The California Undercurrent as a source of upwelled waters in a coastal filament

Wind-driven coastal upwelling drives high biological productivity in the California Current System (CCS). The classic slab model of the mean overturning upwelling cell is comprised of two branches: the upward transport of cold, salty, nutrient-rich water at the coast and the offshore transport of coastal water in a thin Ekman layer. However, instantaneous cross-shore transport is highly variable, typically occurring within intermittent jets, squirts, eddies and filaments. In June 2017, the California Current Ecosystem Long Term Ecological Research (CCE-LTER) program conducted a process cruise (P1706) to investigate the role of such (sub)mesoscale mechanisms on cross-shore fluxes from the coastal margin to deeper water of the CCS. An autonomous Spray glider and a towed SeaSoar completed repeat sections from Point Conception to Monterey Bay and measured the spatiotemporal evolution of a mesoscale upwelling filament, which was characterized by enhanced cross-filament gradients (both physical and biological) and cross-shore velocity. Several P1706 transects lie at the climatological boundary between the California Undercurrent and California Current and are thus well-suited for assessing the cross-shore exchange of these two water masses. Climatological data from the California Underwater Glider Network provides necessary information for water mass differentiation. The analysis reveals that the cold, salty side of the filament carries recently upwelled California Undercurrent water and corresponds with higher chlorophyll-a fluorescence values than the warm, fresh side. Thus, the offshore-flowing filament represents the convergence of heterogeneous water masses with different geographic origins and thermohaline characteristics, which has implications for (sub)mesoscale community structure gradients.