The nature of the cold filaments in the California Current system

Data from the Coastal Transition Zone (CTZ) experiment are used to describe the velocity fields and water properties associated with cold filaments in the California Current. Combined with previous field surveys and satellite imagery, these show seasonal variability with maximum dynamic height ranges and velocities in summer and minimum values in late winter and early spring. North of Point Arena (between 39°N and 42°N) in spring‐summer the flow field on the outer edge of the cold water has the character of a meandering jet, carrying fresh, nutrient‐poor water from farther north on its offshore side and cold, salty, nutrient‐rich water on its inshore side. At Point Arena in midsummer, the jet often flows offshore and continues south without meandering back onshore as strongly as it does farther north. The flow field south of Point Arena in summer takes on more of the character of a field of mesoscale eddies, although the meandering jet from the north continues to be identifiable. The conceptual model for the May‐July period between 36°N and 42°N is thus of a surface jet that meanders through and interacts with a field of eddies; the eddies are more dominant south of 39°N, where the jet broadens and where multiple jets and filaments are often present. At the surface, the jet often separates biological communities and may appear as a barrier to cross‐jet transport, especially north of Point Arena early in the season (March‐May). However, phytoplankton pigment and nutrients are carried on the inshore flank of the jet, and pigment maxima are sometimes found in the core of the jet. The biological effect of the jet is to define a convoluted, 100 to 400‐km‐wide region next to the coast, within which much of the richer water is contained, and also to carry some of that richer water offshore in meanders along the outer edge of that region.