Mediterranean climate

For the influence of Mediterranean climates on viticulture, see Mediterranean climate (wine).
  Hot-summer mediterranean climate (Csa)
  Warm-summer mediterranean climate (Csb)

A mediterranean climate /ˌmɛdɪtəˈrniən/ is the climate typical of the lands in the Mediterranean Basin. The lands around the Mediterranean Sea form the largest area where this climate type is found, but it also is found in most of coastal California, in parts of Western and South Australia, in southwestern South Africa, sections of Central Asia, and in central Chile.

Characteristics

The mediterranean climate is characterised by dry summers and mild, moist winters. Mediterranean climate zones are 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 climatological high pressure cells shift towards the poles in the summer and towards the equator in the winter, playing a major role in the formation of the world's subtropical and tropical deserts as well as the Mediterranean Basin's climate.

The Azores High is associated with the Mediterranean climate found in southern Europe and north Africa and the Sahara Desert (as well as that of the Azores, Canary Islands, and other eastern Atlantic islands). The South Atlantic High is similarly associated with the Namib Desert and the mediterranean climate of the western part 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.[1]

Most large, historic cities of the Mediterranean basin, including Athens, Algiers, Barcelona, Beirut, Izmir, Jerusalem, Marseille, Rome and Tunis, lie within mediterranean climatic zones, as do major cities outside of the Mediterranean basin, such as Lisbon, Casablanca, Cape Town, Adelaide, Perth,Shiraz, Los Angeles, San Francisco, Santiago de Chile, Tashkent and Dushanbe.

Köppen climate classification

Under the Köppen climate classification, "dry-summer subtropical" climates (classified as Csa) and "dry-summer temperate" climates (classified as Csb) are often referred to as "mediterranean". Under the Köppen-Geiger system, "C" zones have an average temperature above 10 °C (50 °F) in their warmest months, and an average in the coldest between 18 to −3 °C (64 to 27 °F) (or, in some applications, between 20 to 0 °C [68 to 32 °F]). The second letter indicates the precipitation pattern: "s" represents dry summers: first, 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.[2][3] The third letter indicates the degree of summer heat: "a" represents an average temperature in the warmest month above 22 °C (72 °F), with at least four months averaging above 10 °C (50 °F); "b", an average temperature in the warmest month below 22 °C (72 °F), and again with at least two months averaging above 10 °C (50 °F).

Under this classification, dry-summer climates (Csa, Csb) usually occur on the western sides of continents. Csb zones include areas normally associated not with mediterranean but with Oceanic climates, such as much of the Pacific Northwest, much of southern Chile, parts of west-central Argentina, Portugal and Spain, but also in higher elevations within the Mediterranean regime.[4] 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 and the eastern part of the Azores.

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 at least 10 °C (50 °F), and the average annual precipitation must not exceed 900 mm (35 in). Thus, under this system, many Csb zones (including the Pacific Northwest) become DO Oceanic. However Trewartha's 900 mm (35 in) threshold also disqualifies some locations generally considered to have a "classic" mediterranean climate, notably Naples, which it classes as "humid subtropical" despite its hot, dry summers.

Climate is the statistics of weather, usually over a 30-year interval. It is measured by assessing the patterns of variation in temperature, humidity, atmospheric pressure, wind, precipitation, atmospheric particle count and other meteorological variables in a given region over long periods of time.

Precipitation

It [Chile] has six, months of winter, no more, and in them, except when there is a quarter moon, when it rains one or two days, all the other days have such beautiful suns...
 Pedro de Valdivia to Charles V, Holy Roman Emperor

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 and making rainfall impossible or unlikely except for the occasional 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 precipitation during their winter, autumn and spring seasons, and may go anywhere from 4 to 6 months during the summer without having any significant precipitation.

Toward the equatorial latitudes, winter precipitation decreases as a share of annual precipitation as the climate grades equator-ward into the steppe climate usually characterized as BSHs normally too dry to support non-irrigated agriculture. Toward the polar latitudes, 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. In the northwestern Mediterranean Basin, the rainiest season is divided into a primary maximum during the autumn and a secondary in spring, making for a shorter dry season than in the classic rainforest climate.

Temperature

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 experiences very mild temperatures in the winter, with frost and snow practically 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 mild summers due to the upwelling of cold subsurface waters along the coast producing regular summer fog that does not reach far inland.In low season the temperatures could be up to 20 °C (68 °F).

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[5] (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

Mediterranean biome

The mediterranean forests, woodlands, and scrub biome 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.[6] Aquatic organisms in these regions show distinct long-term patterns in structure and function,[7] and are also highly sensitive to the effects of climate change.[8][9]

Natural vegetation

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:[10]

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.

The fynbos vegetation in the South-western Cape in South Africa is famed for its high floral diversity, and includes such plant types as members of the Restionaceae, Ericas (Heaths) and Proteas. Representatives of the Proteaceae also grow in Australia, such as Banksias. The palette of California native plants is also renowned for its species and cultivar diversity.

Hot-summer Mediterranean climate

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.

