1,720,993 research outputs found
Impacts of the Nile River damming on the thermohaline circulation and water mass characteristics of the Mediterranean Sea
No full textThe long-term changes in the thermohaline circulation and water mass characteristics of the Mediterranean Sea caused by the damming of the Nile in 1964 are investigated using a 3-D primitive equation model (Princeton Ocean Model, POM). The model is first integrated for 65 years under climatological forcing and without taking into account the Nile runoff to obtain an initial steady state. Then the model is integrated for 75 years with the Nile outflow included until it reaches a steady state representative of that existing in the Mediterranean prior to the damming. Finally, the model is integrated for another 75 years eliminating the Nile runoff to reach a new steady state. The model results show that the absence of the Nile freshwater input induces a saltier surface layer in the vicinity of the Rhodes Gyre thus favoring the preconditioning for the formation of the Levantine Intermediate Water (LIW). This results in about 30% increase of the LIW formation rate. Intermediate waters become saltier, and as they are transported westward they reduce the stability of the water column in the deep-water formation sites, namely the South Adriatic and the Gulf of Lions. Thus saltier and larger amounts of dense waters are formed filling the deep parts of the Mediterranean Sea. According to the model, the Nile damming explains about 45% of the observed salinity increasing trend occurring over the last 40 years in the Western Mediterranean Deep Water (WMDW). Furthermore, model results demonstrate that the Nile damming played a crucial role in the long-term salt preconditioning of the surface/intermediate layers of the Cretan Sea, thus contributing in triggering the Eastern Mediterranean climatic transient in the late 1980s
Hydrological changes in the Mediterranean Sea in relation to changes in the freshwater budget: A numerical modelling study
No full textThe response of the Mediterranean Sea and the various sub-basins to changes in the freshwater budget are investigated in a process-oriented study, using the POM model. The model is first integrated using values of the Nile and Ebro rivers runoff, as well as of the Dardanelles freshwater input, typical of the fifties. The model reaches a steady state representative of that existing in the Mediterranean prior to the major damming period after 90 years of integration. Then the model is integrated using the reduced river runoff values typical of the after-damming period. The additional impact of decadal scale trends in the precipitation rate as well as of intense surface cooling periods/events on the thermohaline circulation during the last 40 years were also examined. The model results show that the dramatic reduction of the Nile freshwater input and to a lesser extent the reduction of the freshwater input from the Dardanelles Straits induced a large increase in the sea surface salinity in the Aegean and Levantine basins in the late sixties/early seventies, in agreement with observations. Furthermore, the Ebro runoff reduction during the same period further enhanced the salinity increase in the Levantine basin as higher salinity surface waters of the western basin reached the eastern basin via the Atlantic Water circulation. This saltier surface layer in the vicinity of the Rhodes Gyre favoured the preconditioning for the formation of the Levantine Intermediate Water, resulting in about 40% increase of its formation rate. This in turn resulted in the production of saltier and larger amounts of deep waters in the various deep-water formation sites. According to the model, the river damming and decreased precipitation since the eighties explain about 95% of the observed salinity increase in the Western Mediterranean Deep Water over the last 40 years. The major contributor to this increase was proved to be the Nile damming. The salt increase in the surface layer is proved to be insufficient to produce alone the two climatic transient events in the deep waters of the Eastern Mediterranean in the late sixties and early nineties, respectively. Surface cooling was found to be important, resulting in large deep water formation and thus allowing the propagation of the increased surface salinity signal to the deep layers. However, model results demonstrate that the river damming played an important role in the long-term salt preconditioning of the surface/intermediate layers, thus contributing in triggering the two events
Inter-annual/decadal variability of north Aegean Sea hydrodynamics over 1960-2000
No full textResults from a high-resolution hindcast model experiment, supported by available observations, reveal an increasing salinity trend in the north Aegean during the Eastern Mediterranean Transient (EMT) period, largely controlled by increases in the flow rate and salinity of water masses of Levantine origin entering the domain through the Myconos-Ikaria strait as a response to an acceleration of the Aegean thermohaline cell. Changes in the Dardanelles inflow (increasing salinity) and in the surface freshwater flux (increasing Evaporation-Precipitation), although both contributing to a higher salt content of the basin during the EMT, play a minor role in the inter-annual/decadal variability of the freshwater budget. A long-term decreasing temperature trend is observed from the 1960s to the early 1990s. It is superimposed on the salinity-preconditioning phase over the 1980s and early 1990s. Both signals are, concomitantly, favouring conditions for intense Dense Water Formation (DWF) in the north Aegean Sea. In addition, the northward displacement of the Black Sea Water front over the EMT, leads to the expansion of convective cells towards the north and to higher formation rates associated with both colder and saltier surface waters
Satellite-derived variability of the Aegean Sea ecohydrodynamics
