An upper level atmospheric teleconnection between grid points: 0 , 55 N; 10 E, 55 N (North Sea) and 50 E, 45 N; 60 E, 45 N (northern Caspian) was identi®ed. This teleconnection, referred as the North Sea-Caspian Pattern (NCP) is evident at the 500 hPa level. The NCP is more pronounced during winter and the transitional seasons. An index (NCPI) measures the geopotential heights differences between the two poles of the NCP. Time series of the NCPI are presented and analysed. Except for September, no signi®cant temporal trends were found. Negative and positive phases of the NCP (NCP(À) and NCP( ), respectively) were de®ned using standardized scores. A classi®cation of all months into NCP(À), NCP( ) or normal conditions during the analysis period (1958±1998) was prepared and analysed. No signi®cant correlation was found between the NCPI and the NAO index. The anomalous circulation during either NCP(À) or NCP( ) conditions is de®ned and its possible impact on the regional climate is discussed. Preliminary results show below normal temperatures and above normal precipitation in the Balkans and the Middle East during NCP( ), and the opposite for NCP(À).
ABSTRACT:The interannual variations and the spatial distribution of rainfall in the Mediterranean and semi-arid regions of Israel are analysed with respect to variations in the occurrence of the typical synoptic systems of the Eastern Mediterranean. The synoptic analysis is based on a daily, semi-objective synoptic classification (Alpert et al., 2004a). The study covers the months November-March, in which 90% of the annual rainfall is obtained, mostly resulting from Cyprus lows. The interannual variations of the rainfall are well explained by the synoptic types, and the occurrences of Cyprus lows are highly correlated with the rainfall. It was found that the daily and seasonal rainfall are highly dependent on the depth of the cyclone. Moreover, deep lows are more effective for the mountainous regions, due both to the enhanced orographic effect and to the fact that stronger winds, associated with deep lows, are more efficient in transporting rain-producing clouds from the Mediterranean Sea inland. The location of the cyclone determines the spatial distribution of the rain it produces over Israel. The cyclones located east of Cyprus were found productive mainly for the southern parts of the study region, while those located to the west and north of Israel were found productive for the north of the country. The high sensitivity of the rainfall to the location of the surface cyclones emphasizes the major role that lower level moisture transport plays in rain formation.
A large body of data has been used to study the distribution of instantaneous rainfall intensity in Israel and its variations in space and in time. Time resolution is k3 minutes. Besides the direct uses that the numerical results may have for practical purposes, the results have been used here to characterize rainfall in a climatologically more meaningful way. This includes reference to the atmospheric processes and surface-conditioned effects that influence rainfall formation, and the identification of regional and seasonal variations in these processes. Thus, it has been shown that contrary to prevalent views, orographic rainfall in Israel is characterized by low intensity, apparently resulting from a continuous advection of moist air through a zone of gradual topographically forced uplifting.In the presentation of results, emphasis has been put also on the significance of results from the point of view of rain-conditioned processes occurring on the earth surface, such as in hydrology, geomorphology, etc. In this context, the water yield from high intensity rainfall has been shown to reflect a high degree of concentration, both areally (through convergence in the atmosphere and run-off processes on the ground surface) and in time (in warm seasons and in certain parts of the day). These features, as well as the relative frequency of high-intensity rains itself, has been shown to be more pronounced in the arid Negev than in the more humid parts of the country. This explains at least in part the increased uncertainty characterizing rainfall in the arid zone.KEY WORDS Rainfall intensity Regional and seasonal variations of rainfall intensity Orographic effect on rainfall Arid zone rainfallRainfall intensity is one of the three major features used to characterize rainfall, along with its accumulated depth (in mm) and its duration or frequency (in hours or days per year). However, as it is a more complex variable than depth or duration, standard rainfall statistics rarely contain information on intensity even where appropriate observations are available. On the other hand, intensity is in many ways a more meaningful characteristic of rainfall than the other parameters. From the general climatological point of view, rainfall intensity is meaningful in terms of the rainfall types prevalent in given regions and/or seasons and of the synoptic and physical processes leading to it. In addition, intensity determines the effectiveness of rainfall from the point of view of hydrological, erosional and biotic processes taking place on the earth surface on various scales, and its contribution to the water budget in various domains, e.g. the soil profile, an ecological niche, a watershed, etc. Thus, detailed information on rainfall intensities is useful in a number of fields in the earth sciences.
The mean synoptic situation associated with dust outbreaks from the Sahara into the central Mediterranean was examined on a daily basis for the month of July from 1979 to 1992. Composite patterns of wind, geopotential heights, and temperature for dusty days versus those for all days were analyzed. Dusty days were defined as days with the Total Ozone Mapping Spectrometer Aerosol Index (TOMS‐AI) in the area around the Apennine peninsula (36°N–46°N, 10°E–18°E) equal to or greater than their monthly average plus 1 standard deviation. It was found that the strength and position of two essential features of the circulation patterns, such as the trough emanating southward from the Icelandic low and the eastern cell of the subtropical high, are the governing factors in making suitable flows for the Saharan dust transportation toward Italy. The deep, well‐developed trough near the Atlantic coasts of Europe and Africa, penetrating well to the south, and the strong eastern cell of the subtropical high situated to the northeast from North Africa near the Mediterranean coast, cause strong south‐southwestern flows with the potential to carry dust northward into the Mediterranean. In extreme cases the dust can reach Europe north of the Alps and even northern Europe, reaching the shores of the Baltic. These warm flows, accompanied by high dust load, also cause considerable warming in the central Mediterranean region of the order of 6–8 K at 700 hPa. Alternatively, the weak western trough and the weak eastern subtropical cell cause westerlies, which are inconsistent with the Mediterranean dust intrusions. Analysis of the extreme intrusion cases in July 1988, based on TOMS‐AI data, and several others in July 2001–2003, based on lidar measurements in Rome, demonstrates the synoptic situation that allows the Saharan dust to reach Italy.
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