| ABSTRACT | Prolonged dry spells (PDSs) that occur during the rainy season have severe environmental implications, including water shortage, damage to agriculture and ecosystems and increased potential for forest fires. This holds in particular for vulnerable regions, such as the Levant, which is already subjected to decrease in rainfall and lengthening of dry spells, in agreement with predictions of climatic models for the coming decades. This is the first extensive study of PDSs that defines the atmospheric patterns responsible for their occurrence, in a scale of thousands of kilometers. A total of 178 PDSs, of over 7 days, were found and analyzed in the months November-March during the 62 years of the study period.
A subjective inspection of upper-level GPH, SLP and lower-level temperature anomalies pointed at the existence of three PDS types, each associated with different climatic regimes. The 'subtropical', associated with an expansion of the subtropical high over the majority of the Mediterranean; the 'baroclinic', induced by a pronounced stagnant ridge, being a part of a Rossby wave, and the 'polar', resulting from an intrusion of lower-level continental polar air. Quantitative indices were derived for objectively classifying each of the PDS types and composite maps for each type indicated substantial differences in the synoptic configuration among them.
For the subtropical type the dynamic factor of subsidence is found the dominant in preventing rain. The baroclinic type is induced by both dynamic and thermodynamic factors, and its highest frequency, 2/3 of the PDSs, evidence that the Levant, in spite of its southern location, is still dominated by the mid-latitude regime, Rossby waves, in the rainy season even during long dry spells. The polar type develops while an upper-level blocking high is formed over eastern Europe, accompanied by a pronounced polar trough east of the Levant and lower-level continental polar intrusion into the entire Middle East. It is shown that the thermodynamic factor is the dominant for producing this type.
The classification of PDSs according to synoptic scenarios enables the analysis of future changes in the occurrence and duration of droughts through the output of climate models. |