Finite-Amplitude Evolution of Instabilities Associated with the Coastal Upwelling Front

Abstract

A primitive equation model is used to study the finite-amplitude evolution of instabilities associated with the coastal upwelling front. Simulations of increasing complexity are examined that represent idealizations of summer conditions off the Oregon coast, including cases with steady and with time-variable wind in a domain with alongshore-uniform bathymetry and with time-variable wind in a domain with realistic Oregon coast bathymetry. The numerical results indicate that the fastest-growing mode in this system has approximately an 8?10-km alongshore wavelength but that, once the disturbances grow to finite amplitude, the predominant alongfront scale increases rapidly because of nonlinear effects. Separation of the total kinetic energy into contributions from the alongshore average flow and perturbation about that average shows that the initial growth of the perturbation kinetic energy is due to potential energy conversion, but transfer of energy from the kinetic energy of the alongshore average flow becomes important once the disturbances reach large amplitude. The time-variable wind simulations again show initial growth of small-scale instabilities followed by evolution to larger scales. In this case, however, even after larger-scale disturbances have developed on the upwelling front, smaller-scale patterns amplify along the front in response to each upwelling-favorable wind event. Realistic coastal bathymetry introduces additional alongshore topographic scales into the problem, but the formation of instabilities on small scales and evolution to larger scales are still ubiquitous. Where instabilities encounter strong curvature in the upwelling front produced by bathymetric effects, the upwelling front becomes highly contorted and horizontal variability is significantly enhanced.