Numerical modelling of the seasonal variability of hydrophysical fields in the Black Sea

The results of research into the seasonal variability of hydrothermodynamic characteristics of the Black Sea are presented. These have been obtained using field data on the fields of temperature, salinity, and wind, and by mathematical modelling. The seasonal variability of the current and heat content fields and of the cold intermediate layer and other hydrophysical characteristics is discussed.Translated by Mikhail M. Trufanov.     

渤海湾M2分潮的季节变化:增强调和分析的应用

M₂分潮的季节变化对沿海的海洋环境有着重要影响。增强调和分析(EHA)既可以提取主要分潮时变的振幅和迟角,同时可以得到其他分潮不随时间变化的振幅和迟角。本文利用EHA分析渤海湾两个站点的水位数据,研究了渤海湾M₂分潮的季节变化。为了评估EHA方法的准确性,在理想实验中设计了人造“水位数据”。利用EHA分析得到的M₂分潮时变振幅和迟角以及S₂、K₁、O₁分潮不随时间变化的振幅和迟角均比其他方法得到的结果更接近给定值,表明了EHA的有效性和可用性。当使用EHA分析渤海湾实际海平面观测数据时,得到的M₂分潮振幅具有明显的季节变化特征:夏季较大,冬季较小。敏感性实验表明,分析所得渤海湾M₂分潮振幅的季节变化趋势不受实验设置的影响,是鲁棒的,能够反映该海域真实的M₂分潮季节变化。此外,渤海湾M₂分潮振幅的季节变化可能是东亚季风通过影响平均海平面、层化和涡动黏性系数的季节变化而引起的。     

Distribution of surface-active substances in the northern Adriatic Sea

Spatial distribution and seasonal variability of surfactant activity (SA) of seawater were studied in the northern Adriatic Sea and compared to the temperature and salinity patterns in the 2-year period from February 1998 to January 2000, based on monthly surveys. Surface-active substances (SAS) were determined with alternating current voltammetry (in-phase mode) using o-nitrophenol as an electrochemical probe. A general characteristic of the SA seasonal variability for the northern Adriatic is the sinusoidal change of SA within the period of 1 year; similar behaviour was observed for the seasonal variations of temperature. Maximum SA values appeared during the summer period, while minimum SA values were measured in winter, during the period of well-mixed water layers and lower phytoplankton production. In May and October, the highest SA values were obtained in the upper seawater layer, which was ascribed to the influence of high inflow of nutrient-rich Po River freshwater. Riverine inputs indirectly favour autochthonous production processes, which result in increased concentration of organic matter (OM), particularly surface-active substances.     

Climatic structure of the wind fields in the Black-Sea region

For the Black-Sea region, we perform the dynamic reanalysis of the data on atmospheric circulation for the period 1958–2001 by using the HadRM3P model with a space resolution of 25 km. We estimate the mean climatic atmospheric fields of vorticity and divergence of the wind velocity and study their space structure and seasonal variability. The climatic estimates of the annual course of vorticity of the wind velocity over the sea are presented. The obtained large annual average values of vorticity of the velocity reveal the predominant role played by the wind action in the generation not only of the seasonal variability but also of the mean cyclonic circulation of waters in the Black Sea.     

Mesoscale, seasonal and interannual variability in the Mediterranean Sea using a numerical ocean model

In this paper, we present the results from a 1/8° horizontal resolution numerical simulation of the Mediterranean Sea using an ocean model (DieCAST) that is stable with low general dissipation and that uses accurate control volume fourth-order numerics with reduced numerical dispersion. The ocean model is forced using climatological monthly mean winds and relaxation towards monthly climatological surface temperature and salinity. The variability of the circulation obtained is assessed by computing the volume transport through certain sections and straits where comparison with observations is possible. The seasonal variability of certain currents is reproduced in the model simulations. More important, an interannual variability, manifested by changes in currents and water mass properties, is also found in the results. This may indicate that the oceanic internal variability (not depending on external atmospheric forcing), is an important component of the total variability of the Mediterranean circulation; variability that seems to be very significant and well documented by in situ and satellite data recovered in the Mediterranean Sea during the last decade.     

