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This study introduces a new conceptual model to explain the recently observed changes in winter precipitation over Israel. The model is based on our earlier published work (where a connection was reported between the occurrence of...
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This study introduces a new conceptual model to explain the recently observed changes in winter precipitation over Israel. The model is based on our earlier published work (where a connection was reported between the occurrence of major rain days (MRDs) in different parts of the country and three prototypes, A, B and C, of the 500 hPa trough axis orientation prevailing on MRDs) and on additional results obtained by an extension of that work in the present paper. The first part of the present study is devoted to the extension of our early work. Composite techniques have been used on National Center for Atmospheric Research 9NCAR)-National Meteoreological Center grid-point data for the rain seasons 1981-82 to 1985-86 to identify the sea-level pressure (SLP) distribution associated with each of the three 500 hPa prototypes. Prototype A (trough axis oriented from northwest to southeast, earlier shown to be associated with MRDs in northern Israel) was found in the present work to be associated with a surface low in the vicinity of Antalya, southern Turkey. Prototype B (trough axis oriented from north to south, earlier shown to be associated with MRDs in central Israel) was found in the present work to be associated with a surface low over southeastern turkey. Prototype C (trough axis oriented from northeast to southwest, earlier shown to be associated with MRDs in southern Israel) was found to be associated with elevated surface pressure over northwestern Turkey and a trough over eastern Turkey. In the second part of the study we used our results to construct a conceptual model of the mechanism responsible for the relative increases in seasonal (winter) rainfall over the southern part of the country and the decrease over the north. Using National Centers for Environmental Prediction-NCAR reanalysis data for the period 1982-2000, we demonstrated that the direct atmospheric agent responsible for this change in the spatial rainfall distribution is an increased frequency of occurrence of 500 hPa troughs oriented from northeast to southwest (Prototype C) accompanied by prominent positive SLP anomalies centred over turkey. Our analysis further shows that these atmospheric systems are consistent with the persistence of a positive phase of the North Atlantic Oscillation on the one hand and with latest IPCC predictions of precipitation patterns over the eastern Mediterranean basin on the other hand.
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The Eastern Mediterranean (EM) Basin is exposed to frequent dust storms during all seasons but summer. The dust is originating from two large sources: a western source, namely North African (mainly Sahara Desert) and an eastern so...
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The Eastern Mediterranean (EM) Basin is exposed to frequent dust storms during all seasons but summer. The dust is originating from two large sources: a western source, namely North African (mainly Sahara Desert) and an eastern source, the Arabian Peninsula.The characteristics of 53 dust storms (2007–2013) were examined in relation to their origin – western (30) vs. eastern (23). Analysis was done using ground monitoring stations, satellite data and subjective synoptic classification. Significantly higher ground concentration (722 μg/m3for western events vs 242 μg/m3for eastern events), higher Aerosol Optical Thickness (AOT) levels (0.50 for western events vs 0.32 for eastern events), longer duration (25 h for western events vs 15 h for eastern events) and stronger surface wind speed (6 m/s for western events vs 4 m/s for eastern events) were found for western origin dust storms. Dust top height was higher for eastern origin events (1.7 km for western events vs 2.3 km for eastern events, non significant). The associated synoptic circulation types dominant during the western origin dust storms are winter cold-core cyclones while for the eastern origin events the Red Sea Trough is the dominant type and occur mainly during fall.The main assumed reasons for these distinguished characteristics (higher ground concentration, AOT levels, longer duration, and surface wind speed) are that western origin cyclones can induce both mobilization and transport of dust. Together with longer dust path and local sources contribution will create a potential for high concentration and long-lasting event as compared to eastern origin events where there is a separation between main mobilization process that occur during summer and the transport westwards necessary to deliver dust to the EM and occurs during non-summer events.
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The complex multiscale system circulation of the Levantine Eastern Mediterranean (EM) basin is analyzed by applying various spatiotemporal statistical methods on the AVISO 14 years (1993-2006) data set of satellite altimetry sea l...
