A cyclonic mesoscale circulation that forms over the coastal waters of southern California. It is most common in the spring and early summer months but can occur at any time of the year. A Catalina eddy circulation is typically associated with a deepening of the marine layer and an associated improvement in the air quality in the Los Angeles Basin. In many cases the marine layer can deepen above 1 km and allow marine air to spill through gaps in the coastal mountains and reach interior desert regions. Catalina eddies are often observed to develop on the coast downwind of the coastal mountains during a period of offshore flow. Cyclonic vorticity develops over the bight of southern California in response to lowered sea level pressure produced by offshore, downslope flow across the Santa Ynez and San Rafael Mountains. Southeastward displacement and offshore expansion of the cyclonic circulation typically occur later in Catalina eddy events.