Submesoscale Influence on Nearshore Lagrangian Transport: How Important is Resolution to Simulated Coastal Connectivity?

Physical, biological, and ecological investigations of the coastal ocean often require realistic simulation of Lagrangian material transport in the nearshore (from the shoreline to approximately 10 km offshore). A useful metric these simulations can provide is the probability of material being transported from a source point to a destination, over a given time scale since material release, referred to as the oceanographic “connectivity” between the two sites. Many historical and recent investigations of coastal connectivity utilize circulation models with $\approx$ 1 km resolution. However, recent high-resolution simulations of the coastal ocean have revealed a shelf populated with small-scale, rapidly evolving currents that arise at resolutions $⪅$ 100 m. Here, we show a resolution sensitivity to simulated Lagrangian transport and coastal connectivity with a hierarchy of Regional Oceanic Modeling System (ROMS) simulations of the Santa Barbara Channel at $\Delta x=$ 1, 0.3, 0.1, and 0.036 km. At higher-resolution ($\Delta x⪅$ 100 m), rapid along-shore and vertical transport occurs in regions less than 1 km from the shoreline due to submesoscale shelf currents that open up new transport pathways on the shelf: submesoscale fronts and filaments, topographic wakes, and narrow along-shore jets. Shallow-water fronts and filaments induce early-time downwelling and subsequent dispersal at depth of surface material; this is not captured at coarser-resolution ($\Delta x=$ 1 km). The submesoscale-driven early-time downwelling causes differences in three-dimensional and two-dimensional (surface) trajectories. In general, three-dimensional trajectories are more dispersive than two-dimensional, due to a separation in their respective trajectories. Overall, the results of this work caution biophysical studies that attempt to simulate large-scale connectivity of nearshore-sourced biological material utilizing circulation models with $\Delta x⪆$ 1 km, previously perceived as "high-resolution".