Normal-Mode Instabilities of a Time-Dependent Coastal Upwelling Jet

Abstract

The linear stability of a nearly time-periodic, nonlinear, coastal upwelling-downwelling circulation, over alongshore-uniform topography, driven by a time-periodic wind stress is investigated using numerical methods. The near-periodic alongshore-uniform basic flow is obtained by forcing a primitive equation numerical model of coastal ocean circulation with periodic wind stress. Disturbance growth on this nearperiodic flow is explored in linear and nonlinear model simulations. Numerous growing normal modes are found in the linear analyses at alongshore scales between 4 and 24 km. These modes vary in cross-shore structure and timing of maximum disturbance growth rate. One group of modes, in the 6.5-8.5-km alongshore-scale range, bears strong resemblance to the ensemble average disturbance structures observed in perturbed nonlinear model simulations. These modes are of a mixed type, exhibiting both strong baroclinic and barotropic energy exchange mechanisms, with maximum disturbance growth occurring during the transition from upwelling favorable to downwelling favorable winds. Nonlinear disturbance growth is characterized by similar structures at these same scales, but with significant exchange of energy between disturbances at different alongshore scales, such that overall disturbance energy accumulates at the longest (domain) scales, and gradually propagates offshore mainly in the pycnocline over numerous forcing cycles.

Generation of the Cape Ghir upwelling filament: A numerical study

“…Nevertheless, the role of high-frequency wind variations cannot be discarded: off the Oregon coast, experiments with temporally variable winds (Durski and Allen, 2005), with relaxed and sustained winds (Durski et al, 2007) or with a timeperiodic winds (Durski et al, 2008) underlined their role in the creation of alongshore-scale instabilities and large-scale disturbances in the upwelling front. Experiments with more realistic winds will be performed in a future work.…”

Remote sensing observation and numerical modeling of an upwelling jet in Guangdong coastal water

“…The spatial distribution of upwelling determines the offshore extent of nutrient enhancement. In addition to the coastline [ Barth and Smith , 1998; Barth et al , 2000; Dale and Barth , 2001; Relvas and Barton , 2005; Durski et al , 2008], the bottom topography is a significant factor specifying the spatial distribution of upwelling as well [ Song et al , 2001; Song and Chao , 2004]. Alongshore undulations of an upwelling can be attributed to the alongshore topographic variation [ Glenn et al , 1996].…”

“…However, the 2‐D assumption cannot resolve the horizontal, fine structure of the coastal upwelling dynamics in alongshore direction. In fact, due to irregularities of the bottom topography and coastline, the coastal upwelling exhibits significant variations in both the alongshore and cross‐shore directions [ Barth and Smith , 1998; Barth et al , 2000; Dale and Barth , 2001; Relvas and Barton , 2005; Durski et al , 2008]. On the Oregon coastal water, bottom topography dominates over the coastline in determining the alongshore distribution of upwelling [ Peffley and O ' Brien , 1976].…”

The Baroclinic Adjustment of Time-Dependent Shear Flows

“…The relatively few works that have investigated timedependent shear flows have almost entirely focused on the extreme cases in which the shear flows are time periodic (Davis 1976) or white noise (Farrell andIoannou 1996, 1999). Two notable exceptions are Durski et al (2008), who recently studied an aperiodic time-dependent coastal upwelling jet, and Inoue and Smyth (2009), who investigated the mixing efficiency of unsteady shear flows. Because the majority of realistic flows lie somewhere between the periodic and white-noise limits, these examples are useful idealizations, but additional research is needed to understand more realistic time-dependent shear flows.…”