The dominant features of the mean North Atlantic pressure pattern are the Icelandic Low and the Azores High. These are present at all seasons (see Figure 7.9), although their location and relative intensity change considerably. The upper flow in this sector undergoes little seasonal change in pattern, but the westerlies decrease in strength by over half from winter to summer. The other major pressure system influencing European climates is the Siberian winter anticyclone, the occurrence of which is intensified by the extensive winter snow cover and the marked continentality of Eurasia. Atlantic depressions frequently move towards the Norwegian or Mediterranean seas in winter, but if they travel due east they occlude and fill long before they can penetrate into the heart of Siberia. Thus the Siberian high pressure is quasi-permanent at this season, and when it extends westward severe conditions affect much of Europe. In summer, pressure is low over all of Asia and depressions from the Atlantic tend to follow a more zonal path. Although the storm tracks over Europe do not shift poleward in summer, the depressions at this season are less intense and reduced airmass contrasts produce weaker fronts.
Wind velocities over western Europe bear a strong relationship to the occurrence and movement of depres sions. The strongest winds occur on coasts exposed to the northwest airflow that follows the passage of frontal systems, or at constricted topographic locations that guide the movement of depressions or funnel airflow into them (Figure 10.1). For example, the Carcassonne Gap in southwest France provides a preferred southern route for depressions moving eastward from the
Source: From Troen and Petersen (1989).
Atlantic. The Rhône and Ebro valleys are funnels for strong airflow in the rear of depressions located in the western Mediterranean, generating the mistral and cierzo winds, respectively, in winter (see C.3, this chapter). Throughout western Europe, the mean velocity of winds on hilltops is at least 100 per cent greater than that in more sheltered locations. Winds in open terrain are on average 25 to 30 per cent stronger than in sheltered locations; and coastal wind velocities are at least 10 to 20 per cent less than those over adjacent seas (see Figure 10.1).
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