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21.
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. 相似文献
22.
We calculate the spectral energy distribution (SED) of electromagnetic radiation and the spectrum of high-energy neutrinos from BL Lac objects in the context of the synchrotron proton blazar model. In this model, the high-energy hump of the SED is due to accelerated protons, while most of the low-energy hump is due to synchrotron radiation by co-accelerated electrons. To accelerate protons to sufficiently high energies to produce the high-energy hump, rather high magnetic fields are required. Assuming reasonable emission region volumes and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs), which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there is a significant contribution to the high-frequency hump of the SED from pion photoproduction and subsequent cascading, including synchrotron radiation by muons. In contrast, in HBLs we find that the high-frequency hump of the SED is dominated by proton synchrotron radiation. We are able to model the SED of typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We also calculate the expected neutrino output of typical BL Lac objects, and estimate the diffuse neutrino intensity due to all BL Lacs. Because pion photoproduction is inefficient in HBLs, as protons lose energy predominantly by synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino intensity. We suggest that nearby LBLs may well be observable with future high-sensitivity TeV γ-ray telescopes. 相似文献
23.
Bed shear stress in the southern North Sea as an important driver for suspended sediment dynamics 总被引：3，自引：0，他引：3
Emil Vassilev Stanev Mikhail Dobrynin Andrey Pleskachevsky Sebastian Grayek Heinz Günther 《Ocean Dynamics》2009,59(2):183-194
This paper addresses the spatial and temporal patterns of drivers for sediment dynamics in coastal areas. The basic assumption
is that local processes are dominating. The focus is put on the bed shear stress in the southern part of North Sea giving
the basic control for deposition–sedimentation and resuspension–erosion. The wave-induced bed shear stress is formulated using
a model based on the concept that the turbulent kinetic energy associated with surface waves is a function of orbital velocity,
the latter depending on the wave height and period, as well as on the water depth. Parameters of surface waves are taken from
simulations with the wave spectrum model WAM (wave model). Bed shear stress associated with currents is simulated with a 3D
primitive equation model, Hamburg Shelf Ocean Model. Significant wave height, bed shear stress due to waves and currents,
is subjected to empirical orthogonal functions (EOF) analysis. It has been found that the EOF-1 of significant wave height
represents the decrease of significant wave height over the shallows and, due to fetch limitation, along the coastlines. Higher
order modes are seesaw-like and, in combination, display a basin-scale rotational pattern centred approximately in the middle
of the basin. Similar types of variability is also observed in the second and third EOF of bed shear stress. Surface concentrations
of suspended matter derived from MERIS satellite data are analysed and compared against statistical characteristics of bed
shear stress. The results show convincingly that the horizontal distribution of sediment can, to a larger extent, be explained
by the local shear stress. However, availability of resuspendable sediments on the bottom is quite important in some areas
like the Dogger Bank. 相似文献
24.
This work deals with the analysis of simulations carried out with a primitive equation numerical model for the region of the
East Frisian Wadden Sea. The model, with 200-m resolution, is forced by wind, air–sea heat, and water fluxes and river runoff
and is nested in a German Bight 1-km-resolution numerical model, the latter providing tidal forcing for the fine resolution
model. The analysis of numerical simulations is focused both on responses due to moderate conditions, as well as to extreme
events, such as the storm surge Britta, for which the model demonstrates very good skills. The question addressed in this
paper is how well the model output can be compressed with the help of empirical orthogonal function analysis. It is demonstrated
that, for the short-time periods of the order of a spring–neap cycle, only a few modes are necessary to almost fully represent
the circulation. This is just an illustration that the circulation in this region is subject to the dominating tidal forcing,
creating clear and relatively simple response patterns. However, for longer periods of about several months, wind forcing
is also very important, and correspondingly, the circulation patterns become much more complex. Possible applications of the
results in hindcasting and forecasting of hydrodynamics and sediment dynamics in the coastal zone are considered. 相似文献
25.
