Monitoring Agricultural Drought in the Near East - Monitoring and Predicting Agricultural Drought: A Global Study - page 212

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Line
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-3.1
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Norm
PgEn

Figure 16.3 Cumulative deviations of annual precipitation for three locations in Syria.

blocking episodes by mid-latitude high-pressure cells. The strength of the
blocking is, to a large extent, linked with the North Atlantic Oscillation
(NAO; Lamb and Peppler, 1987), a large-scale mode of climate variability
in the Northern Hemisphere at monthly, seasonal, interannual, and decadal
timescales. According to Hurrell (1995), the NAO is the dominant mode of
interannual-decadal climate variability for the Atlantic sector, accounting
for 20-60% of the variance over the last 150 years. A simple index of the
NAO is the sea-level air pressure difference between the Azores, an island
group in the Atlantic Ocean at about 38° northern latitude, and Iceland.
A positive NAO phase is characterized by the strengthened westerly winds
across the mid-latitude North Atlantic, leading to mild and relatively wet
winters in northern Europe (van Loon and Rogers, 1978) but to anoma-
lously dry conditions in the Iberian Peninsula (Zorita et al., 1992) and the
Maghreb countries (Lamb and Peppler, 1987). A negative NAO tends to be
linked with moist air in the Mediterranean and cold air in northern Europe.
The NAO index varies from year to year but tends to remain in one phase
for intervals lasting several years. Since the late 1970s, the winter NAO
index tends to be positive (Hurrell, 1995), while precipitation in subtrop-
ical northwestern Africa and the Iberian Peninsula is on a declining trend
(Ward et al., 1999).
In the eastern Mediterranean, both the Atlantic Ocean and the Mediter-
ranean Sea are the primary source regions for the formation of winter
precipitation in the form of mid-latitude cyclones (Turkes, 1996). These
migratory low-pressure systems have four primary cyclonic centers near
Crete, Cyprus, southern Italy, and the Gulf of Genoa. Cullen and de-
Menocal (2000) found a physical link between precipitation in the eastern

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