Geoscience Reference
In-Depth Information
is interposed between the temperate maritime
type and the arid subtropical desert climate.
The boundary between the temperate maritime
climate of Western Europe and that of the
Mediterranean may be delimited on the basis of
the seasonality of rainfall. However, another
diagnostic feature is the relatively sharp increase
in solar radiation across a zone running along
northern Spain, southeast France, northern Italy
and to the east of the Adriatic (
Figure 10.25
). The
Mediterranean regime is transitional in a special
way, because it is controlled by the westerlies in
winter and by the subtropical anticyclone in
summer. The seasonal change in position of the
subtropical high and the associated subtropical
westerly jet stream in the upper troposphere
are evident in
Figure 10.25
. The type region
is peculiarly distinctive, extending more than
3000km into the Eurasian continent. In addition,
the configuration of seas and peninsulas produces
great regional variety of weather and climate. The
Californian region, with similar conditions (see
Figure 10.20
), is of very limited extent, and
attention is therefore concentrated on the
Mediterranean basin itself.
The winter season sets in quite suddenly in the
Mediterranean as the summer eastward extension
of the Azores high pressure cell collapses. This
phenomenon may be observed on barographs
throughout the region, but particularly in the
western Mediterranean, where a sudden drop
in pressure occurs on about 20 October and is
accompanied by a marked increase in the
probability of precipitation. The probability of
receiving rain in any five-day period increases
dramatically from 50-70 percent in early October
to 90 percent in late October. This change is
associated with the first invasions by cold fronts,
although thundershower rain has been common
since August. The pronounced winter precipita-
tion over the Mediterranean largely results from
the relatively high sea surface temperatures during
that season, the sea temperature in January being
about 2
convective instability along the cold front,
producing frontal and orographic rain. Incursions
of arctic air are relatively infrequent (there being,
on average, six to nine invasions by cA and mA air
each year), but penetration by unstable mP air
is much more common. It typically gives rise to
deep cumulus development and is crucial in the
formation of Mediterranean depressions. The
initiation and movement of these depressions
(
Figure 10.26
) is associated with a branch of the
polar front jet stream located at about 35
N. This
jet develops during low index phases, when a
blocking anticyclone at about 20
°
W distorts the
westerlies over the eastern Atlantic. This leads to
a deep stream of arctic air flowing southward over
the British Isles and France.
Low pressure systems in the Mediterranean
have three main sources. Atlantic depressions
entering the western Mediterranean as surface
lows make up 9 percent and 17 percent form as
baroclinic waves south of the Atlas Mountains
(the so-called Saharan depressions; see
Figure
10.27
). The latter are important sources of rainfall
in late winter and spring). Fully 74 percent
develop in the western Mediterranean in the lee of
the Alps and Pyrenees (see
Chapter 7H.1
). The
combination of the lee effect and that of unstable
surface air over the western Mediterranean
explains the frequent formation of these
Genoa-
type depressions
whenever conditionally unstable
mP air invades the region. These depressions are
exceptional in that the instability of the air in the
warm sector gives unusually intense precipitation
along the warm front. The unstable mP air
produces heavy showers and thunderstorm rainfall
to the rear of the cold front, especially between 5
and 25
°
E. This warming of mP air produces air
designated as
Mediterranean
. The mean boundary
between this Mediterranean air mass and cT air
flowing northeastward from the Sahara is referred
to as the Mediterranean front (see
Figure 10.26
).
There may be a temperature discontinuity as great
as 12-16°C across it in late winter. Saharan
depressions and those from the western
Mediterranean move eastward, forming a belt of
°
C higher than the mean air temperature.
Incursions of colder air into the region lead to
°
