Large-scale vorticity generation and kinetic energy budget along the U.S. West Coast

TL;DR

This study analyzes the energy and vorticity generation along the U.S. West Coast, revealing shoreline as the primary source of kinetic energy and vorticity, supported by observational data and lattice Boltzmann simulations.

Contribution

It provides a detailed energy budget analysis of the region, highlighting shoreline effects and estimating eddy viscosity, which are novel insights into coastal ocean dynamics.

Findings

Kinetic energy and vorticity are mainly generated along shorelines.

Net wind stress work is negligible in open water regions.

Effective eddy viscosity is about 10^-2 m^2/s along shorelines.

Abstract

We attempt to evaluate energy budget over a restricted but extremely well studied oceanic region along the shorelines of Oregon and California. The analysis is based on a recently updated geostrophic flow field data set covering 22 years with daily resolution on a grid of 0.25$^\circ\times$0.25$^\circ$, and turbulent wind stress data from the ERA-Interim reanalysis over the same geographic region with the same temporal and spatial resolutions. Integrated 2D kinetic energy, enstrophy, wind stress work and { kinetic energy tendency} are determined separately for the shore- and open water regions. The empirical analysis is supported by 2D lattice Boltzmann simulations of freely decaying vortices along a rough solid wall, which permits to separate the pure shoreline effects and dissipation properties of surface flow fields. Comparisons clearly demonstrate that kinetic energy and vorticity { of the geostrophic flow field} are mostly generated along the shorelines and advected to the open water regions, where the net wind stress work is almost negligible. Our results support that the geostrophic flow field is quasistationary on the timescale of a couple of days, thus total forcing is practically equal to total dissipation. Estimates of unknown terms in the equation of oceanic kinetic energy budget are based on other studies, nevertheless our results suggest that { an effective} eddy kinematic viscosity is in the order of magnitude $10^{-2}$ m$^2$/s along the shorelines, and it is lower { by a factor of two} in the open water region.