Nearshore Lagrangian Connectivity: Submesoscale Influence and Resolution Sensitivity

Abstract

Realistic simulation of nearshore (from the shoreline to approximately 10‐km offshore) Lagrangian material transport is required for physical, biological, and ecological investigations of the coastal ocean. Recently, high‐resolution simulations of the coastal ocean have revealed a shelf populated with small‐scale, rapidly evolving currents that arise at resolutions ⪅100 m. However, many historical and recent investigations of coastal connectivity utilize circulation models with ≈1‐km resolution. Here we show a resolution sensitivity to simulated Lagrangian transport and coastal connectivity with a hierarchy of Regional Oceanic Modeling System simulations of the Santa Barbara Channel at Δx= 1, 0.3, 0.1, and 0.036 km. At higher resolution ( normalΔx⪅ 100 m), rapid alongshore 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 alongshore 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 (Δx= 1 km). Differences in three‐dimensional and two‐dimensional transport are explored in a higher‐resolution simulation: In general, three‐dimensional trajectories are more dispersive than two‐dimensional, due to a separation in their respective trajectories.

Characteristics and Dynamics of Density Fronts over the Inner to Midshelf under Weak Wind Conditions

“…5a) with maxima near 6908 (fronts aligned with the shoreline) and a minimum near u f 5 08 (fronts shore-normal oriented). The prevalence of alongshore-oriented fronts is generally consistent with modeled inner to midshelf density gradients preferentially aligned across isobath (Romero et al 2013;Dauhajre et al 2017). Based on the u f distribution, fronts are categorized into alongshore oriented, cross-shore oriented, and inclined fronts.…”

“…In numerical models in the Gulf of Mexico, the density gradient of TTW generated fronts and filaments can strengthen rapidly on hourly time scales consistent with an asymptotic model assuming weak near-surface stratification (Barkan et al 2019). The TTW mechanism was invoked to explain the strengthening of coastal density filaments and fronts during winter and spring in a highresolution numerical model (Dauhajre et al 2017). However, to what extent the DF and TTW mechanisms are generally applicable to generation of density fronts in coastal regions within 10 km from shore is unclear.…”

“…The submesoscale flows were elevated for stronger root-mean-square (rms) surface alongshore density gradients at length scales , 15 km, which were enhanced by the large scale (over '15 km alongshelf) convergent northward alongshore flow, suggesting cross-shore-oriented fronts. In high-resolution numerical models, density gradients are preferentially perpendicular to bathymetric contours in depths , 50 m (Romero et al 2013;Dauhajre et al 2017), suggesting alongshore-oriented fronts and filaments. In general, the kinematics (i.e., occurrence likelihood, orientations, lengths, and density gradient magnitude) of coastal (within 10 km of shore) density fronts in regions with little freshwater input are poorly understood.…”

“…Previous studies have shown that submesoscale fronts and filaments in the open ocean (hundreds of kilometers from shore) can affect the transport of biogeochemical tracers and contaminants (e.g., Franks 1992;Nagai et al 2015;Mahadevan 2016;Lévy et al 2018). Submesoscale density fronts are ubiquitous on continental shelves in high-resolution coastal models (e.g., Romero et al 2016;Dauhajre et al 2017Dauhajre et al , 2019, observed within 10 km from shore (e.g., Ohlmann et al 2017;Connolly and Kirincich 2019), and detected in satellite sea surface temperature (SST) images (e.g., Castelao et al 2006;Kahru et al 2012). Dye and SST observations reveal frontal variability within 1 km from shore (Hally-Rosendahl et al 2015;Grimes et al 2020).…”

Clustering of Floating Tracer Due to Mesoscale Vortex and Submesoscale Fields

“…Ocean circulation at the scales smaller than the mesoscale is dominated by the broad range of submesoscale processes, which have been intensively studied (Berti et al, 2011(Berti et al, , 2016Haza et al, 2016;Huntley et al, 2015;Jacobs et al, 2016;McWilliams, 2016;Ohlmann et al, 2019;Schroeder et al, 2012;Zhong & Bracco, 2013). Interactions between submesoscale and mesoscale motions are essential in the formation and breakdown of coherent mesoscale vortices, but the theoretical understanding is hindered by overwhelming computational costs due to the spatial resolution requirements (Dauhajre et al, 2019). An efficient way (though, with obvious limitations) to study these interactions is by employing kinematic models for submesoscales, whereas retaining dynamical models for mesoscales-this is the approach adopted in our study and applied to the tracer clustering phenomena.…”