Influences of the Juan de Fuca Eddy on circulation, nutrients, and phytoplankton production in the northern California Current System

A diagnostic circulation model and water mass analyses are used to examine variability in the structure and circulation of the Juan de Fuca Eddy, a highly productive region at the northern end of the California Current. Results from three years of field studies demonstrate that the eddy increases in spatial extent from early to late summer as the vertically averaged contribution of California Undercurrent source water grows from ∼60% in June to ∼80% in September. Typical near‐surface eddy radii range from ∼15 km in the early summer to ∼30 km in September and increase with depth. Below 100 m, eddy radii are ∼40 km. Fresher water, associated with the estuarine outflow from the Juan de Fuca Strait, is advected around the eddy margin. During southward wind conditions, the combination of cyclonic geostrophic flow and wind‐driven currents in the surface Ekman layer cause the eddy to be “leaky” on its southern perimeter. Eddy surface circulation becomes more retentive (up to ∼32 d observed) during periods of weak winds or frequent northward reversals. The presence of the eddy facilitates large inputs of dissolved inorganic nutrients into the region through two mechanisms: doming of California Undercurrent water within the eddy and enhanced cross‐shelf advection of Juan de Fuca Strait outflow. The combination of these sources results in a persistent, broad (100 km offshore) region of elevated macronutrients. The retentive circulation patterns combined with persistent nutrient supply may favor the development of toxigenic diatom blooms of Pseudo‐nitzschia species in this region.