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The complex multiscale system circulation of the Levantine Eastern Mediterranean (EM) basin is analyzed by applying various spatiotemporal statistical methods on the AVISO 14 years (1993-2006) data set of satellite altimetry sea level anomalies. The Rossby deformation radius at the EM is at the order of 10 km, which is also the horizontal satellite resolution. Therefore, the geostrophic currents, derived from the satellite altimetry measurements, represent well the basin and the mesoscale circulations. The long-term averaged mean dynamic topography (MDT) is found to capture the different eddy activity regions in the EM; however, these regions are highly turbulent. The instantaneous currents in these regions can be different from the MDT-derived currents, by the same order of magnitude. Furthermore, the turbulence intensity appears to be nonperiodic, despite of the seasonal and long-term steric periodic variability. Because these eddies interact with the basin circulation flow, they modify it continuously. The major exception of this sporadic behavior is the Ierapetra eddy, southeast of Crete. It appears to be forced by shear of the summer Etesian winds, created when the Crete mountain ridges block these winds and cause a funneling effect. It has a persistent seasonal signature that dominates the variance of the entire Levantine basin.
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The complex multiscale system circulation of the Levantine Eastern Mediterranean (EM) basin is analyzed by applying various spatiotemporal statistical methods on the AVISO 14 years (1993-2006) data set of satellite altimetry sea l...
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The complex multiscale system circulation of the Levantine Eastern Mediterranean (EM) basin is analyzed by applying various spatiotemporal statistical methods on the AVISO 14 years (1993-2006) data set of satellite altimetry sea level anomalies. The Rossby deformation radius at the EM is at the order of 10 km, which is also the horizontal satellite resolution. Therefore, the geostrophic currents, derived from the satellite altimetry measurements, represent well the basin and the mesoscale circulations. The long-term averaged mean dynamic topography (MDT) is found to capture the different eddy activity regions in the EM; however, these regions are highly turbulent. The instantaneous currents in these regions can be different from the MDT-derived currents, by the same order of magnitude. Furthermore, the turbulence intensity appears to be nonperiodic, despite of the seasonal and long-term steric periodic variability. Because these eddies interact with the basin circulation flow, they modify it continuously. The major exception of this sporadic behavior is the Ierapetra eddy, southeast of Crete. It appears to be forced by shear of the summer Etesian winds, created when the Crete mountain ridges block these winds and cause a funneling effect. It has a persistent seasonal signature that dominates the variance of the entire Levantine basin.
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摘要 :
The complex multiscale system circulation of the Levantine Eastern Mediterranean (EM) basin is analyzed by applying various spatiotemporal statistical methods on the AVISO 14 years (1993-2006) data set of satellite altimetry sea l...
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The complex multiscale system circulation of the Levantine Eastern Mediterranean (EM) basin is analyzed by applying various spatiotemporal statistical methods on the AVISO 14 years (1993-2006) data set of satellite altimetry sea level anomalies. The Rossby deformation radius at the EM is at the order of 10 km, which is also the horizontal satellite resolution. Therefore, the geostrophic currents, derived from the satellite altimetry measurements, represent well the basin and the mesoscale circulations. The long-term averaged mean dynamic topography (MDT) is found to capture the different eddy activity regions in the EM; however, these regions are highly turbulent. The instantaneous currents in these regions can be different from the MDT-derived currents, by the same order of magnitude. Furthermore, the turbulence intensity appears to be nonperiodic, despite of the seasonal and long-term steric periodic variability. Because these eddies interact with the basin circulation flow, they modify it continuously. The major exception of this sporadic behavior is the Ierapetra eddy, southeast of Crete. It appears to be forced by shear of the summer Etesian winds, created when the Crete mountain ridges block these winds and cause a funneling effect. It has a persistent seasonal signature that dominates the variance of the entire Levantine basin.
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Near-surface temperature difference (i.e., the difference between land surface skin and 2-m air temperatures-ΔT) drives numerous ecological and biophysical processes on Earth, constituting an essential parameter in process-based ...
