The Mediterranean climate is characterised by dry summers and mild, rainy winters. They are located on the western sides of continents, between roughly 30 and 45 degrees north and south of the equator. Mediterranean climate zones are associated with the five large subtropical high-pressure belts of the oceans: the Azores High, South Atlantic High, North Pacific High, South Pacific High, and Indian Ocean High. These high-pressure belts, called anticyclones, rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. Anticyclones cause air in the surrounding atmosphere to diverge and descend, sinking the air in opposing directions until there are no clouds present; leaving clear skies. Climatological high-pressure anticyclones migrate by latitude according to the direct angle of the Sun, shifting northward in the summer and toward the equator in the winter. These semi-permanent anticyclones play a major role in the formation of the world's subtropical and tropical deserts as well as the rainless summers associated with the Mediterranean climate.
The Azores High (also known as the Bermuda High), is associated with the Mediterranean climate found in the Mediterranean Basin, the Sahara Desert, and Arabian Desert (as well as that of the Azores, Canary Islands, and other western Atlantic islands). The South Atlantic High is similarly associated with the Namib Desert and Kalahari Desert, and the Mediterranean climate of the western part of South Africa. The North Pacific High is related to the North American deserts: the Mojave Desert, Great Basin Desert, Sonoran Desert, and Chihuahuan Desert, and maintains the Mediterranean climate found in the interior of California. The South Pacific High correlates to the Atacama Desert and Central Chile's climate, while the Indian Ocean High controls southwestern Australia's climate.
Under the Köppen climate classification, "hot dry-summer" climates (classified as Csa) and "cool dry-summer climates (classified as Csb) are often referred to as "mediterranean". Under the Köppen climate system, the first letter indicates the climate group (in this case temperate climates). Temperate climates or "C" zones have an average temperature above 0 °C (32 °F), but below 18 °C (64 °F), in their coolest months. The second letter indicates the precipitation pattern ("s" represents dry summers). Köppen has defined a dry summer month as a month with less than 30 mm (1.2 in) of precipitation and with less than one-third that of the wettest winter month. Some, however, use a 40 mm (1.6 in) level. The third letter indicates the degree of summer heat: "a" represents an average temperature in the warmest month above 22 °C (72 °F), while "b" indicates the average temperature in the warmest month below 22 °C (72 °F).
Under the Köppen classification, dry-summer climates (Csa, Csb) usually occur on the western sides of continents. Csb zones in the Köppen system include areas normally not associated with mediterranean climates but with Oceanic climates, such as much of the Pacific Northwest, much of southern Chile, parts of west-central Argentina, and parts of New Zealand. Additional highland areas in the subtropics also meet Cs requirements, though they, too, are not normally associated with mediterranean climates, as do a number of oceanic islands such as Madeira, the Juan Fernández Islands, the western part of the Canary Islands, the eastern part of the Azores and England
Under Trewartha's modified Köppen climate classification, the two major requirements for a Cs climate are revised. Under Trewartha's system, at least eight months must have average temperatures of 10 °C (50 °F) or higher (subtropical), and the average annual precipitation must not exceed 900 mm (35 in). Thus, under this system, many Csb zones in the Köppen system become Do or Oceanic climate, and only the classic dry-summer warm winter, low annual rainfall locations are included in the Mediterranean type climate.
During summer, regions of mediterranean climate are dominated by subtropicalhigh-pressure, with dry sinking air capping a surface marine layer of varying humidity and making rainfall unlikely. The Mediterranean climates closer to the equator usually have a dearth of atmospheric disturbances during the summer and fall, due to them being closer to the Horse Latitudes and the Tropic of Cancer, and so the weather is often very stable and dry. In many Mediterranean climates there is a strong diurnal character to daily temperatures in the warm summer months, due to the great loss of ultraviolet radiation from the sun at night.
In winter, Mediterranean climate zones come into contact with the westerlies and associated periodic storms can reach into the mediterranean climate zones. Since the high-pressure anticyclones are no longer there to deflect storms away from these regions, thunderstorms and heavy rain become possible. As a result, areas with this climate receive almost all of their precipitation during their winter and spring seasons, and may go anywhere from 3 to 6 months during the summer without having any significant precipitation. In the lower latitudes, precipitation usually decreases in both the winter and summer because they are closer to the Horse Latitudes which brings calm winds and clear skies; thus bringing smaller amounts of rain. Toward the polar latitudes, total moisture usually increases; the Mediterranean climate in Southern Europe has more rain. The rainfall also tends to be more evenly distributed throughout the year in Southern Europe, while in the Eastern Mediterranean (the Levant) and in Southern California the summer is nearly or completely dry and the dry season most severe. Places where evapotranspiration is higher, steppe climates tend to prevail, but still follow the weather pattern of the Mediterranean climate.
Mediterranean climate distribution in the Americas
The majority of the regions with mediterranean climates have relatively mild winters and very warm summers. However winter and summer temperatures can vary greatly between different regions with a mediterranean climate. For instance, in the case of winters, Lisbon and Los Angeles experience mild temperatures in the winter, with frost and snowfall almost unknown, whereas Dushanbe has colder winters with annual frosts and snowfall. Or to consider summer, Athens experiences rather high temperatures in that season (48 °C (118 °F) has been measured in nearby Eleusis). In contrast, San Francisco has cool summers with daily highs around 21 °C (70 °F) due to the continuous upwelling of cold subsurface waters along the coast. The coast of California also produces regular summer fog, due to cool ocean air coming into contact with high-pressure.
