Prolonged dry spells in the Levant region: climatologic-synoptic analysis

Abstract

Prolonged dry spells (PDSs) during the rainy season have severe environmental implications, including water shortage, damage to agriculture and increased potential for forest fires. This holds in particular for vulnerable regions, such as the Levant, 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 comprehensive study which identifies atmospheric patterns responsible for PDS occurrence on thousands of kilometres scale. A total of 178 PDSs, of >7 days, were found within the 62 seasons studied.A subjective inspection of upper-level geopotential height (GPH), sea-level pressure (SLP) and lower-level temperature anomalies point at three types, each associated with a definite climatic regime. The 'subtropical' type is associated with an expansion of the subtropical high over the majority of the Mediterranean, accompanied by northward migration of the Mediterranean cyclone track. The 'baroclinic', the most frequent type, is induced by a pronounced stagnant ridge over the eastern Mediterranean, being a part of Rossby wave, accompanied by a pronounced trough/cut-off low over the western Mediterranean. The 'polar' type results from intrusion of lower-level continental polar air associated with upper-level trough east of the Levant and blocking high over central Europe. Quantitative indices were derived for objective classification of the types, based on the climatic regimes defined subjectively, and the centers of action representing each.Composite maps for each type indicate substantial differences in the synoptic configuration and the factors explaining absence of rain. For the subtropical type, the dynamic factor of subsidence is dominant. For the polar, the thermodynamic factor of continental dry advection is dominant and for the baroclinic, both dynamic and thermodynamic factors are important.Classification of PDSs according to synoptic scenarios enables analysis of future changes in the occurrence and duration pattern of PDSs, using output of climate models.

Explaining Extreme Events of 2014 from a Climate Perspective

“…Uncertainties in the models' estimated forced response and the influence of intrinsic variability remain, due to limitations of climate models, uncertainties in the forcings, and (probably to a much lesser extent) uncertainties in the observed temperatures. It is hypothesized that droughts over the Levant region can be linked to three types of synoptic regimes (Saaroni et al 2014). The regimes include 1) an expansion of the subtropical high over the majority of the Mediterranean Basin, 2) a pronounced stagnant ridge/block, and 3) an intrusion of lower-level continental polar air.…”

Extreme dry spell detection and climatology over the Mediterranean Basin during the wet season

“…The high proportion of complex events in the northeast Mediterranean pattern means that more sectors are involved, thus increasing the number of grid points flagged 1. Saaroni et al [2015] noted that many very long drought events typically occur from September to October, and from March to April, in the Levant, and therefore proposed a "rainy season" from mid-November to mid-March. In contrast, the West Mediterranean pattern has only one complex event, but the VLDS events in this sector, centered over the Iberian Peninsula, generally affect a much larger region, extending from the Maghreb to Italy.…”

Convective rainfall in a dry climate: relations with synoptic systems and flash-flood generation in the Dead Sea region

“…Syed et al, 2003;Segond et al, 2007). Although antecedent soil-moisture conditions are known to play a role in runoff generation, studies conducted in semi-arid and arid areas ascribed them only a minor influence on flood response, due to low infiltration capacities of the ground, high evaporation rates (Ries et al, 2017;Syed et al, 2003;Yair and Lavee, 1985), and long dry spells between rainfall events (Saaroni et al, 2014). Hence, high-resolutions in space and time, and over large areas, are required to adequately represent rainfall spatio-temporal distributions.…”