On the dynamics of strait flows: an ocean model study of the Aleutian passages and the Bering Strait

A high-resolution numerical ocean circulation model of the Bering Sea (BS) is used to study the natural variability of the BS straits. Three distinct categories of strait dynamics have been identified: (1) Shallow passages such as the Bering Strait and the Unimak Passage have northward, near barotropic flow with periodic pulses of larger transports; (2) wide passages such as Near Straits, Amukta Pass, and Buldir Pass have complex flow patterns driven by the passage of mesoscale eddies across the strait; and (3) deep passages such as Amchitka Pass and Kamchatka Strait have persistent deep return flows opposite in direction to major surface currents; the deep flows persist independent of the local wind. Empirical orthogonal function analyses reveal the spatial structure and the temporal variability of strait flows and demonstrate how mesoscale variations in the Aleutian passages influence the Bering Strait flow toward the Arctic Ocean. The study suggests a general relation between the barotropic and baroclinic Rossby radii of deformations in each strait, and the level of flow variability through the strait, independent of geographical location. The mesoscale variability in the BS seems to originate from two different sources: a remote origin from variability in the Alaskan Stream that enters the BS through the Aleutian passages and a local origin from the interaction of currents with the Bowers Ridge in the Aleutian Basin. Comparisons between the flow in the Aleutian passages and flow in other straits, such as the Yucatan Channel and the Faroe Bank Channel, suggest some universal topographically induced dynamics in strait flows.     

A high-resolution modelling study of the Turkish Straits System

High-resolution modelling, for the first time, is used to study the basic hydrodynamics of the Turkish Straits System (TSS). Hydraulic controls in the Bosphorus and Dardanelles Straits are found to be essential in determining the coupled response of the TSS, which directly influences the interaction between the Mediterranean and Black Seas. The mixed baroclinic—barotropic response of the system is investigated as a function of the net barotropic flux and density stratification imposed at external boundaries, in the absence of atmospheric and tidal effects. The intense surface jet issuing from the Bosphorus is found to drive the basin-wide circulation of the Marmara Sea, varying with the net flux. The temporal response of the Bosphorus and Dardanelles Straits picks up rather fast, within a day or two, thanks to hydraulic controls within straits, while the surface currents in the Marmara Sea only approach steady state after a few months. Model stratification and circulation features are validated against independent measurements and a stand-alone model of the Bosphorus.     

Interactions between wind and tidally induced currents in coastal and shelf basins

This paper addresses the impact of atmospheric variability on ocean circulation in tidal and non-tidal basins. The data are generated by an unstructured-grid numerical model resolving the dynamics in the coastal area, as well as in the straits connecting the North Sea and Baltic Sea. The model response to atmospheric forcing in different frequency intervals is quantified. The results demonstrate that the effects of the two mechanical drivers, tides and wind, are not additive, yet non-linear interactions play an important role. There is a tendency for tidally and wind-driven circulations to be coupled, in particular in the coastal areas and straits. High-frequency atmospheric variability tends to amplify the mean circulation and modify the exchange between the North and the Baltic Sea. The ocean response to different frequency ranges in the wind forcing is area-selective depending on specific local dynamics. The work done by wind on the oceanic circulation depends strongly upon whether the regional circulation is tidally or predominantly wind-driven. It has been demonstrated that the atmospheric variability affects the spring-neap variability very strongly.     

Low-frequency variability of a semi-closed sea induced by the circulation in an adjacent ocean in a wind-driven,quasi-geostrophic,eddy-resolving simulation