No full textEight years of AVHRR-derived sea surface temperature (SST) and SeaWiFS-derived surface chlorophyll (Chl) data (1998–2005) are used to investigate key processes affecting the spatial and temporal variability of the two parameters in the Aegean Sea. Seasonal mean SST and Chl maps are constructed using daily data to study seasonal dynamics whereas empirical orthogonal function (EOF) and correlational analysis is applied to the 8-day composite SST and Chl anomaly time-series in order to study the variability and co-variability of the two parameters from subseasonal to interannual time-scales. The seasonal mean fields show that Black Sea cold and chlorophyll-rich waters enter through the Dardanelles Strait and they are accumulated in the north-eastern part of the Aegean Sea, steered by the Samothraki anticyclone. Large chlorophyll concentrations are encountered in the hydrological front off the Dardanelles Strait as well as in coastal areas affected by large riverine/anthropogenic nutrient loads. The SST seasonal mean patterns reveal strong cooling that is associated with upwelling along the eastern boundary of the basin during summer due to strong northerly winds, a process which is not present in the surface chlorophyll climatology. The Chl dataset presents much stronger sub-seasonal variability than SST, with large variations in the phase and strength of the phytoplankton seasonal cycles. EOF analysis of the anomaly time-series shows that SST non-seasonal variability is controlled by synoptic weather variations and anomalies in the north–south wind-stress component regulating the summer coastal upwelling regime. Mean SST and Chl patterns, and their associated variations, are not closely linked implying that Black Sea and riverine inputs mainly control the intra-annual and interannual variability of the surface chlorophyll in the Aegean Sea rather than mixing and/or upwelling processes
Plankton dynamics controlled by hydrodynamic processes near a submarine canyon off NW corsican coast: A numerical modelling study
No full textpeer reviewedA three-dimensional (3D) non-linear high-resolution hydrodynamic model coupled to a coastal plankton ecosystem model is used to estimate the impact of hydrodynamic processes on the evolution of the spring phytoplankton bloom in the vicinity of a submarine canyon. Model results for the plankton distribution showed a clear 3D character around and in the canyon, with large horizontal and vertical gradients, induced by the hydrodynamic constraints. Phytoplankton concentrations were significantly larger all along the slope domain with maximum values obtained over the canyon. Upwelling of deep water rich in nitrate takes place both upstream (with respect to the current direction normal to the central axis of the canyon) and downstream of the canyon enhancing primary production. As phytoplankton-rich water enters into the western part of the canyon it is downwelled and trapped by the cyclonic circulation leading to accumulation of phytoplankton biomass there. The effect of wind events was to induce an upward nitrate flux into the upper layer through vertical turbulent diffusion, allowing the start of a short-live phytoplankton bloom. Maximum surface nitrate concentrations were found along the slope and particularly upstream and downstream of the canyon just after the wind stopped. Enhanced turbulent diffusion combined with upwelling motion in these areas resulted in larger upward nitrate transports, further enhancing primary production. (c) 2006 Elsevier Ltd. All rights reserved
Effects of extreme meteorological conditions on coastal dynamics near a submarine canyon
No full textpeer reviewedA 3-D hydrodynamic model is applied to assess shelf/slope exchanges in the Calvi Canyon region (Corsica, NW Mediterranean) during the violent storm that affected the Western Europe in December 1999. Simulations are carried out using high-frequency sampling meteorological data to take into account the short-term variability of the atmospheric conditions. It is shown that the combined effects of canyon topography and of the wind forcing during the storm are responsible for a large increase of both cross-shore and vertical transports in the area. Strong downwelling motion is simulated all along the continental slope with vertical velocities up to 2cm s(-1) within the canyon. High turbulent diffusion levels are obtained leading to the complete mixing of the water column within the canyon. Results suggest that increased turbulent diffusion and downwelling circulation in the canyon during the storm should result in a large transport of coastal water towards the abyssal plain. (C) 2004 Elsevier Ltd. All rights reserved
Water fluxes at an ocean margin in the presence of a submarine canyon
Full text linkpeer reviewedA 3-D, unsteady, nonlinear, high-resolution model is used to estimate shelf/slope exchanges through Calvi Canyon (NW Corsica, Mediterranean Sea) in various regimes of stratification and wind patterns. To evaluate the alongshore and cross-shore fluxes within the canyon area as well as the water exchanges between the canyon and Calvi Bay, volume transports are computed at the sides of two closed, interconnected boxes encompassing the canyon on the shelf and slope domains. Model results show that water transports between Calvi Bay and the open sea are determined by flow modifications in the canyon area. The mean horizontal flow deviates southwestward upstream of the canyon, generating an onshore transport in the western part of Calvi Bay. Within the canyon, the circulation is cyclonic and is responsible for an offshore transport downstream of the canyon and in the eastern part of the bay. The effect of stratification is shown to limit the vertical extent of the influence of canyon topography so that the alongshore flow above the canyon is quasi-undisturbed in strong stratified conditions, resulting in weak cross-shore exchange. Wind events are shown to be responsible for a strong increase of cross-shore transports between the bay and the canyon area. (C) 2002 Elsevier Science B.V All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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