Lifecycle of a Submesoscale Front Birthed from a Nearshore Internal Bore

The ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entails 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 approximately 4 h, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of alongshore velocity creates enhanced vertical vorticity (Rossby number approximate to 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise.

Automated summary

The study presents observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean, and discusses the process of separation from the bore, as well as the subsequent propagation as a gravity current. The study also highlights the development of lateral shear instabilities along the front, which may accelerate its demise.