What phenomenon is responsible for the five seasons in Southern California?

The Santa Ana Winds.

These winds are strong and extremely dry, and when they are mixed with the warm air of the region they create optimal fire conditions.

The climate of California varies widely, from hot desert to polar, depending on latitude, elevation, and proximity to the coast. California's coastal regions, the Sierra Nevada foothills, and much of the Central Valley have a Mediterranean climate, with warmer, drier weather in summer and cooler, wetter weather in winter. The influence of the ocean generally moderates temperature extremes, creating warmer winters and substantially cooler summers in coastal areas.