The Mediterranean

The characteristic west coast climate of the subtropics is the Mediterranean type with hot, dry summers and mild, relatively wet winters. It 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 can 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 3000 km into the Eurasian continent. In addition, the

Global Mediterranean Type Climate

Figure 10.25 Average annual means of daily global irradiation on a horizontal surface (kWh/m-2) for western and central Europe calculated for the period 1966 to 1975. 10-year means of monthly means of daily sums, together with standard deviations (shaded band), are also shown for selected stations.

Source: Palz (1984). Reproduced by permission of the Directorate-General, Science, Research and Development, European Commission, Brussels, and W. Palz.

Figure 10.25 Average annual means of daily global irradiation on a horizontal surface (kWh/m-2) for western and central Europe calculated for the period 1966 to 1975. 10-year means of monthly means of daily sums, together with standard deviations (shaded band), are also shown for selected stations.

Source: Palz (1984). Reproduced by permission of the Directorate-General, Science, Research and Development, European Commission, Brussels, and W. Palz.

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 to 70 per cent in early October to 90 per cent 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 precipitation over the Mediterranean results largely from the relatively high sea-surface temperatures at that season, the sea temperature in January being about 2°C higher than the mean air temperature. Incursions of colder air into the region lead to 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 the westerlies over the eastern Atlantic are distorted by a blocking anticyclone at about 20°W. 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 per cent and 17 per cent 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 per cent develop in the western Mediterranean in the lee of the Alps and Pyrenees (see Chapter 9H.1). The combination of the lee effect and that of unstable surface air over the western Mediterranean explains the frequent

Mediterranean Weather Trough

Figure 10.26 The distribution of surface pressure, winds and precipitation for the Mediterranean and North Africa during January and July. The average positions of the subtropical westerly and tropical easterly jet streams, together with the monsoon trough (MT), the Mediterranean front (MF) and the Zaire air boundary (ZAB), are also shown.

Source: Partly after Weather in the Mediterranean (HMSO, 1962) (Crown Copyright Reserved).

Figure 10.26 The distribution of surface pressure, winds and precipitation for the Mediterranean and North Africa during January and July. The average positions of the subtropical westerly and tropical easterly jet streams, together with the monsoon trough (MT), the Mediterranean front (MF) and the Zaire air boundary (ZAB), are also shown.

Source: Partly after Weather in the Mediterranean (HMSO, 1962) (Crown Copyright Reserved).

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 airmass 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 to 16°C across it in late winter. Saharan depressions and those from the western Mediterranean move eastward, forming a belt of low pressure associated with this frontal zone and frequently drawing cT northward ahead of the cold front as the warm, dust-laden scirocco (especially in spring and autumn when Saharan air may spread into Europe). The movement of Mediterranean depressions is modified both by relief effects and by their regeneration in the eastern Mediterranean through fresh cP air from Russia or southeast Europe. Although many lows pass eastward into Asia, there is a strong tendency for others to move northeastward over the Black Sea and Balkans, especially as spring advances. Winter weather in the Mediterranean is quite variable as the subtropical westerly jet stream is highly mobile and may occasionally coalesce with the southward-displaced polar front jet stream.

With high index zonal circulation over the Atlantic and Europe, depressions may pass far enough to the north that their cold-sector air does not reach the Mediterranean, and then the weather there is generally settled and fine. Between October and April, anticyclones are the dominant circulation type for at least 25 per cent of the time over the whole Mediterranean area and in the western basin for 48 per cent of the time. This

Mediterranean Weather Trough
Figure 10.27 Tracks of Mediterranean depressions, showing average annual frequencies, together with airmass sources. Source: After Weather in the Mediterranean (HMSO, 1962) (Crown Copyright Reserved).

is reflected in the high mean pressure over the latter area in January (see Figure 10.26). Consequently, although the winter half-year is the rainy period, there are rather few rain-days. On average, rain falls on only six days per month during winter in northern Libya and southeast Spain; there are twelve rain-days per month in western Italy, the western Balkan Peninsula and the Cyprus area. The higher frequencies (and totals) are related to the areas of cyclogenesis and to the windward sides of peninsulas.

Regional winds are also related to the meteorological and topographic factors. The familiar cold, northerly winds of the Gulf of Lions (the mistral), which are associated with northerly mP airflow, are best developed when a depression is forming in the Gulf of Genoa east of a high-pressure ridge from the Azores anticyclone. Katabatic and funnelling effects strengthen the flow in the Rhône valley and similar localities, so that violent winds are sometimes recorded. The mistral may last for several days until the outbreak of polar or continental air ceases. The frequency of these winds depends on their definition. The average frequency of strong mistrals in the south of France is shown in Table 10.3 (based on occurrence at one or more stations from Perpignan to the Rhône in 1924 to 1927). Similar winds may occur along the Catalan coast of Spain (the tramontana, see Figure 10.28) and also in the northern Adriatic (the bora) and northern Aegean Seas when polar air flows southward in the rear of an eastward-moving depression and is forced over the mountains (cf. Chapter 6C.1). In Spain, cold, dry northerly winds occur in several different regions. Figure 10.28 shows the galerna of the north coast and the cierzo of the Ebro valley.