The wind-driven circulation in the northwestern Pacific and the South China Sea (SCS) is simplified as a two-layer, quasi-geostrophic model in two rectangular basins connected by an idealized strait. This model is used to investigate the impact of the western boundary current (WBC) on the adjacent marginal sea. The variability of the circulation in the two basins is investigated with a high resolution and at low viscosity, which allows the numerical solution to resolve mesoscale eddy forcing. The model ocean is driven by the time-independent asymmetric wind stress acting on the idealized Pacific (large basin) only. Under the reference parameters used in this study, the WBC can intrude into the idealized SCS (small basin) in the form of a loop current, shedding eddies regularly. The rate of eddy shedding is nearly constant throughout the entire integration time of the model; however, the intensity of the eddy-shedding exhibits multiple timescale variability ranging from quasi-biennial to decadal timescale. A set of sensitivity experiments demonstrate that our results are robust against changes to model parameters and geometry. Multivariate spectral analysis is used to extract the spatiotemporal feature of the variability. Joint analysis for the two basins shows that the circulation in the idealized SCS is significantly impacted by the variability at decadal (15-year), interannual (5–7-year and quasi-biennial), and quasi-annual timescales. The spatial structures of the modes of variablility suggest that the variance in position of the WBC, combined with mesoscale activity, act to influence the low-frequency modes of the idealized SCS. The structural differences between the modes strongly impacting the idealized SCS and those having weak influence are also presented.     

On the variability of the flow along the Meso-American Barrier Reef system: a numerical model study of the influence of the Caribbean current and eddies

A high resolution (3–8 km grid), 3D numerical ocean model of the West Caribbean Sea (WCS) is used to investigate the variability and the forcing of flows near the Meso-American Barrier Reef System (MBRS) which runs along the coasts of Mexico, Belize, Guatemala and Honduras. Mesoscale variations in velocity and temperature along the reef were found in seasonal model simulations and in observations; these variations are associated with meandering of the Caribbean current (CC) and the propagation of Caribbean eddies. Diagnostic calculations and a simple assimilation technique are combined to infer the dynamically adjusted flow associated with particular eddies. The results demonstrate that when a cyclonic eddy (negative sea surface height anomaly (SSHA)) is found near the MBRS the CC shifts offshore, the cyclonic circulation in the Gulf of Honduras (GOH) intensifies, and a strong southward flow results along the reef. However, when an anticyclonic eddy (positive SSHA) is found near the reef, the CC moves onshore and the flow is predominantly westward across the reef. The model results help to explain how drifters are able to propagate in a direction opposite to the mean circulation when eddies cause a reversal of the coastal circulation. The effect of including the Meso-American Lagoon west of the Belize Reef in the model topography was also investigated, to show the importance of having accurate coastal topography in determining the variations of transports across the MBRS. The variations found in transports across the MBRS (on seasonal and mesoscale time scales) may have important consequences for biological activities along the reef such as spawning aggregations; better understanding the nature of these variations will help ongoing efforts in coral reef conservation and maintaining the health of the ecosystem in the region.     

Modeling of the Bosphorus exchange flow dynamics

The fundamental hydrodynamic behavior of the Bosphorus Strait is investigated through a numerical modeling study using alternative configurations of idealized or realistic geometry. Strait geometry and basin stratification conditions allow for hydraulic controls and are ideally suited to support the maximal-exchange regime, which determines the rate of exchange of waters originating from the adjacent Black and Mediterranean Seas for a given net transport. Steady-state hydraulic controls are demonstrated by densimetric Froude number calculations under layered flow approximations when corrections are applied to account for high velocity shears typically observed in the Bosphorus. Analyses of the model results reveal many observed features of the strait, including critical transitions at hydraulic controls and dissipation by turbulence and hydraulic jumps. It is found that the solution depends on initialization, especially with respect to the basin initial conditions. Significant differences between the controlled maximal-exchange and drowned solutions suggest that a detailed modeling implementation involving coupling with adjacent basins needs to take full account of the Bosphorus Strait in terms of the physical processes to be resolved.     

Hydrodynamic modelling of mesoscale eddies in the Black Sea

A three dimensional structure of mesoscale circulation in the Black Sea is simulated using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System. A number of sensitivity tests reveal the response of the model to changes in the horizontal resolution, time steps, and diffusion coefficients. Three numerical grids are examined with x-fine (3.2 km), fine (6.7 km) and coarse (25 km) resolution. It is found that the coarse grid significantly overestimates the energy of the currents and is not adequate even for the study of basin-scale circulation. The x-fine grid, on the other hand, does not give significant advantages compared to the fine grid, and the latter is used for the bulk of simulations. The most adequate parameters are chosen from the sensitivity study and used to model both the basin-scale circulation and day-to-day variability of mesoscale currents for the months of May and June of 2000. The model is forced with actual wind data every 6 h and monthly climatic data for evaporation, precipitation, heat fluxes and river run-off. The results of the fine grid model are compared favourably against the satellite imagery. The model adequately reproduces the general circulation and many mesoscale features including cyclonic and anticyclonic eddies, jets and filaments in different parts of the Black Sea. The model gives a realistic geographical distribution and parameters of mesoscale currents, such as size, shape and evolution of the eddies.     

