Seasonal variability of upwelling and downwelling surface current patterns in a small coastal embayment

Abstract Upwelling in coastal embayments is important to a variety of physical and biological processes. Despite their ubiquity, circulation patterns in small bays (width and length scales ≤ 20 km) in eastern boundary current upwelling systems are relatively understudied compared to their larger counterparts. In this study, we apply a conditional averaging technique to investigate upwelling- and downwelling-driven circulation in a small coastal embayment located in Central California (San Luis Obispo Bay). We also investigate intraseasonal differences in the current patterns. Conditional averaging reveals distinct intraseasonal differences and features that are obscured by traditional seasonal averages when examining the upwelling jet separation and onshore advection, horizontal divergence patterns, and particle trajectories. We show that the upwelling circulation and resulting upwelling jet separation and onshore advection reinforce a convergent upwelling shadow front at this site, with important ecological ramifications. While tuned specifically for San Luis Obispo Bay, these findings can be used as a baseline for similar small upwelling bays, highlight the importance of conditional averages (versus traditional temporal averages), and underline the importance of coastal upwelling seasonality beyond the common bimodal upwelling and non-upwelling description.