Assessing the ecological status of the North-western Adriatic Sea within the European Water Framework Directive: a comparison of Bentix, AMBI and M-AMBI methods

In this paper we looked for the most suitable method for benthic ecological quality status (EcoQS) assessment of the North-western Adriatic Sea. Taking into account the historical and present information, we inter-compared our expectations in terms of EcoQS, with the Bentix, AMBI and M-AMBI classification metrics results, and investigated the sensitivity of these indices to the spatial, seasonal and inter-annual variability of benthic communities. The sampling sites were located at increasing distances with respect to the Po River mouth, along an (inverse) gradient of eutrophication. Moreover, a 2-year monthly monitoring plan (October 2004–September 2006) was carried out at the CEBEB site, located in a central position with respect to the investigated area. The results highlighted that the three indices differed in the sensitivity to spatial and temporal variability of the benthic communities. The strong seasonal and inter-annual changes in the benthic communities had a significant effect on the Bentix, whereas no relevant modifications were observed on the EcoQS assignation obtained using the M-AMBI and AMBI methods. The environmental quality ratios (EQR) values and EcoQS classification obtained by the Bentix and AMBI did not match expectations, as they were not inversely correlated with the site distances with respect to the Po mouth, whereas the M-AMBI clearly discriminated the most and the least eutrophicated sites. It thus appears that the M-AMBI could be considered to be the most valuable method for the ecological-status assessment of the North-western Adriatic Sea benthic ecosystem.     

Reanalysis of seasonal and interannual variability of Black Sea fields for 1993–2012

A retrospective analysis has been done for the hydrophysical fields of the Black Sea for 1993–2012 with the assimilation of undisturbed monthly average profiles of temperature and salinity that were obtained by using an original procedure of joint processing of satellite altimetry and rare hydrological observations. The accuracy of the reconstructed fields of temperature and salinity of the Black Sea is evaluated by comparison with the data of sounding from the hydrological stations and the Argo floats. A comparative analysis is performed for the integral characteristics of the fields of temperature, salinity, and kinetic energy with the same characteristics of the reanalysis for 1992–2012 that assimilated the average annual profiles of temperature and salinity, surface temperature and altimetry level of the sea after being adjusted with respect to climate seasonal variability. The proposed procedure of the reanalysis execution allows a more precise reconstruction of the interannual variability of temperature and salinity stratification in the main pycnocline. The correlation between the annual and seasonal variability of the eddy of the wind friction tangential stress and the average kinetic energy at the levels is revealed.     

黑潮延伸体海洋锋强度和位置变率的对比研究

This study compares the seasonal and interannual-to-decadal variability in the strength and position of the Kuroshio Extension front(KEF) using high-resolution satellite-derived sea surface temperature(SST) and sea surface height(SSH) data. Results show that the KEF strength has an obvious seasonal variation that is similar at different longitudes, with a stronger(weaker) KEF during the cold(warm) season. However, the seasonal variation in the KEF position is relatively weak and varies with longitude. In contrast, the low-frequency variation of the KEF position is more distinct than that of the KEF strength even though they are well correlated. On both seasonal and interannual-to-decadal time scales, the western part of the KEF(142°–144°E) has the greatest variability in strength, while the eastern part of the KEF(149°–155°E) has the greatest variability in position. In addition, the relationships between wind-forced Rossby waves and the low-frequency variability in the KEF strength and position are also discussed by using the statistical analysis methods and a wind-driven hindcast model. A positive(negative) North Pacific Oscillation(NPO)-like atmospheric forcing generates positive(negative) SSH anomalies over the central North Pacific. These oceanic signals then propagate westward as Rossby waves, reaching the KE region about three years later, favoring a strengthened(weakened) and northward(southward)-moving KEF.     

Seasonal variability of the Antarctic Coastal Current and its driving mechanisms in the Weddell Sea

Insight into the dynamics of the Antarctic Coastal Current (ACoC) is achieved by quantifying the contributions of its driving mechanisms to the seasonal variability of its barotropic and baroclinic components. These mechanisms are sought out in the local wind, the sea-ice concentration, wind curl of the Weddell Gyre (Sverdrup transport) and the thermohaline forcing related to warming/cooling and ice melting and freezing. These driving mechanisms induce most of the seasonal variability of both the barotropic and baroclinic components of the ACoC by deepening the pycnocline towards the coast and sharpening the baroclinic profile following thermal wind balance. The resulting coastal current has mainly a barotropic transport (82%) and a major annual cycle, which explains 37% of this component's variability (tides and other high-frequency events generate 40%). The wind contributes with 58% of the seasonal variability of the barotropic component and 23% of the baroclinic; the sea-ice concentration contributes with 8% and 18%, respectively; Sverdrup transport with 4% and 30% and the thermohaline forcing with 30% and 29%. The results of this study are obtained with analysis of fifteen CTD sections (potential density and geostrophic velocities) of RV-Polarstern obtained between 1992 and 2005, as well as composite, spectral and harmonic analyses of 9 years of time series from moored instruments (current speed and temperature), wind speed, atmospheric pressure and sea-ice concentration of satellite imagery.     

