Lifecycle of a Submesoscale Front Birthed from a Nearshore Internal Bore

Abstract

The ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entail complex nonlinear dynamics, even in isolation. Here we present shipboard, moored, and remote observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean. The internal bore is observed to flatten as it shoals, leaving behind a gravity current front that propagates significantly slower than the bore. We posit that the generation and separation of the front from the bore is related to particular stratification ahead of the bore, which allows the bore to reach the maximum possible internal wave speed. After the front is calved from the bore, it is observed to propagate as a gravity current for ≈4 hours, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of along-shore velocity creates enhanced vertical vorticity (Rossby number ≈ 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise.

Interactions Between Internal Tidal Bores and Submesoscale Currents on the Continental Shelf

“…This computational study and recent observational studies (Haney et al, 2021;Kumar et al, 2021) make apparent the coexistence of distinctly generated, small-scale nearshore currents (e.g., surf-zone vortices, internal tidal bores, surface layer fronts and filaments). We are now learning that this coexistence breeds a spectrum of interactions that can modulate the evolution of a particular coherent structure with anticipated, yet unexplored, consequences for energetic and material fluxes.…”

“…The TTW SMCs in this study are depth-filling and commonly advected by larger-scale currents. They can be contrasted with the near-surface, propagating submesoscale front observed in Haney et al (2021) that was birthed from a bore and resembled a gravity current. Despite different generation mechanism and governing dynamics, both of these types of SMC (gravity current, TTW) exhibit large, positive relative vorticity (ζ ≫ f) that can distinguish them from bores (ζ ≈ 0, see Figure 4c).…”

“…We output fields with a 30 min average to better resolve the rapid evolution of interactions between internal tidal bores and submesoscale currents. The dense ISDE data set (August–November 2017; Kumar et al., 2021) highlights the prevalence of internal tidal bores (Becherer et al., 2020; Haney et al., 2021; McSweeney, Lerczak, Barth, Becherer, Colosi, et al., 2020; McSweeney, Lerczak, Barth, Becherer, MacKinnon, et al., 2020), topographic wakes (Kovatch et al., 2021), and surf‐zone plumes (Moulton et al., 2021) in this same region. We do not intend this study to serve as a direct comparison with the ISDE observations due to the different time periods (discussed further in Section 6.2).…”

“…Haney et al. (2021) provide striking evidence of a shoaling internal tidal bore “birthing” a submesoscale front that resembles a gravity current. Kumar et al.…”

Airborne Observations of Shoaling and Breaking Internal Waves

“…Spatio‐temporal evolution of an internal wave can be inferred through remote sensing approaches, such as synthetic‐aperture radar (SAR, e.g., see Romeiser & Graber, 2015) or X‐band radars (e.g., Celona et al., 2021; Haney et al., 2021). These indirect observational techniques rely on the modulation of surface roughness due to wave‐current interaction (Alpers, 1985; Craig et al., 2012) to infer the propagation speed and currents induced by the internal wave.…”