Water intrusions and particle signatures in the Black Sea: a Biogeochemical-Argo float investigation

Continuous observations during 3 years with a vertical resolution of 1 dbar from two Bio-Argo floats in the Black Sea that were equipped with oxygen optodes, chlorophyll fluorometers, and backscattering sensors are analyzed. The particle backscattering coefficient, b _bp provides a proxy for the concentration of suspended particles. The observations clearly identify thermal and b _bp intrusions down to ~700–800 m in the Bosporus inflow area. In this area, b _bp is more than five times larger than elsewhere, which could indicate bacterial abundance and possible biological involvement in the precipitation of Mn-containing particles. The b _bp anomalies become much shallower than the temperature anomalies with increasing distance to the east of the strait. Their maxima are located between the onset of the suboxic zone and the upper part of the anoxic layer. Unlike well-known intrusions that are caused by inflow, open ocean intrusions are shallower and often characterized by multiple layers of backscatter maxima with thicknesses of only 15–20 m. The ratio between backscattering coefficients measured at two wavelengths, which gives a proxy for particle size, shows that the relative amount of larger size particles in the anoxic layer increases with depth. The particle concentrations and their size distribution display different vertical variability, which indicates the complex transformation of biological matter. The lower concentration of particles and lower chlorophyll-a during the extremely warm 2016 reveals an overall positive correlation between the two properties. The trends in the particle backscattering coefficient in the suboxic zone during 2013–2016 could indirectly reveal a biogeochemical response to temperature changes.     

Advances in research of the mid-deep South China Sea circulation

The South China Sea(SCS) is a large marginal sea connecting the Indian and Pacific oceans.Under the factors of monsoons,strait transport,and varied bathymetry,the SCS presents a three-layer structure and strong diapycnal mixing which is far greater than that in the open ocean.Theoretical analysis and observations reveal that internal tides,internal solitary waves,and strong winds are the sources of the strong mixing in the northern SCS.A major consequence of the strong mixing is an active mid-deep circulation system.This system promotes exchange of water between the SCS and adjacent oceans,and also regulates the upper layer of wind-driven circulation,making the 3 dimensional SCS circulation clearly different from that in other tropical and subtropical marginal seas.The mass transport capacity of the mid-deep circulation has a substantial impact on marine sedimentation,the biogeochemical cycle,and other processes in the SCS.This paper summarizes the recent advances in middeep sea circulation dynamics of the SCS,and discusses the opportunities and challenges in this area.     

Numerical analysis of the Black Sea currents and mesoscale eddies in 2006 and 2011

Two prognostic experiments taking into account real atmospheric forcing for 2006 and 2011 were carried out based on the eddy-resolving numerical model with a horizontal resolution of 1.6 km for the Black Sea. The main dynamic features such as the Rim Current, the Sevastopol, and Batumi anticyclones are reproduced in both experiments. The model results are confirmed via observation data. We accomplished the analysis of simulated circulation and energetics. The results demonstrate that both the vertical viscosity and vertical diffusion along with the energy inflow from the wind have been the main contributors to the annual and seasonal budgets of kinetic and potential energies of the Black Sea circulation. It is shown that two regimes of the Black Sea general circulation are implemented depending on a magnitude of wind contribution to the kinetic energy in winter. Intensive mesoscale eddy formation was observed along the Anatolian, Caucasian, and Crimean coasts. The analysis of the Black Sea circulation and eddy energetics allowed us to conclude that the generation and development of the mesoscale coastal eddies is associated with the barotropic instability in case of intensive coastal currents and is associated with both the barotropic and baroclinic instability in case of weak coastal currents.     

