Environmental Engineering Reference
In-Depth Information
which are approximately 2° C above mean air temperatures. Some 10 per cent of
depressions enter the western Mediterranean from the Atlantic, and 20 per cent originate
from the Sahara. The remainder, however, form as Mediterranean depressions in the lee
of the Alps and Pyrenees from northerly cold and conditionally unstable air streams. The
warming of this mP or mA air gives intense instability, with high precipitation along the
warm front, and heavy showers and thunderstorms along the cold front. The boundary
between these Mediterranean depressions and cT air flowing north from the Sahara is
referred to as the Mediterranean Front. Depression tracks are complicated by relief
effects, and by the influences of other air streams entering the basin from outside. Winter
weather in the Mediterranean is variable, also, owing to the mobility of the Subtropical
Westerly Jet Stream, which can move northwards for long periods. When this happens
anticyclonic circulation is dominant, giving fine and settled weather, usually associated
with the positive phase of the North Atlantic Oscillation.
The fact that anticyclonic circulation can re-establish itself during the winter - for 25
per cent of the time over the whole Mediterranean, but for 50 per cent of the time
Figure 26.2
The distribution of evergreen oak and pine in the
Mediterranean basin.
Source: After Quezel (1985).
