Mediterranean California is a Level I ecoregion of North America designated by the Commission for Environmental Cooperation (CEC) in its North American Environmental Atlas. The region is present only in California and Baja California.
Climate
Very few places in the world have the Mediterranean climate of California. It is one of the more rare in the world, with only five locations: the Mediterranean Basin, Southwest Australia, the Cape Province—Western Cape of South Africa, the Chilean Matorral, and the California chaparral and woodlands ecoregion of California and the Baja California Peninsula. The region is typified by warm dry summers and mild wet winters. This is unusual as most climates have more precipitation in the summer. There are three variations to the Mediterranean climate in California, a cool summer/cool winter variation, a cool summer/cool winter with summer fog variation, and a hot summer/cool winter variation. The average temperatures for the cool summer variations are below in the summer and between in the winter. Average summer temperatures for the hot summer variation are above . Average annual precipitation for this climate is per year.
Hydrology
Defined by the Pacific Coast on the west, the Sierra Nevada (mountains) and the deserts of California on the east, and the Northern California Coast Ranges on the north, the Mediterranean California ecoregion has unique physical characteristics that play a large role in the natural systems of the region, including hydrology.
Precipitation
The unusual precipitation pattern of the Mediterranean climate is due to subtropical high-pressure systems in the summer and the polar jet stream in the winter. Rainfall in the summer is uncommon because the marine layer becomes capped with dry sinking air. The marine layer is an air mass over a large body of water brought about by a temperature inversion from the cooling effect of the water on the warmer air. The marine layer is often accompanied by fog. The polar jet stream in the winter brings with it rain and snow. The jet stream is an extremely powerful air current flowing west to east often at over 100 miles per hour.
The precipitation in the region is closely associated with winter frontal storms from the Pacific Ocean, which bring cool air and rain to the area. The annual rainfall varies in different elevations, but the average range is between annually. Much of the rain in Central and Northern California flows out the Sacramento and San Joaquin Rivers, which with numerous tributaries run through an upper part of the ecoregion.
Fog is also an important aspect of the hydrologic cycle in this ecoregion; the cooling of air over the warm seawater create a dense fog that covers large areas of the coast. This fog affects the vegetation and overall environment on the coast. On the contrary, fire also influences this region. The fire-flood sequence that occurs post-fire can greatly effect populations of species in the region. The combination of the geophysical characteristics, little rainfall, and the bodies of water in the region make it a unique, distinct environment.
Geology
Mediterranean climate California's geology is characterized by the meeting of the North American Plate and Pacific Plate, with much of its region near or influenced by the San Andreas Fault along the junction. When the two plates collided the Pacific Plate was pushed under the North American Plate, and the California Coast Ranges and Sierra Nevada were uplifted. The Coast Ranges are largely metamorphic rock formed from the submergence of the Pacific Plate, and the Sierra are uplifted granite batholiths. Not along the San Andreas Fault, the granitic Peninsular Ranges system also uplifted with the collision, and runs from Southern California, down the Baja California Peninsula, into Baja California Sur state, northwest Mexico. The Transverse Ranges are another major Southern California mountain system primarily in the Mediterranean climate zone. Large earthquakes can do considerable damage to populated areas, and to the state's water, transportation, and energy infrastructure.