Low-frequency current variability observed at the shelfbreak in the northeastern Gulf of Mexico: May–October, 2004

High-resolution current measurements were made in the Northeastern Gulf of Mexico by the Naval Research Laboratory (NRL) as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. The major goal of SEED is to understand the mechanisms that transfer properties across the shelf slope. Fourteen acoustic Doppler current profilers (ADCPs) were deployed just west of the DeSoto Canyon on the shelf and down the slope from May to November, 2004 to measure nearly full water column current profiles. Currents were found more variable on the shelf than on the slope but in the mean strongly tended to follow bathymetry, particularly on the slope. During the SEED time period currents were driven by both local and remote winds, by cyclonic eddies associated with the Loop Current extension and Loop Current rings, by smaller eddies associated with the cyclonic eddies, by frontal meanders or streamers associated with the eddies, and by tropical storms. Currents were highly barotropic, accounting for more than 80% of the eddy kinetic energy (EKE). Current magnitudes generally increased from west to east, towards the DeSoto Canyon. Tropical storms had a relatively minor short-term effect upon mass transports. Cross-shelf transports were much smaller than the along-shelf transports. Onshore transports were stronger on the western side of the array while offshore transports were stronger on the eastern side of the array near the DeSoto Canyon. Offshore transports generally occurred during eastward flow periods, onshore transports during westward flow periods, and both during eddy periods. Mesoscale eddies also provided contributions to cross-shelf exchange. Large scale circulation features could be determined from the first two empirical-orthogonal function (EOF) modes which accounted for 83% of the variance and were strongly related to the integrated wind stress.     

State analysis using the Local Ensemble Transform Kalman Filter (LETKF) and the three-layer circulation structure of the Luzon Strait and the South China Sea

A new circulation model of the western North Pacific Ocean based on the parallelized version of the Princeton Ocean Model and incorporating the Local Ensemble Transform Kalman Filter (LETKF) data assimilation scheme has been developed. The new model assimilates satellite data and is tested for the period January 1 to April 3, 2012 initialized from a 24-year simulation to estimate the ocean state focusing in the South China Sea (SCS). Model results are compared against estimates based on the optimum interpolation (OI) assimilation scheme and are validated against independent Argo float and transport data to assess model skills. LETKF provides improved estimates of the western North Pacific Ocean state including transports through various straits in the SCS. In the Luzon Strait, the model confirms, for the first time, the three-layer transport structure previously deduced in the literature from sparse observations: westward in the upper and lower layers and eastward in the middle layer. This structure is shown to be robust, and the related dynamics are analyzed using the results of a long-term (18 years) unassimilated North Pacific Ocean model. Potential vorticity and mass conservations suggest a basin-wide cyclonic circulation in the upper layer of the SCS (z?>??570 m), an anticyclonic circulation in the middle layer (?570 m?≥?z?>??2,000 m), and, in the abyssal basin (<?2,000 m), the circulation is cyclonic in the north and anticyclonic in the south. The cyclone–anticyclone abyssal circulation is confirmed and explained using a deep-layer reduced-gravity model as being caused by overflow over the deep sill of the Luzon Strait, coupled with intense, localized upwelling west of the strait.     

利用多年卫星测高资料研究南海上层环流季节特征

利用10年高精度卫星测高海面高异常网格资料,联合EGM96稳态海面地形模型,构成南海海域合成海面地形的时间序列,并计算了各个时期的南海表层地转流场. 利用卫星跟踪漂流浮标观测结果与相应时期南海地转流场进行对比验证,结果显示本文结果可以很好地反映南海海域一些中小尺度的环流特征. 根据南海各季节多年平均表层环流场结构,对南海环流周年变化规律和季节特征进行了初步的探讨. 研究结果表明,南海表层环流始终处在不断演变过程之中,在时间和空间上都表现出明显的多尺度特征.     

Variations in the Black Sea water salinity from its latest salinization to the present state: Estimation based on mathematical modeling

The available published data were used to reveal the principal conditions and factors that control the formation and evolution of the Black Sea water salinity. Formalization of the major processes that contribute to the formation of sea salinity structure enabled, based on the present-day data on water balance, the reproduction of profiles of water salinity and the coefficient of vertical water exchange K _Z after 2, 4, 6, 8, and 10 ka since the beginning of water exchange with the Sea of Marmara through the Bosphorus. The time during which the present-day profile of the Black Sea water salinity had formed was evaluated. The results of simulation for different formation stages of the Black Sea salinity structure were used to determine the major ways of salt input into the sea (with river runoff and waters of the Lower Bosphorus Current) and its removal from the sea (the Upper Bosphorus Current). It was established that the Black Sea water salinity regime has virtually attained a stationary state.Translated from Vodnye Resursy, Vol. 32, No. 2, 2005, pp. 154–164.Original Russian Text Copyright © 2005 by Leonov.     