The paleo-evolution of the Black Sea level during the Lateglacial–Holocene transition is still unclear, which motivates us
to provide new estimates for that period based on the analysis of water budget. Hydrological conditions in the Black Sea catchment
area are reconstructed here using water balance equation, available data, and constraints based on simple theory relating
the runoff ratio with climatic characteristics. In order to estimate the impact of the aridity of climate we consider two
alternative scenarios: (1) climate change under constant in time gradient in precipitation and evaporation over land and sea,
and (2) climate change accounting for changes in the horizontal distribution of precipitation and evaporation. Hydrological
data are compiled from available present-day data and paleo-observations. A number of sensitivity experiments is carried out
revealing that the hydrological conditions in the Black Sea watershed should have evolved towards a very arid climate (similar
to the present-day climate in the Caspian Sea area) in order to initiate a drop of sea level of ∼100 m below the sill depth
of the Bosporus Straits, as speculated in some recent research. Estimates of sea level changes reveal a qualitative agreement
with the coast-line evolution inferred from paleo-observations. The Younger Dryas draw down of the Black Sea starts at about
13.3 to 13 kyr BP, with a maximum low-stand of 104 m at 11.5 kyr BP. In an arid climate scenario the sea level reaches the
outlet at about 8.8 to 7.4 kyr BP. Approximately at that time, Mediterranean sea level was about 10 m higher, making flooding
events of the Black Sea possible. However, the nature and exact timing of the Holocene reconnection depends also on other
(not well known) factors; one of them is the Bosporus sill depth, motivating further research in this field. Estimates of
the water transport through the Bosporus Straits are also provided for the time of Lateglacial–Holocene transition. 相似文献
26.
C R Joiris L Holsbeek D Bolbat C Gascard T Stanev A Komakhidze W Baumgartner A Birkun 《Marine pollution bulletin》2001,42(10):905-911
This paper reports on mercury (Hg) concentrations in different tissues (liver, muscle, kidney, blubber and brain) of harbour porpoises Phocoena phocoena found dead in the Black Sea between 1997 and 1999, mainly bycaught in fishing nets (n = 79). Total Hg and organic Hg (MeHg) were determined. The main factor affecting Hg accumulation was, as expected, age, with MeHg concentration increasing with age. Accumulation of high concentrations of inorganic Hg in the liver was probably due to a slow demethylation process implying the formation of tiemannite (HgSe). In older adults, liver concentrations reached 35 microg g(-1) dry weight ('ppm') total Hg and 3 microg g(-1) dw MeHg. A geographical comparison with existing data from other regions showed a generally low Hg contamination of Black Sea porpoises, one order of magnitude lower than, e.g. in the North Sea. 相似文献
27.
E.V. Stanev J. Staneva J.L. Bullister J.W. Murray 《Deep Sea Research Part I: Oceanographic Research Papers》2004,51(12):2137-2169
Data from field observations and numerical model simulations are used to understand and quantify the pathways by which passive tracers penetrate into the Black Sea intermediate and deep layers. Chlorofluorocarbon (CFC) concentrations measured during the1988 R.V. Knorr cruise show strong decrease with increasing density in the Black Sea and illustrate the very slow rate of ventilation of deep water in this basin. We develop a 3D numerical model based on the Modular Ocean Model (MOM), and calibrate it in a way to produce consistent simulations of observed temperature, salinity and CFCs. One important feature is the implementation of a special parameterization for convection, which is an alternative of the convective adjustment in MOM and handles the penetration of the Bosporus plume into the halocline. The model forcing includes interannually variable wind, heat and water fluxes constructed from Comprehensive Ocean–Atmosphere Data Set and ECMWF atmospheric analysis data and river runoff data. The analysis of observations and simulated data are focused on correlations between thermohaline and tracer fields, dynamic control of ventilation, and the relative contributions of sources at the sea surface and outflow from the Bosporus Strait in the formation of intermediate and deep waters. A simple theory is developed which incorporates the outflow from the strait along with the vertical circulation (vertical turbulent mixing and Ekman upwelling) and reveals their mutual adjustment. The analyses of simulated and observed CFCs demonstrate that most of the CFC penetrating the deep layers has its source at the sea surface within the Black Sea rather than from the Marmara Sea via the Bosporus undercurrent. Under present-day conditions, the surface CFC signals have reached only the upper halocline. Intrusions below 600 m are not simulated. The major pathways of penetration of CFCs are associated with cold-water mass formation sites, Bosporus effluent, as well as with the diapycnal mixing in the area of Rim Current. Future CFC sampling strategies coherent with the unique conditions in the Black Sea are discussed. 相似文献
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Ocean Dynamics - The interactions between barotropic tides and mesoscale processes were studied using the results of a numerical model in which tidal forcing was turned on and off. The research... 相似文献