Origin of the Catalina Eddy

Abstract

During late springs through early fall the normal regime of low-level westerly and northwesterly flow within the bight of southern California is occasionally interrupted by periods of southerly flow, elevated marine layers, and increased low-level cloudiness. This transition, often termed the Catalina Eddy, is limited to narrow zone of ?100 km from the coastal mountains and brings cooler temperatures and improved air quality. This paper describes the initiation and evolution of Catalina Eddy events using both composite and case study approaches. It is found that this phenomenon is produced by the interaction between the synoptic-scale flow and the formidable topography of the region. As a result of synoptic-scale pressure falls along the central California coast and/or mesoscale lee troughing southeast of Point Conception, an alongshore pressure gradient with lower pressure to the north becomes established. The result of such a pressure gradient in a coastal zone with an adjacent topographic barrier is the establishment of southerly flow within a Rossby radius (?100 km) of the coastal mountains. Since relatively geostrophic northerlies remain offshore, considerable cyclonic vorticity is established in the coastal zone. During the early stages of an eddy event there may be little or no stratus in the southern California bright even through an eddy circulation is present. As the southerlies and the associated marine layer deepen, coastal stratus develops and thickens. In many, but not all, eddy cases a well-defined stratus eddy forms during the night as the stratus-laden southerly flow surges westsward south of Point Conception and then is advected south by the strong northerly flow at and to the west of the Point. Catalina Eddy events continue as long as the supporting alongshore pressure gradient remains. As the synoptic pattern evolves and the alongshore pressure gradient weakens and reverses, the normal wind regime in the bight is reestablished. Although current operational models do not possess the resolution to directly simulate eddy events, they often accurately predict the larger scale flow responsible for eddy formation.