Sediment dynamics in the Black Sea: numerical modelling and remote sensing observations

Here, we address the sediment dynamics in the Black Sea based on analysis of remote sensing data from the Medium Resolution Imaging Spectrometer and numerical simulations with Nucleus for European Modelling of the Ocean model. Boundary conditions consist of realistic meteorological forcing, including significant wave height generated by wave prediction model. A number of sensitivity runs was analysed with the aim to find the most suitable parameters governing sediment fluxes. The comparison between numerical simulations and remote sensing data gives credibility to the quality of simulations. The combined effect of wind waves and currents in the bed layer controls the sediment resuspension that appears to be the major basin-wide source of sediment. Sensitivity experiments included or excluded different forcing terms, e.g. sediment flux from rivers enable to determine the spatial extensions of different point sources. It is concluded that wind-wave forcing is manifested in the sediment dynamics through episodic high energy events contributing to the increase of horizontal sediment fluxes over the northwestern shelf. Both satellite images and numerical model simulations demonstrated that the penetration of suspended sediment into the basin interior was governed by the dynamics of coastal and open-ocean eddies. While fine sediment at sea surface could cross the continental slope propagating into the open ocean, coarser fractions follow the bottom and their penetration into the open ocean is limited. The conclusion is thus that the deposition patterns correlate with the specific shape of Black Sea topography, and the largest depositions are observed in the area of continental slope.     

Modeling the water resources of the Black and Mediterranean Sea river basins and their impact on regional mass changes

For the first time, a dedicated release of the hydrology and water use model WaterGAP3, has been developed to spatially explicit calculate hydrological fluxes within river basins draining into the Mediterranean and Black Sea. The main differences between the new regional version of the global WaterGAP3 model and the previously applied global version WaterGAP2 can be found in the spatial resolution, snow modeling, and water use modeling. Comparison with observations shows that WaterGAP3 features a more realistic representation of modeled river runoff and inflow into both seas. WaterGAP3 generates more inflow to both seas than WaterGAP2. In the WaterGAP3 simulation, contributions to the total runoff into the Black Sea from individual discharge regions show in general a good agreement to climatology derived runoff, but lesser importance of Georgian rivers for the basin's water. After the successful model validation WaterGAP3 has been applied to correct estimates of seawater mass derived from the GRACE gravity mission and to account for freshwater inflow into both basins. The performance of the WaterGAP3 regional solution has been evaluated by comparing the seawater mass derived from GRACE corrected for the leakage of continental hydrology, to an independent estimate derived from steric-corrected satellite altimetry with steric correction from regional oceanographic models. The agreement is higher in the Mediterranean Sea than in the Black Sea. Results using WaterGAP3 and WaterGAP2 are not significantly different. However the agreement with the altimetry-derived results is higher using WaterGAP2, due to the smaller annual amplitude of the continental hydrology leakage from WaterGAP3. We conclude that the regional model WaterGAP3 is capable of realistically quantifying water mass variation in the region, further developments have been identified.     

Transport through Prince William Sound: numerical study in a nowcast/forecast system

Using 1-year simulated data from extended Prince William Sound (PWS) nowcast/forecast system, both barotropic and baroclinic transports through two-strait, semi-enclosed PWS are examined. With major tidal constituents removed, hourly time series of volume transports through two straits are significantly correlated with net transport well balanced by the time rate of change of the PWS spatial-mean sea level. A transition frequency band occurs within the coherence function of hourly volume transports, which is characterized by a nearly 180° phase shift between low-frequency (>30 h) and high-frequency (<6 h) bands. The transition band is implicitly related to the horizontally divergent and horizontally non-divergent flows inside the Sound. Further investigation of monthly and annual mean volume transports indicates strong seasonal variability of flows through two straits. On the other hand, baroclinic transport through PWS demonstrates the transition between a two-layered flow structure during the wintertime and a well-defined three-layered structure, i.e., inflow in both the surface and bottom layer with outflow in the intermediate layer, in the remainder of the year. This three-layer exchange flow is determined to be mainly buoyancy-driven, geostrophic flow, and thus largely affected by seasonal variability of buoyancy over the shelf and PWS.     

