the wave elevation quadratic transfer function (QTF) in bi-directional waves, which is defined as the second-order wave elevation caused by two incident waves of unit amplitude from two directions, can be determined. The developed model was subsequently used to investigate the wave interaction with a cylinder situated near a vertical wall. The image principle was applied to transform the original problem into an equivalent one of wave diffraction caused by two symmetrical cylinders in open seas exposed to bi-directional incident waves. The second-order wave run-up on the cylinder can then be determined using the wave elevation QTF obtained from an analysis of the equivalent problem. A detailed numerical analysis was then conducted. Numerical results revealed that the presence of the vertical wall can apparently disturb the wave diffraction process from the cylinder, and lead to significantly amplified second-order wave elevation within the region between the wall and cylinder. In addition, the respective contribution from the first- and second-order components to the overall wave elevation around the cylinder was discussed.     

波浪能在航船上的应用分析

为节约能源损耗,推进对海洋中波浪能的充分利用,概述了波浪能利用技术在船舶上的应用,分析现有波浪能装置在趸船或海洋平台上的应用,提出解决航行船舶利用波浪能的难题的总体思路,总结船舶使用波浪能利用技术的发展趋向;将波浪能发电装置应用于航行中的船舶,可以有效降低船舶航行时的碳排放,达到船舶节能减排的目的,实现绿色航运.     

Hydrodynamic modelling of a multi-body wave energy converter using the Moving Frame Method

Significant wave energy conversion may be achieved with multiple floats with several modes of motion combining constructively to generate power. With the M4 system, power take off is at hinges to absorb resulting pitch motions. Complexity increases with number of floats and conventional vectorial multi-body dynamics may be conveniently replaced by the recently formulated moving frame method based on the calculus of variations to avoid free-body diagrams and joint reaction forces. Furthermore by utilising group theory to gather spatial rotations and angular velocities in a common structure, the obtained notation is general and straight-forward to apply for single and multi-body systems. This paper lays the foundations for incorporating hydrodynamic forces into the moving frame method. The method is applied to 3, 6 and 8 float cases in regular and irregular waves with results compared to the vectorial method and experimental measurements, showing close agreement. It is suggested that this is a more natural and general approach for complex multi-body, multi-hinge hydrodynamics systems.     

离岸式波浪能发电装置锚泊系统研究进展

基于国内外关于波浪能发电装置的锚泊系统研究现状,介绍了锚泊系统的组成,结构形式的分类以及相关的研究方法,并分析各自的优缺点,综合国内外关于波浪能发电装置的锚泊系统的实例,丰富了锚泊系统的设计基础知识,有利于掌握国内外关于波浪能发电装置的锚泊系统方面的最新研究动态,形成科学的系统的设计方案。     

Surface chlorophyll in the Black Sea over 1978–1986 derived from satellite and in situ data

Absolute values of chlorophyll a concentration and its spatial and seasonal variations in the Black Sea were assessed by using satellite CZCS and in situ data. Since the satellite CZCS had operated for the 1978–1986 period, CZCS data was used for assessing the past state of the Black Sea just before the onset of drastic changes observed in late 1980s. The approach used for the calculation of the absolute values of chlorophyll a concentration from CZCS data was based on the direct comparison of in situ chlorophyll a data and those of CZCS and by applying the algorithm developed for the transformation of CZCS data into chlorophyll a values. CZCS Level 2 data related with pigment concentration having a spatial resolution of 1 km at nadir were used. The daily Level 3 files were derived by binning Level 2 values into 4-km grid cells and the monthly and seasonal Level 3 files were created by averaging the daily Level 3 files over the corresponding period. In situ chlorophyll a data were obtained by spectrophotometric and fluorometric methods in 15 scientific cruises over the 1978–1986 period. Total number of ship-measured data used for the comparison with those CZCS values was 590.Chlorophyll a concentration (Chl) was derived from CZCS values (C) with regression equations Chl=kC; the coefficient of transformation k was calculated from six different data sets by taking into account distinctions between subregions and seasons. The reasons for difference in the k values have been analyzed.Statistical comparison of the chlorophyll a values measured in situ and those derived from CZCS data was based on log-transformed data and gave the following results: regression SLOPE=0.842, regression INTERCEPT=−0.081, coefficient of determination (R²)=0.806, root–mean–square ERROR=0.195. The mean monthly chlorophyll a distributions derived from CZCS data over 1978–1986 have been constructed and the mean seasonal chlorophyll a values in different regions have been calculated and analyzed. The significant difference in chlorophyll concentration between the western shelf regions and the open part of the Black Sea has been demonstrated, especially in warm season. At almost all seasons, the highest chlorophyll concentration is observed in the western interior shelf region which is under strong influence of Danube. The summer mean chlorophyll concentration in this region is 18 times higher than that in the open parts and about nine times higher than in the eastern shelf region. The greatest seasonal variations are observed in the open part of the Black Sea: chlorophyll concentration in cold season is four to six times higher than in summer and three to five times higher than in April and October. To the contrary, in the western interior shelf regions, the concentration is higher in May–October (about twice than that in November–March). Seasonal variations in the western outer shelf regions are smoothed out as compared with both the western interior shelf and the open regions.     