Seasonal and long-term dynamics of the chlorophyll concentration in the Black Sea according to satellite observations

The spatial and temporal variability of the chlorophyll (Chl) concentration in the surface water layer of the Black Sea in 1998–2008 has been analyzed using the data obtained by the SeaWiFS satellite sensor. In the deep-sea areas, the seasonal pattern of the Chl concentration is represented by a U-shape curve. The maximal concentrations are observed in the winter-spring and autumn periods, while the minimal, in the summertime. In the northwestern Black Sea, the maximal concentrations are registered in mostly the summer and autumn periods. Pronounced interannual variability is found for the summer concentrations of Chl observed for an 11-year period. After a cold winter, the concentration of Chl in the spring period is 3–5 times higher compared to the mild-winter years. In December–March, a negative correlation between the water temperature and the average Chl concentration is registered.     

Climatic parameters of wind-field variability in the Black Sea region: Numerical reanalysis of regional atmospheric circulation

A reanalysis of atmospheric circulation in the Black Sea region is performed with a high spatial resolution of 25 × 25 km for the period from 1958 to 2001. Climatic wind speed fields are estimated, as are their spatial structure and seasonal variability. Mesoscale regions of cyclonic and anticyclonic speed vorticity, which are connected with edge effects and orography, are distinguished. To single out the monsoon mechanism in the annual votricity cycle, numerical experiments on the sensitivity that the regional atmospheric circulation has to the sea-surface temperature perturbations have been carried out. Large-scale regional peculiarities of the surface wind field are considered for different seasons. The vorticity of the surface wind speed and strength is assessed. Large annual average values of the vorticity are obtained which are comparable with the range of seasonal variability; they evidence the defining role that wind plays in the generation and seasonal variability, as well as the average cyclonic water circulation in the Black Sea.     

Seasonal cycle of phosphate in the open ocean

The seasonal cycle of phosphate in the world ocean is described using all historical data (over 170,000 profiles) held at the U.S. National Oceanographic Data Center and World Data Center-A for Oceanography. Generally, phosphate concentrations are depressed in the season of highest primary production, in conformance with phosphate's role as a major nutrient. Mean phosphate concentration in the North Pacific are twice those in the North Atlantic. The largest seasonal differences occur in the sub-polar North Atlantic and Pacific, where changes in concentration are as large as a factor of two. Temperate and equatorial regions exhibit less seasonal variability. High latitudes, upwelling areas, and river mouths exhibit a notable seasonal signal in phosphate. Enrichment of phosphate from the Amazon and Orinoco rivers appears to dominate the seasonal signal in the tropical Atlantic. In fact, the extent of the rivers’ effects extend so far north into the North Atlantic gyre that it obscures the normal pattern of summer depletion occurring elsewhere in the basin. The seasonal signal in the tropical Pacific Ocean is a function of seasonal variability in the winds, which affect the strength of coastal upwelling.     

Seasonal patterns of tidepool macroalgal assemblages in the North of Portugal. Consistence between species and functional group approaches

Macroalgae are useful organisms to monitor the environmental quality and to detect impacts due to anthropogenic activities. However, it is very important to identify the scales of variation in natural assemblages, particularly for the detection of environmental impacts. Otherwise, changes due to anthropogenic impacts may be confused with differences due to natural temporal variability. Another important task is to determine the appropriate level of taxonomical effort needed to detect changes in the assemblage structure. Many taxonomical surrogates, at higher taxonomic levels than that of species, have been proposed but, the consistence in space and time of the results produced by surrogates with those obtained at specific level should be tested. The objectives of this study are to identify the seasonal patterns of tidepool macroalgal communities using objective procedures and to test the consistence between the patterns obtained considering data at the species level and functional groups. Results showed that the seasonal pattern obtained using functional groups and species was consistent. Tidepool macroalgal assemblages showed a seasonal pattern with significant differences between spring–summer and autumn–winter. This pattern can be explained by changes in environmental variables and the seasonal development of the dominating species. Ulva spp. and the non-indigenous species Grateloupia turuturu were the species responsible for this pattern due to their high seasonality in terms of biomass. Finally, the abundance and species diversity within the corticated functional group was proposed as indicator of environmental impacts due to its relatively constant abundance and its sensitivity to environmental impacts.     

Wave mechanism of the seasonal variability of the largescale current field in the northern Tropical Atlantic

A spatial-temporal analysis of the density field in the large-scale hydrological observation areas in the Tropical Atlantic is carried out. The spectral maximum corresponding to the first baroclinic mode of the planetary Rossby wave is discriminated and studied. It is shown that the seasonal transformation of the large-scale circulation of the current field is connected with the propagation of this wave. A simple quasi-geostrophic model is suggested which describes the seasonal variability of the North Equatorial Countercurrent. The results obtained by this model are compared with the hydrological survey data.Translated by Mikhail M. Trufanov.     