Global geoenvironmental problems: Black Sea basin

The issue of the geoenvironmental state of Black Sea basin, the final basin receiving the runoff of many large rivers of the European continent (Danube, Dnieper, Don, etc.) and considerable runoff from the nearby watersheds, including Turkish, is considered. The extent and geoenvironmental consequences of the large-scale changes in the hydrology, water balance, and hydrochemistry of the sea basin under the effect hydrotechnical interception and industrial pollution of river runoff are discussed. The genetic role of Black Sea rifting in one of the world’s largest oil-and-gas-bearing basins with its inherent thermal and geochemical processes, geoenvironmental conditions, and risks is emphasized. Improved information is given about the genesis of hydrogen sulfide dissolved in seawater and related to the processes of geochemical sulfate reduction of silt water sulfates under the effect of hydrocarbon gases, which mostly form in zones of deep catagenesis of sedimentary formations and lower crust rock metamorphism. It is emphasized that the ecological stability of a sea basin showing such complex and contradictory relationships with natural and technogenic objects and processes can be ensured only by joint and well coordinated efforts of the countries of the Black Sea and nearby regions aimed on rational nature and water management, including when laying pipelines on Black Sea bed and developing the power resources of the Black Sea oil-and-gas-bearing basin.     

A mesoscale eddy detection method of specific intensity and scale from SSH image in the South China Sea and the Northwest Pacific

Mesoscale eddies exist almost everywhere in the ocean and play important roles in the ocean circulation of the world. These eddies may cause sound spread singular regions and bring great influences to the upwater ship and underwater aircraft. Due to the lack of hydrographic survey datasets, study of mesoscale eddies has been greatly restricted. Fortunately, satellite altimeter provided an effective way to study mesoscale eddies. An automatic detection algorithm is introduced to detect mesoscale eddies of specific intensity and spatial/temporal scale based on satellite sea surface height (SSH) data and the algorithm is applied in a strong eddy activity region: the South China Sea and the Northwest Pacific. The algorithm includes four steps. The first step is preprocessing of the SSH image, which includes elimination of error SSH data and interpolation. The second step is to detect suspected mesoscale eddies from preprocessed SSH images by dynamic threshold adjustment and morphological method, and the suspected mesoscale eddy detection includes two procedures: suspected mesoscale eddy core region detection and suspected mesoscale eddy brim extraction. The third step is to pick out mesoscale eddies satisfied with specified criteria from suspected mesoscale eddies. The criteria include three items, that is, intensity criterion, spatial scale, criterion and temporal scale criterion. The last step is algorithm performance analysis and verification. The algorithm has the capability of adaptive parameter adjustment, and can extract mesoscale eddies of interested intensity and spatial/temporal scale. The paper can provide a basis for analyzing space-time characteristics of mesoscale eddy in the South China Sea and the Northwest Pacific.     

(137)Cs baseline levels in the Mediterranean and Black Sea: a cross-basin survey of the CIESM Mediterranean Mussel Watch programme

The common mussel Mytilus galloprovincialis was selected as unique biomonitor species to implement a regional monitoring programme, the CIESM Mediterranean Mussel Watch (MMW), in the Mediterranean and Black Seas. As of today, and upon standardization of the methodological approach, the MMW Network has been able to quantify (137)Cs levels in mussels from 60 coastal stations and to produce the first distribution map of this artificial radionuclide at the scale of the entire Mediterranean and Black Seas. While measured (137)Cs levels were found to be very low (usually < 1 Bq kg(-1) wet wt) (137)Cs activity concentrations in the Black Sea and North Aegean Sea were up to two orders of magnitude higher than those in the western Mediterranean Basin. Such effects, far from representing a threat to human populations or the environment, reflect a persistent signature of the Chernobyl fallout in this area.