A Mediterranean climate is a climate that resembles the climate of the lands bordering the Mediterranean Sea. Outside the Mediterranean, this climate covers relatively small areas of the Earth, and generally occurs on the western coasts of continental landmasses, roughly between latitudes 30° to 45° north and south of the equator.
Location
Besides the Mediterranean Basin, regions which have a Mediterranean climate include much of California and non-coastal southern Oregon (most areas between the Umpqua River and San Diego), the Western Cape in South Africa, central Chile, southern Western Australia and the coastal areas of central and south-east South Australia.
Isolated examples of microclimates which approximate the Mediterranean climate may occur outside the Mediterranean climate zones, but these generally are a result of localized processes that are not characteristic of the Mediterranean climate regime. For example, in the Pacific Northwest of North America, the dry summers typical of a Mediterranean climate continue northward all the way into parts of southern British Columbia, although the overall temperature ranges and vegetation in these areas are more typical of Oceanic climate zones.
Mediterranean climates on land tend to grade off poleward toward zones of Oceanic climate in which summer rains are significant, and equatorward into dry-summer steppes where the winter rains are inadequate (an example of such a steppe climate is to be found immediately south and east of San Diego, California). However, in California, the poleward transition involves cooler temperatures and more total precipitation than in other Mediterranean zones.
Perth, Australia has what might be regarded as a typical Mediterranean climate. Hot, dry summers (an average daily maximum temperature is 30°C/86°F, with 34 mm/1.33 inches of rain in December-February,[1]) and cool, wet winters (an average daily minimum of 8°C/46°F and 450 mm/17.7 inches of rainfall in June-August.[2])
The Mediterranean climate is associated with the five large subtropical high pressure cells of the oceans, the Azores High, South Atlantic High, North Pacific High, South Pacific High, and Indian Ocean High. These high pressure cells shift polarward in the summer and equatorward in the winter, playing a major role in the formation of the world's tropical deserts and the zones of Mediterranean climate polarward of the deserts. For example, the Azores High is associated with the Sahara Desert and the Mediterranean Basin's climate. The South Atlantic High is similarly associated with the Namib Desert and the Mediterranean climate of the Western Cape of South Africa. The North Pacific High is related to the Sonoran Desert and California's climate, while the South Pacific High is related to the Atacama Desert and central Chile's climate, and the Indian Ocean High is related to the deserts of western Australia (Great Sandy Desert, Great Victoria Desert, and Gibson Desert) and the Mediterranean climate of southwest and south-central Australia.[3]
The Mediterranean forests, woodlands, and shrub biome is closely associated with Mediterranean climate zones. Particularly distinctive of the climate are sclerophyll shrublands, called maquis in the Mediterranean Basin, chaparral in California, matorral in Chile, fynbos in South Africa, and mallee and kwongan shrublands in Australia.
Rainfall
During summer, regions of Mediterranean climate are dominated by subtropical high pressure cells, with dry sinking air capping a surface marine layer of varying humidity but making rainfall impossible or unlikely but for the odd thunderstorm, while during winter the polar jet stream and associated periodic storms reach into the lower latitudes of the Mediterranean zones, bringing rain, with snow at higher elevations. As a result, areas with this climate receive almost all of their yearly rainfall during the winter season, and may go anywhere from 2-5 months during the summer without having any significant precipitation.
Toward the equatorial end, winter precipitation diminishes. Toward the polar end, total moisture usually increases; in Europe there is more summer rain further north while along the American west coast the winters become more intensely wet and the dry seasons shorter as one moves north.
Temperature
All regions with Mediterranean climates have relatively mild winters, but summer temperatures are variable depending on the region. For instance, Athens, Greece experiences rather high temperatures in the summer (48.0 ºC has been measured in Eleusina), whereas San Francisco has cool, mild summers due to the upwelling of cold subsurface waters along the coast. Because all regions with a Mediterranean climate are near large bodies of water, temperatures are generally moderate with a comparatively small range of temperatures between the winter low and summer high (although the daily range of temperatures during the summer is large, except along the immediate coasts due to dry and clear conditions). Temperatures during winter only occasionally reach freezing and snow only rarely occurs at sea level, but often in surrounding mountains due to wet conditions. In the summer, the temperatures range from mild to very warm, depending on distance from the open ocean, elevation, and latitude. Even in the warmest locations with a Mediterranean-type climate, however, temperatures usually don't reach the highest readings found in adjacent desert regions due to cooling from water bodies, although strong winds from inland desert regions can sometimes boost summer temperatures quickly resulting in a much increased forest fire risk.
Inland locations sheltered from or distant from sea breezes can experience severe heat during the summer. Locations inside the Sacramento Valley of northern California, for example, are subject to summer temperatures characteristic of hot deserts (often around 40°C/104F), although winters are rainy enough to allow lusher vegetation than is typical in deserts. Unlike the coastal climates that are designated Csb in the Köppen climate classification—characteristic of places with cooler summers—the hotter, typically inland areas have the Csa classification that indicates a hot summer. Porto, Portugal, experiences the typical Mediterranean pattern of cool, rainy winters and very dry summers, but has relatively mild average summer temperatures.
On another note, locations that are slightly higher latitude or elevation and are cut off from milder ocean winds may have somewhat colder winters and more distinct seasons with occasional snow. This "temperate Mediterranean" climate is most noticeable in the Rogue and Umpqua Basins of southwestern Oregon, central Spain, southeastern France away from the immediate coastline, northern Italy, and northern Greece. In these areas, plants that are commonly associated with milder Mediterranean climates, such as citrus, olive, oleander and eucalyptus, can be frozen to death in a severe winter. Hardy pine and deciduous oak are far more common in these frosty-winter areas.
Areas of high altitude adjacent to locations with Mediterranean climates, such as the "Mesetas" or plateaus of central Spain, may have the cold winters that are characteristic of a continental climate; under Köppen's scheme such places might earn the designation Dsa, Dsb or even Dsc. An example of a hyper humide Mediterranean Snow climate Dfsc is the highest summit on Orjen, Zubacki kabao in the subadriatic Dinaric Alps in Montenegro.