AMediterranean climate (/ˌmɛdɪtəˈreɪniən/MED-ih-tə-RAY-nee-ən), also called adry summer climate, described byKöppen andTrewartha asCs, is atemperate climate type that occurs in the lower mid-latitudes (normally 30 to 44 north and south latitude). Such climates typically have dry summers and wet winters, with summer conditions being hot and winter conditions typically being mild. These weather conditions are typically experienced in the majority of Mediterranean-climate regions and countries, but remain highly dependent on proximity to the ocean, elevation, and geographical location.
The dry summer climate is found throughout the warmermiddle latitudes, affecting almost exclusively the western portions of continents in relative proximity to the coast. Theclimate type's name is in reference to the coastal regions of theMediterranean Sea, which mostly share this type of climate, but it can also be found in the Atlantic portions ofIberia and Northwest Africa, the Pacific portions of the United States and Chile, extreme west areas of Argentina, the southwest tip of South Africa, parts ofSouthwest andSouth Australia, and parts ofCentral Asia. They tend to be found in proximity (both poleward and near the coast) ofdesert andsemi-arid climates, and equatorward ofoceanic climates.
Mediterraneanclimate zones are typically located along the western coasts of landmasses, between roughly 30 and 45 degrees north or south of theequator. The main cause of Mediterranean, or dry summer, climate is thesubtropical ridge, which extends towards the pole of the hemisphere in question during the summer and migrates towards the equator during the winter. This is due to the seasonal poleward-equatorward variations of temperatures.[1]
The resultingvegetation of Mediterranean climates are thegarrigue ormaquis in the European Mediterranean Basin, thechaparral in California, thefynbos inSouth Africa, themallee in Australia, and thematorral in Chile. Areas with this climate are also where the so-called "Mediterranean trinity" of major agricultural crops have traditionally been successfully grown (wheat,grapes andolives). As a result, these regions are notable for their high-quality wines, grapeseed/olive oils, and bread products.[2]
Continental Portugal has a clear contrast between the cool to warm (Csb) and hot (Csa) summers.
Under theKöppen climate classification, "hot dry-summer" climates (classified asCsa) and "cool dry-summer" climates (classified asCsb) are often referred to as just "Mediterranean". Under the Köppen climate system, the first letter indicates the climate group (in this case temperate climates). Temperate climates or "C" zones average temperature above 0 °C (32 °F) (or −3 °C (27 °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 as a month within the high-sun months of April to September, in the case of the Northern Hemisphere and October to March, in the case of the Southern Hemisphere, and it also must contain exactly or less than one-third that of the wettest winter month. Some, however, use a 40 mm (1.6 in) level.[3][4] 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). There is a "c" with 3 or less months' average temperature above 10 °C (50 °F), but this climate is rare and is very isolated.[3]
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 withOceanic climates, such as much of thePacific Northwest, much of southernChile, parts of west-centralArgentina, and parts ofNew Zealand.[5] Additional highland areas in the subtropics also meetCs requirements, though they, too, are not normally associated with Mediterranean climates. The same goes for a number ofoceanic islands such asMadeira, theJuan Fernández Islands, the western part of theCanary Islands, and the eastern part of theAzores.[3]
UnderTrewartha's modified Köppen climate classification, the two major requirements for aCs 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), as well as satisfying Köppen's precipitation requirements.[citation needed]
[Chile] has four 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 a beautiful sunshine that it is not necessary to resort to fire. The summer is so temperate, with delicious airs, that man can go all day under the sun.
Poleward extension and expansion of the subtropicalanticyclone over theoceans bringsubsiding air to the region in summer, with clear skies and high temperatures. When the anticyclone moves Equator-ward in winter, it is replaced by traveling,frontal cyclones with their attendant precipitation.[6]
During summer, regions of the Mediterranean climate are strongly influenced by the subtropical ridge which keeps atmospheric conditions very dry with minimal cloud coverage. In some areas, such ascoastal California, the coldocean current has a stabilizing effect on the surrounding air, further reducing the chances for rain, but often causing thick layers of marinefog that usuallyevaporate by mid-day. Similar todesert climates, in many Mediterranean climates there is a strongdiurnal character to daily temperatures in the warm summer months due to strong solar heating during the day from sunlight and rapid cooling at night.