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Near-surface temperature difference (i.e., the difference between land surface skin and 2-m air temperatures-ΔT) drives numerous ecological and biophysical processes on Earth, constituting an essential parameter in process-based Earth System Modeling. ΔT is known to be governed by factors like incoming solar radiation and wind, which vary according to synoptic-scale circulation via horizontal pressure gradient and cloud cover. ΔT is also affected by land surface characteristics and vegetation dynamics. Here we assess the role of seasonality, synoptic-scale circulation, and vegetation dynamics, using satellite normalized difference vegetation index (NDVI) and latent heat estimates, on the ΔT in the Eastern Mediterranean (EM). ΔT was calculated using land surface temperatures derived from the Moderate Resolution Imaging Spectroradiometer and air temperatures measured at 94 meteorological stations in the EM for 2006-2010. The effect of circulation on ΔT is demonstrated for four synoptic categories covering all seasons, showing that the influence of synoptic-scale circulation may be sometimes stronger than that of the seasonal signal in this region. ΔT showed negative relationships with NDVI indicating that vegetation is attenuating the near ground temperature difference, with a gradual effect increasing from southern drylands to the more humid northern vegetated areas in the EM. The relationship between ΔT and NDVI was stronger for specific synoptic classes than for seasonal division, implying the combined role of vegetation cover dynamics and synoptic-scale conditions on ΔT. Findings from this study show promise for continuous spatiotemporal estimations of ΔT from land surface temperature and NDVI satellite data.
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The scientific interest in Eastern Mediterranean (EMed) processes of major importance has been revived in recent years due to the predominance of internal variability manifested in a decadal time scale leading to the alternating a...
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The scientific interest in Eastern Mediterranean (EMed) processes of major importance has been revived in recent years due to the predominance of internal variability manifested in a decadal time scale leading to the alternating activation of the two dense water sources (i.e. the Adriatic and Aegean Seas). Analysis of available hydrographic data during the 2003-2012 period reveals an anticorrelated almost decadal oscillation in the thermohaline properties of the upper and intermediate water masses in both the Ionian and the Levantine/Aegean Seas. This event is the manifestation of an ongoing internal mechanism initially introduced by Theocharis et al. (in press) which periodically disturbs the upper thermohaline EMed conveyor belt and changes the respective water mass pathways, thus driving the alternating activation of the twodense water sources through salinity preconditioning. Since 2004-2005, the salinity of the upper/intermediate layer in the eastern part of the EMed gradually increased up to 2010, while at the same time it decreased in the North Ionian Sea. During the same period we observed the activation of the Aegean Sea as a Dense Water Formation area even though the atmospheric forcing conditions were not favorable. After 2010 the salinity trend reversed in both regions. This suggests that in the near future the salinity preconditioning of the Adriatic Sea will be favored again following the respective water mass pathway changes.
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The circulation of the southeastern Levantine Basin (Eastern Mediterranean) as determined from several oceanographic cruises carried out in April, May, and August 2001 and in March, May, and August 2002 was investigated within the...
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The circulation of the southeastern Levantine Basin (Eastern Mediterranean) as determined from several oceanographic cruises carried out in April, May, and August 2001 and in March, May, and August 2002 was investigated within the framework of the CYCLOPS experiment. The May cruises were part of CYCLOPS while the others, which provide supporting data, were part of the ongoing CYBO project. The May 2002 data provided the location of the eddy and physical description of the circulation, which were crucial for the in situ phosphate/SF_6 addition experiment of CYCLOPS. The major flow features observed in the region south of Cyprus displayed strong variability within the period of the observations. Throughout the entire study period, the warm core, anticyclonic eddy, known as the Cyprus eddy, was a dominant component of the circulation. During the 18 months covered by the cruises, the eddy drifted slowly to the southwest. At the end of the CYCLOPS cruises, the center of this persistent, recurrent eddy was found approximately 100-150 km west of its previously observed location since the 1980s. The mid-Mediterranean jet (MMJ), associated with the rim of this eddy, followed a complex path south of Cyprus depending upon the eddy's shape and location. During most of the study period the MMJ surface currents near the northern periphery of the rim of the eddy were as high as 35-45 cm/s. In May 2002, during the in situ experiment, the near surface seasonal thermocline inhibited downward mixing of the added tracers while the strong horizontal current around the rim of the clearly defined eddy core limited the lateral spreading and dilution of the patch. Finally, the westward shift of the Cyprus eddy allowed space for the generation of a secondary anticyclonic eddy in the eastern part of the domain. The co-existence of the two anticyclonic eddies during 2002 along with the development of a third anticyclonic eddy closer to the coast of Egypt provides strong evidence for the re-establishment of the previously observed, multi-pole, sub-basin scale Shikmona gyre.