Long-term variability of the water transparency (Secchi Depth) in the Sea of Azov

The spatial, seasonal, and interannual variability of the water’s transparency (Secchi depth) in the Sea of Azov is studied using the database of long term transparency measurements for the 1922–2009 period and relevant materials on the meteorological, hydrological, and hydrobiological parameters of the environment. We identified four main types of seasonal variability of the water’s transparency depending on the local conditions. The mean values of the Secchi depth are calculated for the different periods, which are distinguished by the differences in the relative role of the water transparency factors associated both with the climatic variations and the human activities.     

Multitime scale variations of sea surface temperature in the China seas based on the HadISST dataset

The variability of the sea surface temperature(SST) in the China seas has been studied in seasonal,interannual and interdecadal scales based on the monthly data of HadISST spanning from 1870 to 2007. The main results obtained are SST in the China offshore changes most actively at the seasonal scale with the intensity diminishing from north to south,as the temperature differences between summer and winter reaching 17 and 4 C in the northern and southern areas,respectively. Moreover,seasonal variation near the coastal regions seems relatively stronger than that far from the coastline;significant interannual variations are detected,with the largest positive anomaly occurring in 1998 in the overall area. But as far as different domains are concerned,there exists great diversity,and the difference is also found between winter and summer. Differed from the seasonal variations,where the strongest interannual variability takes place,resides to the south of that of the seasonal ones in the northern section,nevertheless in the South China Sea,the most significant interannual variability is found in the deep basin;interdecadal changes of summer,winter and annual mean SST in different domains likewise present various features. In addition,a common dominant warming in recent 20 a are found in the overall China offshore with the strongest center located in the vicinity of the Changjiang Estuary in the East China Sea,which intensifies as high as 1.3 C during the past 130 a.     

基于1987-2010年实测资料对PN断面东海黑潮季节变化的分析

As the spatio-temporal variability of the Kuroshio is highly influenced by mesoscale eddies, representing its seasonal variability characteristics requires sufficiently long term observations to reduce...     

Spatial versus temporal patterns in fish assemblages of a tropical estuarine coastal lake: The Ebrié Lagoon (Ivory Coast)

The fish assemblages of the Ebrié lagoon (Ivory Coast) were sampled by experimental fishing over the entire lagoon using a purse seine net. The sampling was conducted in the two main hydroclimatic seasons for this ecosystem, i.e. in the dry season (March–April) and in the wet season (August–September). The results obtained showed a fish assemblage organized around a consistently occurring group of twenty species. When analysed in terms of ecological categories, the seasonal influence led to a cycle in the assemblages from freshwater to marine around this permanent species pool, with a seasonal renewal of the assemblage. At the scale of the lagoon, there were variations in the composition of the assemblages that clearly distinguished the western part from the eastern one. The limit was situated at the Vridi canal, a wide artificial channel permanently connecting the lagoon to the sea. To the west, the assemblage was characterised by a strong spatial uniformity and low seasonal variability. To the east, the assemblage formed two different entities; one assemblage with pronounced freshwater affinities occurring in a side arm and the other assemblage with great seasonal variability under the alternating influence of seawater in the dry season and freshwater in the wet season. This part of the lagoon functioned somewhat like a typical estuary.     

Seasonal and Inter-Annual Variability of Wind-Driven Upwelling near the East Coast of Sakhalin Island Based on QuikSCAT/SeaWinds Scatterometer Data

Izvestiya, Atmospheric and Oceanic Physics - In this paper, seasonal and interannual upwelling variability in the Sea of Okhotsk off the eastern coast of Sakhalin Island based on wind data obtained...     

Seasonal and interannual variations of the field of sea-surface temperature in the southwest Atlantic according to the data of satellite and contact measurements

We study the seasonal and interannual variations of the field of sea-surface temperature (SST) in the Southwest Atlantic on the basis of the analysis of three data arrays, namely, the Levitus-Boyer-1994 hydrological array, the WOD-98 actual database, and the data of satellite measurements of SST. We perform the comparative analysis of the amplitude-phase characteristics of the annual course of the SST field and discuss the distinctive features of its seasonal and interannual variations. It is shown that the results obtained by using different data arrays are in good agreement. The values of the coefficient of linear correlation are equal to 0.7–0.9. It is discovered that the maximum seasonal variations of SST are observed in the zone of confluence of the Falkland and Brasil Currents in the vicinities of the South Subtropical and Antarctic Polar fronts. The minimum seasonal variations are recorded in the Antarctic region and in the Weddell Sea. The satellite data demonstrate that the level of interannual variability is high in the zones of climatic fronts. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 62–76, September–October, 2006.