In winter, thesubtropical ridge migrates towards the equator and leaves the area, making rainfall much more likely. As a result, areas with this climate receive almost all of their precipitation during their winter and spring seasons, and may go anywhere from four to six months during the summer and early fall without having any significantprecipitation. In the lower latitudes, precipitation typically decreases during both the winter and summer months. Toward the polar latitudes, total moisture levels generally increase; for example, the Mediterranean climate inSouthern Europe tends to experience more rainfall. Rainfall inSouthern Europe is also more evenly distributed throughout the year, while in places such asSouthern California, the summer is nearly or completely dry. In areas wheresteppe climates are found, the overall precipitation pattern may still resemble that of Mediterranean climates, though with drier conditions.
Irregularity of the rainfall, which can vary considerably from year to year, accentuates the droughts of the Mediterranean climate.Rain does not fall evenly, nor does the rain arrive at the same time or within the same intervals. InGibraltar, for instance, rain starts falling nearly half a season earlier than at theDead Sea. InIsrael no rain at all falls in summer but early rains may come in autumn.[7]
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,Funchal experiences mild to warm temperatures in the winter, with frost and snowfall almost unknown,[8] whereasTashkent has cold winters with annual frosts and snowfall;[9] or, to consider summer,Seville experiences rather high temperatures in that season.[10] In contrast,San Francisco has cool summers with daily highs around 21 °C (70 °F)[11] due to the continuousupwelling of cold subsurface waters along the coast.
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 unlike (the relatively rare) dry-summerhumid continental climates (although the daily diurnal range of temperature during the summer is large due to dry and clear conditions, except along the immediate coastlines). Temperatures during winter only occasionally fall below the freezing point andsnow is generally seldom seen. Summer temperatures can be cool to very hot, depending on the distance from a large body of water, elevation, and latitude, among other factors. Strong winds from inland desert regions can sometimes boost summer temperatures up, quickly increasing the risk ofwildfires. Notable exceptions to the usual proximity from bodies of water, thus featuring extremely high summer temperatures and cooler winters, include south-eastern Turkey and northern Iraq (Urfa,Erbil), surrounded by hot deserts to the south and mountains to the north. Those places routinely experience summer daily means of over 30 °C (86 °F) and daily highs above 40 °C (104 °F), while receiving enough rainfall in winter not to fall intoarid orsemi-arid classifications.[12]
As in everyclimatologic domain, the highland locations of the Mediterranean domain can present cooler temperatures in the summer and winter than the lowland areas, temperatures which can sometimes prohibit the growth of typical cold-sensitive Mediterranean plants. Some Spanish authors opt to use the term 'continental Mediterranean climate' (Clima Mediterráneo Continentalizado) for some regions with lower temperatures in winter than the coastal areas,[13] but Köppen'sCs zones show no distinction as long as winter temperature means stay above freezing.
Additionally, the temperature and rainfall pattern for aCsa or even aCsb climate can exist as amicroclimate in some high-elevation locations adjacent to a rare tropicalAs (tropical savanna climate with dry summers, typically in arainshadow region, as in Hawaii).These have a favourable climate, with mild wet winters and fairly warm, dry summers.
TheMediterranean forests, woodlands, and scrubbiome is closely associated with Mediterranean climate zones, as are unique freshwater communities, though vegetation native to the Mediterranean climate zone can also be found in the approximate nearby climate zones, which usually tend to be thehumid subtropical,oceanic and/orsemi-arid zones, depending on the region and location. Particularly distinctive of the climate aresclerophyll shrublands, calledmaquis in the Mediterranean Basin,chaparral in California,matorral in Chile,fynbos in South Africa, andmallee andkwongan shrublands in Australia.[citation needed]
Mediterranean vegetation shows a number of adaptations todrought,grazing, and frequent fire regimes. The small sclerophyllous leaves that characterize many of theperennial shrubs of this biome, help conserve water and prevent nutrient loss. The soils generally are oflow fertility, and many plants havemutualistic relationships withnitrogen-fixing bacteria.[citation needed]
Aquatic communities in Mediterranean climate regions are adapted to a yearly cycle in whichabiotic (environmental) controls of stream populations and community structure dominate during floods,biotic components (e.g. competition and predation) controls become increasingly important as the flood 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 fromdroughts,floods, andfires.[14] Aquatic organisms in these regions show distinct long-term patterns in their structure and function,[15] and are also highly sensitive to the recenteffects of climate change.[1][2]
The native vegetation of Mediterranean climate lands must be adapted to survive long, hot summer droughts in summer and prolonged wet periods in winter. Mediterranean vegetation examples include the following:[16]
Many native vegetations in Mediterranean climate area valleys have been cleared foragriculture and farming. In places such as theSacramento Valley andOxnard Plain inCalifornia, draining marshes and estuaries combined with supplemental irrigation has led to a century of intensive agriculture. Much of theOverberg in the southern Cape ofSouth Africa, was once covered withrenosterveld, but has likewise been largely converted to agriculture, mainly forwheat. In hillside and mountainous areas, away from theurban sprawls, ecosystems and habitats of native vegetation are more sustained and undisturbed.[citation needed]
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 and −3 °C (64 and 27 °F) or,[3] in some applications, between 18 and 0 °C (64 and 32 °F).[4] Regions with this form of the Mediterranean climate typically experience hot, sometimes very hot and dry summers. Winters can be mild, cool or chilly, and some cities in this region receive somewhat regular snowfall (e.g.Kermanshah),[17] while others do not receive any (e.g.Casablanca).
