Contemporary Climate

The Levant is affected by two climate systems. During winter, the westerlies bring in cyclonic low barometric pressures, causing cold air masses to arrive from the Atlantic and the North Sea. These travel over the relatively warm Mediterranean and become saturated by moisture, which is discharged as rain and snow. The rate of movement of the belts southwestward and, therefore, the number, intensity and duration of the rainstorms reaching the region, varies from year to year. When a belt of high pressure remains over the area, rainstorms are less abundant and the year is dry. Because of the configuration of the coastline of the southeastern edge of the Mediterranean Sea, the deserts of northern Egypt, Sinai, the Negev and southern Jordan lie outside the main path of rainstorms approaching from the west.

As can be seen from the multi-annual precipitation map (Fig. 1.1), precipitation usually declines to the south and the east. Yet the topography also has an influence. For example, the rift valleys are in the shadow of the rain coming from the sea and, therefore, are relatively arid, while the mountains receive more rain and snow in winter. The scarcity of rains and the high variance in rainfall from year to year become increasingly great as one goes farther into the desert. Rains in the desert, therefore, are characterized by scarcity and randomness.

Precipitation takes place during the winter months, from November to March. This is an advantage over other regions where rain falls in the summer. The temperatures during the winter are relatively low, which means that evaporation is also low. Consequently, the relative effect of the winter rains is rather high. The development of high-pressure systems often follows the low-pressure systems and causes clear and cold weather conditions. Many of the rainstorms, affected by a barometric low in the northern and central part of this region, enter the desert areas as smaller eddies on the margins of the bigger cone of low barometric pressure. They form small convective cells, a few to tens of kilometers in diameter. This causes rain to fall on a limited area around the center of the cell - other more peripheral areas may remain dry. Such a rainstorm may be of high intensity and last for only a few minutes, or it may continue for up to a few hours. Sometimes precipitation descends as hail, and it may snow at the higher elevations during a cold winter. Rainstorms may be preceded by a barometric high over the desert area. In this case, a flow of dry, hot air from the desert blows dust, which flows in the direction of the barometric low. In the autumn and spring, when dust storms are most abundant, the hot,

Fig. 1.1. Map of the Middle East showing the multi-annual precipitation (mm per year).

dry periods (known locally as khamsin) can come to an abrupt end with a heavy rainstorm. Most dust storms are connected with a barometric high over the continent and lows approaching from the sea.

The Mediterranean Sea acts as a gigantic temperature regulator, because of the high heat capacity of the water. As distance from the sea increases, the regulatory effect decreases. As a result, the temperature differences between day and night, as well as seasonal temperatures, are high. The influence of the Red Sea, the Dead Sea and the Persian Gulf, which are enclosed in narrow depressions, is limited to their very close vicinities. Thus, in the desert areas, the differences between day and night temperatures may reach 15 °C and in some extremes even 20 °C. In summer, the temperature can reach 40 °C during the day, while during the night it drops to about 25 °C. On a winter night, the temperature may fall below 0 °C, while during the day it may reach 20 °C.

Ambient air temperature increases in a directional pattern, similar to that of regional precipitation. (In northern Syria, the average temperature is 5 °C in January and 24 °C in August; in Beirut, it is 13 °C in January and 27 °C in August.)

For inhabitants of these areas, the severity of the high and low temperatures is compensated for by the dryness of the weather during most of the year. This relieves heat stress, since perspiration can evaporate. Humans will feel comparatively comfortable if not exposed to direct sun radiation. However, the dryness causes high evaporation rates from the surface of water bodies and high transpiration rates from vegetation.

During the summer, the weather is less variable, being affected by the semi-permanent surface heat trough centered over Iran and Iraq. This surface trough is coupled with an upper air high-pressure system, producing stable, hot and dry weather. During the autumn (mainly October to November), cool and moist air masses occasionally penetrate the region from the north and produce rainfall. Spring (mainly March to April) is characterized by frequent occurrences of khamsins and dust storms, although some rainfall may occur.

Overall, six main air masses, originating over the following areas, affect the weather over the Levant:

1. The Arctic Ocean;

2. The Atlantic Ocean, south and west of Iceland;

3. Northern Russia and Siberia;

4. Northern Russia, being modified while passing over the Volga-Ural basins;

5. The Atlantic Ocean south of the Azores;

6. The North African and Syrian-Arabian desert.

Air masses of the first four areas originate at high latitudes and are characterized by low temperatures and dryness. The masses acquire moisture as they pass over the Mediterranean Sea. The last two air masses originate at low latitudes and are characterized by high temperatures, and dryness, which they maintain.

Rainfall in the Middle East, on the whole, has an inverse correlation with temperature, except in areas under the influence of the summer rainfall regime (Crown, 1972). A synoptic analysis of excessive rainfalls in Israel (Amiran and Gilead, 1954) shows that they are the result of an influx of deep, moist and cold polar air into the eastern Mediterranean along meridian trajectories, which makes contact with the warm surface air in a Cyprus low. With the build-up of the Siberian anticyclone as winter progresses, this situation becomes less probable. There is less chance of a strong jet stream forming over central Europe and the Mediterranean that would feed sufficient air into such a rainfall-causing circulation system. Such excessive rains are, therefore, restricted to the beginning of the season, i.e., November or December.

Aridity in the Levant has three general causes (Otterman, 1974):

• separation of the region from oceanic moisture sources owing to distance or topography (rain shadow);

• the existence of dry stable air masses that resist convective currents;

• the absence of a course of events that cause convergence to create unstable air masses and provide the lifting necessary for precipitation.

Zangvil (1979) investigated the temporal fluctuations of seasonal precipitation in Jerusalem during the period 1946/47 to 1953/54. He employed time spectrum analysis and filtering techniques. A prominent peak appeared in the spectrum at a period of 3.0-3.3 years. (Rainfall oscillations in California also show a peak around 3 years.) The most prominent peak in the spectra occurred at 3.3 years at most of the East African stations. A more than average rainfall in east Africa during the main rain period of January to April is probably associated with a more intense Hadley circulation. This circulation causes strong westerlies in the same longitude, resulting in reduced rainfall in the eastern Mediterranean. Zangvil (1979) suggests that there is, perhaps, a connection between the El Nino southern oscillation (ENSO) and the rainfall in Jerusalem. The ENSO is a world-wide phenomenon, having a dominant period of 3 to 6 years, which corresponds to Jerusalem rainfall oscillations, the first peak at 3.03.3 years and the secondary one at 5 years. A similar observation for the eastern part of the Iberian peninsula was found by Rodo etal. (1997).

Analyses of the multi-annual trends of variation of precipitation (Alpert et al., 2002; Ben-Gai et al., 1998) have shown that, while there is a general decrease in the overall quantities of precipitation over the Mediterranean region, there is a trend for an increase in the number of rainstorms of high intensity and for either rainier or drier years within the average rainstorms and years.

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