Kinetic modelling of the dispersion of plutonium in the eastern Irish Sea: two approaches

A new model to simulate the dispersion of plutonium in the eastern Irish Sea is presented. The model solves the 3D hydrodynamic equations using normalized σ coordinates in the vertical simultaneously with the suspended matter equation. Pu can be present in three phases: water, suspended matter and bottom sediments. Reduction and oxidation reactions are also included in the model, in terms of reaction rates, since Pu can be present in the marine environment in principally two different oxidation states. Two kinetic models are presented to describe the transfers of radionuclides between the liquid and solid phases: a one-step model consisting of a single reversible reaction and a two-step model consisting of two consecutive reversible reactions. It has been found that both models can properly simulate the contamination of the waters and sediments from the eastern Irish Sea due to the releases from the BNFL Sellafield nuclear fuel processing plant, since the outputs from both models are very similar and in agreement with observations. Also, both models can simulate the speciation of Pu between the reduced and oxidized forms. However, if the dominant source of radionuclides to the water column is redissolution from a contaminated sediment, a process that is actually occurring in the Irish Sea, it has been found that a two-step model must be used. Indeed, a one-step model predicts the redissolution from the sediment to be unrealistically rapid.     

Wave climate evolution in the Bay of Biscay over two decades

As a background, a review of long term evolution of wave climate in the North Atlantic is discussed. Most studies show that interannual evolutions in wave heights may be related to climatic factors, such as the North Atlantic Oscillation Index for example.Data of a waverider, consisting of a time series of 20 years in the Bay of Biscay (off Biscarosse, France), are analysed. Based on these data obtained from 1980, wave heights tend to decrease over the period. Also, interannual evolutions exist, particularly in the wave period. The fact that the annual wave periods at Biscarosse are found to vary more significantly than the annual wave heights led us to assume that it is an indirect effect of the evolution in the location of generating storms relative to the buoy. The relationships between wave parameters and climatic factors such as the North Atlantic Oscillation Index and the Garonne discharge have been derived: they show that the NAO index is negatively correlated with the river discharge and positively correlated with the wave period. This result is in agreement with the general climate scheme associated with NAO cycle proposed by Kushnir et al. [Kushnir, Y., Cardone, V.J., Cane, M., 1995. Link between Atlantic climate variability of surface wave height and sea level pressure. Proc. Fourth Int. Workshop on Wave Incasting and forecasting, Banff, AB, Canada, 59–64.]: NAO+ shifts storm tracks northward and dry weather are encountered in the southern part of Europe (conversely NAO− brings storm tracks and rainfall closer). Concerning wave heights, the lack of dependence on NAO index may be associated with compensation effects between wind intensity and storm tracks: wave energy is partly dissipated while reaching the Bay of Biscay in case of severe but distant storms (NAO+), which is not the case for storms generated nearer to the buoy and associated with moderate westerly winds (NOA−). However, the reason of the decrease in wave heights is not clear.     

Northern Adriatic Sea surface circulation and temperature/pigment fields in September and October 1997

Satellite images of surface chlorophyll-a concentration measured by the sea-viewing wide field-of-view sensor (SeaWiFS) and of sea surface temperature derived from advanced very high resolution radiometer (AVHRR) measurements, combined with in-situ drifter measurements of surface currents, and ancillary wind, Po River discharge and surface salinity data, are used to describe the surface dynamics in the northern Adriatic during the period September–October 1997.The satellite observations revealed very complex mesoscale dynamics, with time scales of a day or two and length scales of about 10 km, including the meandering and instability of basin-scale currents (e.g., the western coastal layer), jets/filaments and eddies. In addition, the two typical patterns of the Po River plume are observed and qualitatively explained in terms of wind forcing. A basin-wide double gyre pattern spreads the rich runoff water across most of the northern Adriatic from mid-September to early October, following Bora wind events and under stratified sea conditions. In contrast, in late October the Po plume is confined to the coast due to weaker winds and de-stratified conditions. This variability in the Po River plume extension is also confirmed by in-situ salinity measurements.     

About process times in some deepwater formation events in the Mediterranean Sea

Experimental data gathered on some Mediterranean deepwater formation sites have been analysed in order to check the Chapman (Ch), Visbeck and Maxworthy scale laws, their final mixing water depth and process times estimates. All the analysed sites have a high probability to be “internally constrained” so that the baroclinic Rossby radius is the dominant horizontal length scale and the final chimney depth is independent of the rate of rotation as indicated by Visbeck and Maxworthy criteria; the Eady instability times are generally very fast (3/f) but the least final mixing process times are too long compared with the meteorological forcing ones. It is attempted here to identify some new criteria to give some insight on the processes time scale, which is still an open problem. Conclusions about the Rhodes gyre appear questionable, because the experimental data appear inconsistent with any theoretical criterion.