Csa climates are mainly found around theMediterranean Sea, southern Australia, southwesternSouth Africa, sections ofCentral Asia, northern sections ofIran andIraq, the interior of northernCalifornia west of theSierra Nevada, along theWasatch Front inUtah, and inland areas of southernOregon west of theCascade Mountains.[3] Southern California's coasts also experience hot summers due to the shielding effect of theChannel Islands. However, unshielded areas of that coastline can have warm-summer Mediterranean climates with hot-summer areas just a few kilometres inland.
Occasionally also termed the "cool-summer Mediterranean climate", this subtype of the Mediterranean climate (Csb) is less common and involves warm (but not hot) and dry summers, with no average monthly temperatures above 22 °C (72 °F) during its warmest month and as usual an average in the coldest month between 18 and −3 °C (64 and 27 °F) or,[3] in some applications, between 18 and 0 °C (64 and 32 °F).[4]
Also, at least four months must average above 10 °C (50 °F).[3]
Cool ocean currents, upwelling and higherlatitudes are often the reason for this cooler type of Mediterranean climate.[citation needed]
The other main reason for this cooler type is theelevation. For instance,Menton on the French coast has a Csa climate whileCastellar, Alpes-Maritimes, the adjacent town just north of Menton, with an elevation between 100 and 1,382 m (328 and 4,534 ft), has a Csb climate instead.[22] The village ofSiah Bisheh inNorthern Iran also has a Csb climate because of its location inside theAlborz mountains.[23]
Winters in this zone are rainy and can be mild to chilly. Some locales in this zone experience some amount of snowfall, while others do not.
Distribution of the relatively rare cold-summer Mediterranean climate (Köppen typeCsc) in Washington, Oregon and California
The cold-summer subtype of the Mediterranean climate (Csc) is rare and predominantly found at scattered high-elevation locations along the west coasts of North and South America having a similar climate.[citation needed] This type is characterized by cool, dry summers, with less than four months with a mean temperature at or above 10 °C (50 °F), as well as with cool, wet winters, with no winter month having a mean temperature below 0 °C (32 °F) (or −3 °C [27 °F], depending on the isotherm used).[3] Regions in the Americas with this climate are influenced by the dry-summer trend (though briefly) that extends considerably poleward along the west coast, as well as the moderating influences of high and relative proximity to the Pacific Ocean. These conditions maintain an unusually narrow temperature range throughout the year for climate zones at such distances from coasts.
In North America, areas withCsc climate can be found in theOlympic,Cascade,Klamath, andSierra Nevada ranges in Washington, Oregon and California. These locations are found at high elevation nearby lower elevation 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, onHaleakalā Summit in Hawaii.[citation needed]
In South America,Csc regions can be found along theAndes in Chile and Argentina. The town ofBalmaceda, Chile is one of the few towns confirmed to have this climate.[citation needed]
Small areas with aCsc climate can be found at high elevations inCorsica.[33]
In Norway, the small fishing village ofRøstlandet, inRøst Municipality, above theArctic Circle has a climate bordering onCsc and is known as a "climatic anomaly" due to abnormally warm temperatures despite its latitude located above 67°N latitude.[34]
^abLawrence, 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 (4):1424–1440.doi:10.1899/09-178.1.S2CID84679634.
^abFilipe, 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.719:331–351.doi:10.1007/s10750-012-1244-4.hdl:2445/48186.S2CID17658477.
^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 (1):51–81.Bibcode:1999AnRES..30...51G.doi:10.1146/annurev.ecolsys.30.1.51.
^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 (1):93–118.Bibcode:2013HyBio.719...93R.doi:10.1007/s10750-012-1373-9.S2CID186919.
^"Victoria Gonzales HTS, British Columbia".Canadian Climate Normals 1971–2000 (in English and French). Environment Canada. 19 January 2011.Archived from the original on 17 May 2013. Retrieved15 February 2013.
