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As brackish turbid waters exit San Francisco Bay, one of the largest estuaries in the U.S. West Coast, they form the San Francisco Bay Plume (SFBP), which spreads offshore and influences the Gulf of the Farallones (GoF), an ecologically significant region in the California Current System that is also home to three National Marine Sanctuaries. This paper provides the first observationally based investigation of the spatio-temporal variability of the SFBP, using a plume tracking algorithm applied to more than two decades (2002-2023) of ocean color data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard satellites Aqua and Terra. The turbid SFBP spreads radially, extending 10-20 km offshore around 50% of the time, and during extreme discharge events (<1% of the time), the plume can reach nearly 60 km offshore to the shelf break. The greatest variability in frequency of plume occurrence was observed 10-20 km offshore and it was largely explained by the seasonal cycle (80% of total variance), linked primarily to seasonal changes in river discharge. Largest plume areas (determined by summing up all pixel areas weighted by their respective fraction of plume occurrence) were observed during winter and smallest during summer, occupying on average 24% and 1.5% of GoF area, respectively. Beyond 20-30 km offshore, variability in frequency of plume occurrence was dominated by the intraseasonal band (50-80% of total variance), attributed to plume response to synoptic wind-forcing and/or filaments and eddies, while the interannual band played a secondary role in the plume variability (<20% of total variance). Finally, a multivariable linear regression model of the turbid SFBP area was created to explore the potential predictability of the plume’s influence in the GoF. The model included the annual and semi-annual cycles and discharge anomalies (deseasoned and detrended), and despite its simplicity, it explained over 78% of total variance of the turbid SFBP area. Therefore, it could be a useful tool for scientists and stakeholders to better understand how management actions on freshwater supply can have consequences offshore beyond the Golden Gate and help guide future management decisions in this ecologically important region. 

Abstract. Offline particle tracking (OPT) is a widely used tool for theanalysis of data in oceanographic research. Given the output of ahydrodynamic model, OPT can provide answers to a wide variety of researchquestions involving fluid kinematics, zooplankton transport, the dispersionof pollutants, and the fate of chemical tracers, among others. In thispaper, we introduce ROMSPath, an OPT model designed to complement theRegional Ocean Modeling System (ROMS). Based on the Lagrangian TRANSport(LTRANS) model (North et al., 2008), ROMSPath is written in Fortran90 and provides advancements in functionality and efficiency compared toLTRANS. First, ROMSPath calculates particle trajectories using the ROMSnative grid, which provides advantages in interpolation, masking, andboundary interaction while improving accuracy. Second, ROMSPath enablessimulated particles to pass between nested ROMS grids, which is anincreasingly popular scheme to simulate the ocean over multiple scales.Third, the ROMSPath vertical turbulence module enables the turbulent(diffusion) time step and advection time step to be specified separately,adding flexibility and improving computational efficiency. Lastly, ROMSPathincludes new infrastructure which enables inputting of auxiliary parameters for addedfunctionality. In particular, Stokes drift can be input and added toparticle advection. Here we describe the details of these updates andperformance improvements.