| Popis: |
Arid climates are generally characterised by a negative water balance (i.e. rainfall amounts are lower than potential evaporation) for at least 10 months per year (see Fig. 2 in Introduction chapter, this volume). Contrary to the most hyper-arid areas (core regions of large continental deserts, e.g. the central Sahara or coastal deserts such as the Atacama), the desert margins are influenced by rainy seasons with a distinctive seasonality. Summer rainfall may show deep intrusions into the arid regions along their equatorward fringe, whereas the poleward margins receive winter rainfall when extratropical atmospheric circulation is intensified and becomes more widespread because of the large thermal and pressure contrast between subtropical and sub-polar latitudes at that time. The sand dune field of the north-western Negev is located at the contact zone of Mediterranean sub-humid to semiarid climates, and the hyper-arid climate of the southern Negev and Sinai, both part of the Saharo-Arabian desert belt. With its northern margin being only about 16 km from the Mediterranean Sea, the area under consideration shows a steep decline in rainfall totals, as interpolated from a few recording stations from north (coastal plain: 200 mm) to south (90 mm) over a horizontal distance of only 50 km. The decrease in rainfall is associated to the frequency of intrusion of Mediterranean cyclonic fronts into the area, depending on the individual track of the pressure system (west–east with more rainfall in the northern part; southwest–northeast with uniform rainfall distribution). In this way, the regional rainfall distribution is the most important macro-scale climatic parameter controlling the environmental gradient along the desert margin, such as in the sand dune field of the north-western Negev, while mesoand micro-scale features may show well-defined interaction with vegetation and relief. However, the dynamics of regional climate have to be understood as the major boundary condition for sub-scale ecological structures and processes, i.e. evapotranspiration. |
