In this article we will discuss about:- 1. Location of Mediterranean Climate 2. Temperature of Mediterranean Climate 3. Air Pressure and Winds 4. Precipitation 5. Natural Vegetation.
Location of Mediterranean Climate:
The Mediterranean type of climates, climatically known as subtropical dry summer climates, is called Mediterranean type because most of the areas falling under this climate are situated around Mediterranean Sea.
The Mediterranean climate or biome is also called as sclerophyll ecosystem or biome because of the development of special features and characteristics in the dominant trees and shrubs to adapt to the typical environmental conditions of this climate-dry summer and wet winter.
Though the Mediterranean type of climate covers only 1.7 per cent area of the globe but this is most clearly defined climate and is easily differentiated from other climatic regions.
The Mediterranean climate has three distinct characteristics:
(i) Wet winter and dry summer season,
(ii) Warm and hot summers, and mild winters, and
(iii) Abundant sunshine throughout the year (90 per cent in summer and 50 to 60 per cent in winter).
This climate has developed between 30°- 40° (some-times upto 45°) latitudes in both the hemispheres in the western parts of the continents (fig. 39.5).
This climatic region includes (A) European, Asiatic and African lands bordering the Mediterranean Sea e.g., (1) European lands — Rhone- Saone Valleys of France, southern Italy, Greece; (2) Asiatic lands-western Turkey, Syria, Lebanon, western Israel; (3) north coastal Africa – Morocco, northern Algeria and Tunisia and the area north of Bengasi in Lybia, (B) central and southern California in the USA, (C) central Chile in South America, (D) the Capetown area of South Africa, and (E) the coastal zones of southern and south-western Australia.
The Mediterranean type of climate owes its origin to the seasonal shifting of wind and pressure belts due to northward (summer solstice) and southward migration (winter solstice) of the sun. Thus, these areas come under the domain of westerlies in winter season.
Since the westerlies are moisture laden because they come from over the oceans and are associated with temperate cyclones, they give sufficient rains during winter season. On the other hand, they come under the influence of subtropical high pressure belt and associated anticyclones during summer season and hence there is no rainfall.
Temperature of Mediterranean Climate:
The average temperature during cool winter season ranges between 5°C and 10°C whereas mean summer temperature varies from 20°C to 27°C and thus the mean annual range of temperature becomes 15°C to 17°C or even more. In fact, the Mediterranean climate is considered as a resort climate because of its pleasant and comfortable winter season.
The Mediterrancean climate whether having coastal or inland location generally records temperature above freezing point during winter season as the average temperature of the coldest month ranges between 4.4°C and 10°C. The mean January temperature is recorded as 7.7°C at Sacramento in California (USA), 6.1°C at Marseille (France), 12.8°C at Perth (Australia) and 6.6°C at Rome.
Sometimes, the temperature becomes so low at nights that frost occurs which is very much injurious to field crops. The valleys and depressions have freezing to subfreezing temperatures in winter and hence valleys are avoided for sensitive crops like citrus fruits which are grown on hill slopes. Los Angeles (California, USA), Naples (Italy) and Sacramento (California, USA) have recorded lowest temperatures of -2.2°C, -1.1°C and-8.3°C respectively.
It may be pointed out that occasional fall of temperature below freezing point is limited to a few minutes to a few hours only. Summer temperature rises above 26°C e.g., Red Bluff of Sacramento valley records 27.5°C whereas average temperatures during summer season are 26.6°C in European Mediterranean lands and 24°C in northwest Africa.
It may be mentioned that high summer temperatures are never uncomfortable due to low relative humidity. Daily maximum temperature in summer goes beyond 26°C. For example, the Great Valley of California records daily maximum summer temperatures ranging between 30°C and 32°C.
Red Bluff and Sacramento have recorded highest daily maximum temperatures of 45.5°C and 46°C respectively so far. The night temperature during summer season falls to 15.5°C, thus diurnal range of temperature becomes as high as 30°C. Daily and annual ranges increase from coastal areas to inland areas.
On the basis of temperature variations the Mediterranean climate is divided into 2 subtypes e.g., Csa and Csb. Csa climate, characterized by hot summer, has inland location whereas Csb climate of mild summer is located along the coastal margins of the continents. It may be pointed out that temperatures of the Mediterranean climatic regions except those around Mediterranean Sea are largely affected by cool oceanic currents e.g., Californian region by cool California current, Chilean region by cool Peru current, Cape Town region by Benguela currents etc.
Air Pressure and Winds of Mediterranean Climate:
In fact, the Mediterranean climate is the outcome of the seasonal shifting of pressure and wind belts. All the pressure belts except polar high pressure belt move northward from their normal positions during northern summer (summer solstice when the sun becomes vertical over the tropic of Cancer on June 21) and thus subtropical high pressure system extends over the regions of Mediterranean climate (30°N to 40°N) and anticyclonic conditions dominate the weather conditions causing subsidence of air from above, horizontal divergence of surface winds, stability of air resulting into dry conditions.
