Why California, Chile, Cape Town and Perth share the Mediterranean's weather
Wine country in California, Chile, South Africa and Australia shares a pattern with the shores of the Mediterranean: rainy, mild winters and long, parched summers. The cause is a band of high pressure that drifts toward the pole each summer and back toward the equator each winter, taking the rain away and then bringing it back.
This dry-summer climate appears mostly on the western edges of continents, in a latitude band running from about 30 to 45 degrees in either hemisphere. Besides the Mediterranean coasts it covers Atlantic Iberia and northwest Africa, the Pacific shores of the United States and Chile, the southwestern tip of South Africa, parts of southern and southwestern Australia and pockets of Central Asia. Such zones usually sit between deserts on the equator side and cooler oceanic climates toward the poles.
In summer the subtropical ridge pushes poleward, sending sinking air that keeps skies clear and hot. In winter it retreats and travelling storm fronts move in with rain. Many places therefore go four to six months without meaningful rainfall. Cold offshore currents can suppress rain further, which is why coastal California gets thick morning fog that burns off by midday and why San Francisco summers peak around 21 degrees Celsius. Rain is also erratic from year to year, deepening droughts; it starts falling in Gibraltar almost half a season before it reaches the Dead Sea.
Temperatures vary widely within the type. Funchal on Madeira almost never sees frost, while Tashkent gets snow every year. Under the Koppen scheme a dry summer month has under 30 millimetres of rain and no more than a third of the wettest winter month, though some use 40 millimetres. The rival Trewartha system additionally demands at least eight months averaging 10 degrees or warmer and annual rainfall no higher than 900 millimetres.
Plants have converged on similar solutions everywhere: small, tough, water-saving leaves, tolerance of grazing and fire, and partnerships with nitrogen-fixing bacteria in poor soils. The resulting scrub goes by many names, including maquis in Europe, chaparral in California, fynbos in South Africa, matorral in Chile and mallee in Australia. These regions also suit the classic trio of wheat, grapes and olives.
Source: Mediterranean climate
