Recurrent frontal slicks of a coastal ocean upwelling shadow

Abstract

Marine ecosystems are greatly influenced by the structure and dynamics of fronts. In coastal upwelling systems, frontogenesis occurs frequently by upwelling and transport of cold water and warming in sheltered “upwelling shadow” retention sites. Monterey Bay, in the California Current upwelling system, hosts a dynamic upwelling shadow environment. Using a decade of satellite synthetic aperture radar (SAR) observations with ancillary remote sensing and in situ data, we describe recurrent surface slicks that develop along the seaward periphery of the Monterey Bay upwelling shadow, and we examine their relationships with fronts. Slick median dimensions, 17.5 km long and 0.8 km wide, describe their elongated structure. Although the typical pattern is a single slick, multiple slicks may concurrently develop in association with different types of fronts. Repeated volume surveys through a front, underlying a slick, revealed lateral mixing and interleaving of regional water types. Velocity fields from coastal HF radar show that slicks may coincide with a variety of surface circulation patterns, that they may extend contiguously across regions having very different surface velocity, and that they may be separated from the shear front of upwelling filaments by 5–10 km. Slicks occur in all seasons and may coincide with both upwelling and downwelling wind forcing. Surfactant accumulation in small‐scale convergence zones is indicated as the primary mechanism of slick formation, not ocean current shear or small‐scale air‐sea coupling. The results of this study emphasize the role of upwelling system fronts in creating small‐scale structure and dynamics that influence plankton ecology.