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Isotopic composition of precipitation in relation to air circulation patterns in the Mediterranean basin

A research project on the isotopic composition of precipitation in the Mediterranean basin in relation to air circulation patterns and climate coordinated by the Isotope Hydrology Section of the International Atomic Energy Agency was initiated in 2001. The objectives of this research programme were the assessment of the variation of the isotopic values along the main directions west-east and south-north ; the correlation between these variations and the main climatic and meteorological parameters ; the determination of physical phenomena in compliant place and the refinement of a process-based understanding to fully realise the value of isotopic data in hydrological and climate studies. The thirteen Mediterranean countries participating to this research programme performed a monthly sampling of precipitation at about 80 stations. Daily or event-based precipitation sampling was also performed at 32 stations The sampling of the water vapour was done at ten stations during 48 hours starting Monday morning with about half-a-day interval. Stable isotopes analyses (O-18 and H-2) were performed for all samples. In order to reduce the number of isotopes analyses to be performed, two sampling periods were chosen for daily/event based and water vapour sampling ; October 2001 to March 2002 and October 2002 to March 2003. The minimum information collected at each station is the basic meteorological data (surface air temperature and rain amount). Synoptic weather maps are available for each event. Position of the inversion layer and back-trajectories for specific events have been also collected in various countries. A common database containing basic meteorological parameters,  18O and  2H was built. The spatial variation of the monthly isotope data was associated to the different air mass classes affecting the countries. A clear relation was noticed in most of the Mediterranean countries between the air masses origins and trajectories and the isotope contents in precipitation and water vapour. On a daily or event basis, a poor correlation between rain amount, air temperature and isotope data was observed at most of the stations. At a monthly basis and for the same stations, temperature and amount effects described in the literature were observed. That means that the daily-based sampling is giving additional information than the monthly sampling and isotope content variations are driven by other parameters. The origin and trajectory of air masses can be better-assessed using data from daily basis sampling. The sequential bases allow the determination of the importance of condensation temperature, evaporation and isotopic exchange. The western part of the Mediterranean basin has a d-excess of 14‰ whereas the eastern part shows an excess of +22‰ . This variation reflects a mixture between Mediterranean and Atlantic (d-excess=10‰ ) air masses. This evolution is not regular from west to east as it also depends on the origin of the air masses. For example, the d-excess in Tunisia is lower than the one observed in Southern part of France since the main origin of the Tunisian precipitation is the Atlantic one, circulating over the North African continent. The southern Mediterranean influence can be observed in Assekrem, located in the Ahaggar (Algeria) and mainly influenced by the Guinean monsoon air masses. The 18O/2H analyses of daily precipitation sampling and water vapour are giving valuable information for climate studies. The preliminary conclusions drawn from the regional scale interpretation of data obtained at various stations is discussed in the present article.

Stable isotopes; Atmospheric Moisture; Regional Climate; Air Masses Trajectories

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