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The Aegean Sea is a region of special interest for the Mediterranean oceanographic community, as one of the dense-water formation sites of the Mediterranean, driving its thermohaline circulation. Early oceanographic literature exh...
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The Aegean Sea is a region of special interest for the Mediterranean oceanographic community, as one of the dense-water formation sites of the Mediterranean, driving its thermohaline circulation. Early oceanographic literature exhibits significantly varying opinions regarding the role of the Aegean as a contributor to the water masses of the eastern Mediterranean. The higher temporal and spatial resolution studies that followed the introduction of Conductivity-Temperature-Depth (CTD) profilers in the 1980s, revealed that the various scenarios were within the interannual variability of dense water formation in the region. A peak in this variability was the appearance of the Eastern Mediterranean Transient event in the early 1990s. This phenomenon showed that the Aegean Sea has the potential to function as a source of dense water for the eastern Mediterranean; however, it takes over this role only sporadically, depending on the meteorological conditions over the eastern Mediterranean and, possibly, central/eastern Europe. The North Atlantic oscillation appears to be a contributor to this bimodal behaviour. Palaeoceanographic information has confirmed the large sensitivity of the Aegean Sea to climatic variability. Based on the available information, possible scenarios are examined for the response of the Aegean to the current climatic trends. Copyright (C) 2004 Royal Meteorological Society.
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The characteristics and interannual variability of the deep water masses in the North and Central Aegean Sea are being investigated through the data sets of the Hellenic Navy Hydrographic Service (HNHS) and the MEDATLAS 1997 proje...
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The characteristics and interannual variability of the deep water masses in the North and Central Aegean Sea are being investigated through the data sets of the Hellenic Navy Hydrographic Service (HNHS) and the MEDATLAS 1997 project. In the period between 1987 and 1993, the densest deep water in the Mediterranean has been produced in the Aegean Sea (with sigma(0) densities reaching up to 29.6 kg/m(3)), contributing to what has been called the Eastern Mediterranean Transient. The examination of time series of mean integrated values of theta, S and sigma(theta) below the depth of 500 dbar reveals the significant deep water density increase after 1987 in all of the deep basins in the area. Data suggest that the density increase of 1987-1988 is mainly attributed to a temperature drop, while in 1993, an even more intense density increase is observed, characterized this time by an abrupt salinity increase. We assume that the increased salinity necessary to produce deep water masses with the observed characteristics was not locally produced but rather advected from the Levantine through the South Aegean. After 1993, no new deep water formation episodes have been observed. A series of Theta-S diagrams derived from HNHS CTD casts covering the period between 1993 and 2000, depict the different characteristics of the deep water masses in the area. As 1993 marks the end of the formation period, observed differences between basins in that year must be attributed to different deep water formation sites. Thereafter, the stagnating deep water in the North and Central Aegean basins has been slowly gaining buoyancy by losing salt and gaining heat. The rate at which this phenomenon takes place varies between different deep basins. It is suggested that these variations are linked to the different volumes of each basin as well as to the general circulation features of the Aegean Sea. (C) 2004 Elsevier B.V. All rights reserved.
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