The generally wet, windy and mild winter season in the Mediterranean is succeeded by a long indecisive spring lasting from March to May, with many false starts of summer weather. The spring period, like that of early autumn, is especially unpredictable. In March 1966, a trough moving across the eastern Mediterranean, preceded by a warm southerly khamsin and followed by a northerly airstream, brought up to 70 mm of rain in only four hours to an area of the southern Negev Desert. Although April is normally a dry month in the eastern Mediterranean, Cyprus having an average of only three days with 1 mm of rainfall or more, high rainfalls can occur, as in April 1971 when four depressions affected the region. Two of these were Saharan depressions moving eastward beneath the zone of diffluence on the cold side of a westerly jet and the other two were intensified in the lee of Cyprus. The rather rapid collapse of the Eurasian high-pressure cell in April, together with the discontinuous northward and eastward extension of the Azores anticyclone, encourages the northward displacement of depressions. Even if higher latitude air does penetrate south to the Mediterranean, the sea surface there is relatively cool and the air is more stable than during the winter.

By mid-June, the Mediterranean basin is dominated by the expanded Azores anticyclone to the west, while to the south the mean pressure field shows a low-pressure trough extending across the Sahara from southern Asia (see Figure 10.26). The winds are predominantly northerly (e.g. the etesians of the Aegean) and represent an eastward continuation of the northeasterly trades. Locally, sea breezes reinforce these winds, but on the Levant Coast they cause surface southwesterlies. Land and sea breezes, involving air up to 1500-m deep, largely condition the day-to-day weather of many parts of the North African coast. Depressions are by no means absent in the summer months, but they are usually weak. The anticyclonic character of the large-scale circulation encourages subsidence, and airmass contrasts are much reduced compared with winter. Thermal lows form from time to time over Iberia and Anatolia, although thundery outbreaks are infrequent due to the low relative humidity.

The most important regional winds in summer are of continental tropical origin. There are a variety of local names for these usually hot, dry and dusty airstreams -scirocco (Algeria and the Levant), lebeche (southeast

Table 10.3 Number of days with a strong mistral in the south of France.

> 11 m s-1 (21 kt) 10 9 13 11 8 9 9 7 5 5 7 10 103

> 17 m s-l(33 kt) 4 4 6 5 3 2 0.6 1 0.6 0 0 4 30

Source: After Weather in the Mediterranean (HMSO, 1962).

Anticyclone Azores Pressure Profiles

Figure 10.28 Areas affected by the major regional winds in Spain as a function of season.

Source: From Tout and Kemp (1985), by permission of the Royal Meteorological Society.

Figure 10.28 Areas affected by the major regional winds in Spain as a function of season.

Source: From Tout and Kemp (1985), by permission of the Royal Meteorological Society.

Spain) and khamsin (Egypt) - which move northward ahead of eastward-moving depressions. In the Negev, the onset of an easterly khamsin may cause the relative humidity to drop to less than 10 per cent and temperatures to rise to as much as 48°C. In southern Spain, the easterly solano brings hot, humid weather to Andalucia in the summer half-year, whereas the coastal levante -which has a long fetch over the Mediterranean - is moist and somewhat cooler (see Figure 10.28). Such regional winds occur when the Azores high extends over western Europe with a low-pressure system to the south.

Many stations in the Mediterranean receive only a few millimetres of rainfall in at least one summer month, yet the seasonal distribution does not conform to the pattern of simple winter maximum over the whole of the Mediterranean basin. Figure 10.29 shows that this is found in the eastern and central Mediterranean, whereas Spain, southern France, northern Italy and the northern Balkans have more complicated profiles with a maximum in autumn or peaks in both spring and autumn. This double maximum may be interpreted as a transition between the continental interior type with summer maximum and the Mediterranean type with winter maximum. A similar transition region occurs in the southwestern United States (see Figure 10.20), but local topography in this intermontane zone introduces irregularities into the regimes.

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Responses

  • kaitlin
    Is europe having a low pressure system?
    8 years ago
  • belba
    What kind of weather does the mediterranean have between april and october?
    6 years ago
  • wilimar
    What causes both the summer and winter patterns of mediterranean climate?
    5 years ago
  • Fiorenzo
    What is the difference of climate and weather mediterranean?
    8 months ago
  • harry
    Is there any major probelms withthe weather mediterranean sea usually?
    2 months ago

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