Csa climates are mainly found around the Mediterranean Sea, southwestern Australia, southwestern South Africa, sections of Central Asia, including Northern Iran, the interior of northern California west of the Sierra Nevada, and inland areas of southern Oregon west of the Cascade Mountains. Southern California's coasts also experience hot summers due to the shielding effect of the Channel Islands. However, unshielded areas of that coastline can have warm-summer mediterranean climates with hot-summer areas just a few miles inland.

Malta
Climate chart (explanation)
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Source: MaltaWeather[11]
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Source: NOAA
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Source: BoM[12]

Warm-summer Mediterranean climate

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.

Csb climates are found in northwestern Iberia, coastal California, western Washington and Oregon, Canada's Vancouver Island, central Chile, parts of southern Australia, sections of southwestern South Africa and sections of the Atlantic coast of Morocco.

Porto
Climate chart (explanation)
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Source: Instituto de Meteorologia[13]
San Francisco
Climate chart (explanation)
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Source: NOAA[14]
Cape Town
Climate chart (explanation)
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Source: WMO[15]

Cold-summer Mediterranean climate

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 Mountains in Chile. The town of Balmaceda, Chile 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.

Balmaceda, Chile
Climate chart (explanation)
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Source: DMC[16]infochile[17]
Haleakala Summit, Hawaii
Climate chart (explanation)
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Source: The Western Regional Climate Center[18]

See also

References

  1. Akin, Wallace E. (1991). Global Patterns: Climate, Vegetation, and Soils. University of Oklahoma Press. p. 35. ISBN 0-8061-2309-5.
  2. Kottek, Markus; Grieser, Jürgen; Beck, Christoph; Rudolf, Bruno; Rube, Franz (June 2006). "World Map of the Köppen-Geiger climate classification updated" (PDF). Meteorologische Zeitschrift. 15 (3): 259–263. doi:10.1127/0941-2948/2006/0130. Retrieved 2011-02-27.
  3. Peel, M. C.; Finlayson, B. L.; McMahon, T. A. (2007). "Updated world map of the Köppen-Geiger climate classification" (PDF). Hydrology and Earth System Sciences. 4 (2): 439–473. doi:10.5194/hessd-4-439-2007. Retrieved 2011-02-27.
  4. Peel, M. C.; Finlayson, B. L.; McMahon, T. A. (2007). "Updated world map of the Köppen-Geiger climate classification". Hydrology and Earth System Sciences. 11 (5): 1633–1644. doi:10.5194/hess-11-1633-2007. Retrieved 2011-02-27.
  5. climate of Spain, National Geographic Institute of Spain
  6. Gasith, A. and V.H. Resh (1999). "Streams in mediterranean Climate Regions: Abiotic Influences and Biotic Responses to Predictable Seasonal Events". Annu. Rev. Ecol. Syst. 30: 51–81. doi:10.1146/annurev.ecolsys.30.1.51.
  7. Resh, V.H.; L.A. Bêche; J.E. Lawrence; R.D. Mazor; E.P. McElravy; A.H. Purcell; S.M. Carlson (2013). "Long-term Population and Community Patterns of Benthic Macroinvertebrates and Fishes in Northern California Mediterranean-climate Streams". Journal of the North American Benthological Society. 719: 93–118. doi:10.1007/s10750-012-1373-9. Retrieved 10 November 2013.
  8. Lawrence, J.E.; K.B. Lunde; R.D. Mazor; L.A. Bêche; E.P. McElravy; V.H. Resh (2010). "Long-Term Macroinvertebrate Responses to Climate Change: Implications for Biological Assessment in Mediterranean-Climate Streams". Journal of the North American Benthological Society. 29: 1424–1440. doi:10.1899/09-178.1. Retrieved 10 November 2013.
  9. Filipe, A.F.; J.E. Lawrence; N. Bonada (November 2013). "Vulnerability of Biota in Mediterranean Streams to Climate Change: A Synthesis of Ecological Responses and Conservation Challenges". Hydrobiologia. 719: 331–251. doi:10.1007/s10750-012-1244-4. Retrieved 10 November 2013.
  10. Dallman, Peter (1998). Plant Life in the World's Mediterranean Climates. Berkeley, CA: University of California Press. ISBN 9780520208094.
  11. "Malta's Climate". MaltaWeather (Meteo Malta & MaltaMedia). Retrieved 2016-09-02.
  12. "Perth Monthly climate statistics". Australia Bureau of Meteorology. Retrieved 2010-08-02.
  13. "Monthly Averages for Porto, Portugal". Instituto de Meteorologia. Retrieved 2010-08-02.
  14. "Climatography of the United States No. 20 (1971–2000)" (PDF). National Oceanic and Atmospheric Administration. Retrieved 2010-05-31.
  15. "Weather Information for Cape Town". World Weather Information Service. Retrieved 2010-08-02.
  16. "Estadistica Climatologica Tomo III (pg 319-343)" (PDF). Dirección General de Aeronáutica Civil. Retrieved January 15, 2013.
  17. "Datos climatológicos Chile Sur". Atmosfera.cl.
  18. "Seasonal Temperature and Precipitation Information". Western Regional Climate Center. Retrieved November 3, 2013.

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