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Production of particulate organic carbon (POC) in nutrient-rich coastal waters over continental shelves, its export to depth, and its transport to deeper ocean waters is a poorly quantified component of the global carbon cycle. A critical step in quantifying this vertical transport is identifying shelf processes that export phytoplankton out of the euphotic zone. During cruises of the Santa Barbara Coastal Long Term Ecological Research project, we discovered substantial chlorophylla(chla)below the euphotic zone in the Santa Barbara Channel, a part of the southern California Current System. Observations from towed, undulating vehicles revealed deep chlorophyll layers near fronts where upwelled waters from central California converged with lower-density waters from the Southern California Bight. The mean fraction ± 1 standard deviation (SD) of chlorophyll biomass below the euphotic zone spanning the entire Santa Barbara Channel was ~7 ± 9% during 13 cruises averaged across all seasons. In one spring cruise, the fraction was ~30%, and in other cruises the layers were absent. Phytoplankton export out of the euphotic zone by subduction was indicated by spatial coherence between chlaand sloping density surfaces. Vertical plumes of chlacrossing density surfaces indicated enhanced gravitational export within cyclonic eddies. Chlain water samples below the euphotic zone, away from fronts and cyclonic flows, suggested additional phytoplankton export. Our results emphasize the importance of subduction in the export of phytoplankton and POC out of the euphotic zone in coastal upwelling systems. 

Abstract Retention of surface water parcels can benefit coastal ecosystems by increasing the residence times of nutrient‐rich waters and marine larvae but can also negatively impact marine life and human health by concentrating oil and other pollutants. To investigate the spatial patterns, temporal variability, and drivers of retention, surface water parcel retention is quantified using particle simulations forced by high‐frequency radar surface current observations in the Santa Barbara Channel (SBC), California, USA. Retention is defined here as the time a particle remains within 20 km of its starting location and typical retention times are ∼4 days. However, the mechanisms driving retention differ across the SBC. In the central SBC, high retention times are driven by a persistent cyclonic eddy, while in the eastern SBC, high retention times are due to weak oscillatory flow. Areas in the western SBC outside of the cyclonic eddy exhibit the shortest mean retention times (<2 days) due to sustained horizontal advection. Stepwise regression is used to assess the drivers of interannual retention anomalies at sub‐SBC scales and is able to predict the greatest (least) amount of retention variability in the western (eastern) SBC. The presence of coherent eddies, both cyclonic and anticyclonic, is a dominant driver of high retention channel‐wide, while some factors such as wind stress and along‐channel flow have counteracting effects on retention at sub‐channel scales. 

Abstract Submesoscale eddies form an important component of the circulation of the Southern California Bight (SCB). Despite their acknowledged significance in influencing ocean physics, biology, and ecological processes, submesoscale eddies have been exceptionally hard to study and observe because of the technical challenges posed by both field and remote platforms. Here, using a decade of high‐frequency radar surface current observations, we describe submesoscale eddies in the SCB. Between 2012 and 2021, a total of ∼235,000 eddies were detected, averaging 452 ± 116 eddies per week. Recurring eddies in certain locations over time, formed hotspots of eddy activity, largely in association with topographical features. On seasonal scales, eddies were more numerous in the summer and early fall. At inter‐annual scales, eddy counts increased by 40% in association with the 2014–2015 marine heatwave and the 2015–2016 El Niño. A domain‐wide diurnal cycle was observed in the formation of eddies and the normalized vorticity. To determine the relative contributions of tides and diurnal winds, an analysis of spectral components and their spatial distribution along the SCB was conducted. The results revealed that while diurnal tides may exert some influence on the diurnal variations, their effect is comparatively minor when compared to diurnal winds. This conclusion was reached by considering the prevalence of theS₁frequency, which is a meteorological tide known to be associated with motions induced by sea‐land breeze. Overall, diurnal variability was more prominent in the southern SCB and less significant toward the north.