Submesoscale Currents on the Continental Shelf
The study of oceanic submesoscale dynamics, currents with spatial-scales of 0.1 - 1 km and time-scales of hours to days, is a rapidly growing branch of physical oceanography due to their prevalence in most regions of the ocean and importance to a growing list of oceanic processes (e.g., setting the stratification of the upper ocean). Previous investigations of submesoscale currents focus primarily in the open-ocean, where the preferred structures of the submesoscale (fronts, filaments, and vortices) exist in the surface boundary layer, well above the sea floor. However, recent, high-resolution regional simulations of the coastal ocean reveal a continental shelf populated with analogous submesoscale coherent structures. The revelation of nearshore submesoscale currents alters historical conceptions of continental shelf circulation that do not predict submesoscale variability in the nearshore.
This dissertation explores the coastal submesoscale regime from multiple perspectives. A phenomenological overview demonstrates that submesoscale fronts, filaments, and vortices are ubiquitous on the shelf, that local bathymetry can control spatial orientation, and that irregular coastal topography can generate submesoscale structures on the shelf. The local circulation of shallow-water fronts and filament is generally consistent with their open-ocean counterparts, defined by strong surface convergence, downwelling, and cyclonic shear that can be described by a diagnostic momentum balance between rotation, pressure gradient force, and vertical eddy diffusion (known as Turbulent Thermal Wind or TTW). Discovery of a previously unknown diurnal cycle in front or filament circulations (that is not predicted by the TTW balance) further extends understanding of submesoscale currents. Creation and analysis of an idealized model that adds acceleration to the TTW balance allows exploration of this diurnal variability. This idealization elucidates controlling one-dimensional (1-D) Ekman layer dynamics on the diurnal phasing. The isolation of the controlling 1-D mechanism motivates the formulation of a simple 1-D model that can accurately predict the phasing of front and filament circulations around the globe.
Simulation of Lagrangian trajectories in high-resolution coastal simulations reveals that submesoscale currents play an important role in the fate and transport of nearshore material. The ageostrophic secondary circulations of ephemeral shelf fronts and filaments can laterally trap and downwell surface material in less than a day. Failure to represent these currents in a simulation of coastal transport will result in a strongly retentive bias to along-shore transport and an under-representation of vertical transport. The results of this work are applicable to any biophysical study of the coastal ocean that require accurate representation of nearshore material fluxes.