Because most 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 temperature during the summer is large due to dry and clear conditions, except along the immediate coasts). Temperatures during winter only occasionally fall below the freezing point and snow is generally seldom seen. In the summer, the temperatures range from mild to very hot, depending on distance from a large body of water, elevation, and latitude. Even in the warmest locations with a mediterranean-type climate, however, temperatures usually do not reach the highest readings found in adjacent desert regions because of cooling from water bodies, although strong winds from inland desert regions can sometimes boost summer temperatures, quickly increasing the risk of wildfires.
As in every climatologic domain, the highland locations of the mediterranean domain can present cooler temperatures in winter than the lowland areas, temperatures which can sometimes prohibit the growth of typical Mediterranean plants. Some Spanish authors opt to use the term "Continental Mediterranean climate" for some regions with lower temperature in winter than the coastal areas (direct translation from Clima Mediterráneo Continentalizado), but most climate classifications (including Köppen's Cs zones) show no distinction.
Additionally, the temperature and rainfall pattern for a Csa or even a Csb climate can exist as a microclimate in some high-altitude locations adjacent to a rare tropical As (summer-drought tropical climate, typically in a rainshadow region). These have a favourable climate with mild wet winters and fairly warm,dry summers
The mediterranean forests, woodlands, and scrubbiome is closely associated with mediterranean climate zones, as are unique freshwater communities. 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. Aquatic communities in mediterranean climate regions are adapted to a yearly cycle in which abiotic (environmental) controls of stream populations and community structure dominate during floods, biotic components (e.g. competition and predation) controls become increasingly important as the discharge declines, and environmental controls regain dominance as environmental conditions become very harsh (i.e. hot and dry); as a result, these communities are well suited to recover from droughts, floods, and fires. Aquatic organisms in these regions show distinct long-term patterns in structure and function, and are also highly sensitive to the effects of climate change.
The native vegetation of mediterranean climate lands must be adapted to survive long, hot summer droughts and prolonged wet periods in winter. Mediterranean vegetation examples include the following:
Much native vegetation in mediterranean climate area valleys have been cleared for agriculture. In places such as the Sacramento Valley and Oxnard Plain in California, draining marshes and estuaries combined with supplemental irrigation has led to a century of intensive agriculture. Much of the Overberg in the southern Cape of South Africa, once covered with renosterveld, has likewise been largely converted to agriculture, mainly wheat. In hillside and mountainous areas, away from urban sprawl, ecosystems and habitats of native vegetation are more sustained.
This subtype of the mediterranean climate (Csa) is the most common form of the mediterranean climate, therefore it is also known as a “typical mediterranean climate”. As stated earlier, regions with this form of a mediterranean climate experience average monthly temperatures in excess of 22.0 °C (71.6 °F) during its warmest month and an average in the coldest month between 18 to −3 °C (64 to 27 °F) or, in some applications, between 18 to 0 °C (64 to 32 °F). Also, at least four months must average above 10 °C (50 °F). Regions with this form of the mediterranean climate typically experience hot, sometimes very hot and dry summers and mild, wet winters. In a number of instances, summers here can closely resemble summers seen in arid and semiarid climates. However, high temperatures during summers are generally not quite as high as those in arid or semiarid climates due to the presence of a large body of water. All areas with this subtype have wet winters. However, some areas with a hot mediterranean subtype can actually experience very chilly winters, with occasional snowfall. Precipitation is heavier during the colder months. However, there are a number of clear, sunny days during the wetter months.
Occasionally also termed “Cool-summer mediterranean climate”, this subtype of the mediterranean climate (Csb) is the less common form of the mediterranean climate. Cool ocean current and upwelling are often the reason for this cooler type of mediterranean climate. As stated earlier, regions with this subtype of the mediterranean climate experience warm (but not hot) and dry summers, with no average monthly temperatures above 22 °C (72 °F) during its warmest month and an average in the coldest month between 18 to −3 °C (64 to 27 °F) or, in some applications, between 18 to 0 °C (64 to 32 °F). Also, at least four months must average above 10 °C (50 °F). Winters are rainy and can be mild to chilly. In a few instances, snow can fall on these areas. Precipitation occurs in the colder seasons, but there are a number of clear sunny days even during the wetter seasons.
Distribution of the relatively rare cold-summer Mediterranean climate (Köppen type Csc) in Washington, Oregon and California.
The cold-summer subtype of the mediterranean climate (Csc) is rare and predominately found at scattered high-altitude locations along the coasts of North and South America. This type is characterized by cool summers, with fewer than four months with a mean temperature at or above 10 °C (50 °F), as well as with mild winters, with no winter month having a mean temperature below 0 °C (32 °F) (or −3 °C [27 °F]), depending on the isotherm used). Regions with this climate are influenced by the dry-summer trend that extends considerably poleward along the west coast of the Americas, as well as the moderating influences of high altitude and relative proximity to the Pacific Ocean.
In North America, areas with Csc climate can be found in the Olympic, Cascade, Klamath, and Sierra Nevada ranges in Washington, Oregon and California. These locations are found at high altitude nearby lower altitude regions characterized by a warm-summer mediterranean climate (Csb) or hot-summer mediterranean climate (Csa). A rare instance of this climate occurs in the tropics, on Haleakalā Summit in Hawaii.
In South America, Csc regions can be found along the Andes in Chile. The town of Balmaceda is one of the few towns confirmed to have this climate.
Small areas with a Csc climate can also be found at high elevations in Corsica.