Sirocco local hot winds blow from Sahara northward picking red sands which are brought to Italy, Spain, southern France and Greece. Summer winds are generally hot and dry. These areas come under the domain of westerlies during winter season when the sun moves southward (northern winter).
The westerlies are associated with temperate cyclones originating in the middle latitudes. Since the westerlies are associated with temperate cyclones and come from over sea surfaces and hence being moist give sufficient rainfall in the coastal areas. Some local winds like Bora. Mistral (cold winds) etc. affect the local weather conditions of the European Mediterranean lands during winters.
Precipitation of Mediterranean Climate:
The mean annual rainfall ranges between 37 cm and 65 cm, the most portion of which (more than 75 per cent) is received during winter season mainly between December and March in the northern Hemisphere and between May to September in the southern hemisphere. The winter rainfall is received through cyclonic storms associated with moist westerlies.
The summer season is almost dry. Because of moderate to scanty rainfall the mediterranean climate is called as subhumid climate. The orographic rainfall is common in those coastal areas (e.g. California) which are backed by mountain ranges parallel to the coasts. Though winter season is quite wet but the sky is seldom overcast for longer duration in continuation and thus abundant sunshine is available even during wet winter season.
For example, Red Bluff (California, USA) has only 11, 12, 10 and 10 rainy days in December, January, February and March respectively. January is the rainiest month in San Bernardino and Los Angeles but there are only 7 rainy days. January being the coldest month of Red Bluff receives 11.5 cm of rainfall while July being the coldest month of Perth (Australia) receives 16 cm of rainfall.
All these indicate the intensity of rainfall during winter months in the Mediterranean climatic regions. Summer season is characterized by scanty rainfall, almost dry weather, clear skies, and bring sunshine. Besides temporal variation, there is also spatial variation of rainfall. Generally, the amount of rainfall increases from south (24 cm at San Deigo, 29 cm at Los Angeles) to north (58 cm at Sansfransisco) in California.
The seasonal regime of rainfall causes fluctuations in the soil-water and soil-moisture regimes during winter and summer seasons. The amount of soil- water increases during winter season because of winter and spring rainfall which is responsible for maximum growth in vegetation but dry summer season causes deficiency in the soil-water content because of loss of water and moisture due to increased evaporation and evapotranspiration because of substantial increase in temperature and of course due to general lack of rainfall during summer season.
Natural Vegetation of Mediterranean Climate:
Though the Mediterranean regions are widely scattered over different continents, there is more or less broad generalization in the overall structure and composition of vegetation community of all the regions of Mediterranean biome. The structure of Mediterranean vegetations is such that they can withstand the aridity of summer season.
Consequently, the leaves have developed sclerophyllous characteristics in that they are stiff and hard and the stems have thick barks. The Mediterranean vegetation community consists of a variety of sclerophyll plant formation classes which range from Mediterranean mixed evergreen forests (in the coastal lands immediately bordering the aseas and the oceans) to woodland, dwarf forest and scrubs.
The vegetation community is dominated by trees and shrubs. The shrubs are differently named in various parts of the Mediterranean biome on the basis of local names e.g., maquis or garigue in southern Europe, chaparral in California, fymbos or fymbosch in South Africa, and mallee scrub in Australia.
The xeromorphic structure such as thickened suiticles, grandular hairs, sunken stomata etc. enables the plants to withstand dry conditions. The sclerophyllous structure of plant leaves enables them to regulate the gaseous exchange according to the availability or scarcity of water during different seasons of the year.
The plants have also developed special types of root systems in accordance with the regional environmental conditions mainly the availability of moisture.
The European Mediterranean regions are characterized by multi-layered structural pattern of vegetation community consisting of:
(i) Topmost layer of evergreen and deciduous oak trees,
(ii) Middle layer of shrubs locally called as maquis or garigue, and
(iii) The ground layer of numerous herbaceous plants.
The Californian Mediterranean lands are characterized by the:
(i) Topmost layer of oak trees,
(ii) The middle layer of chaparrals, equivalent to European maquis, and
(iii) The ground layer of herbaceous plants and grasses.
The South African Mediterranean Biome is characterized by attractive flowering plants of numerous varieties e.g., Erica, Ereesia, Lobellia, Kniphofia species etc. The shrubs are locally called as fymbos. The Australian Mediterranean Biome is characterized by numerous species of eucalyptus.
Fire, both natural and man-included, is normally an annual occurrence in the Mediterranean climatic regions. Burning, mass clearance of natural vegetation for agricultural and commercial purposes, overgrazing of grasslands etc. have led to accelerated rate of soil erosion, increase in the silt load of major rivers and transformation of original natural vegetation.