Scale Evolution in Coastal Upwelling Frontal Instabilities

Scott Durski
Oregon State Univerisity

Nonlinear model studies of instability evolution on a time-evolving coastal upwelling front indicate that disturbances develop on short horizontal scales but rapidly evolve to longer wavelengths. Several mechanism associated with the non-stationarity of the basic state on which the instabilities develop may promote the progression in scale. The upwelling evolution alters the flow structure either when advecting offshore under sustained winds or falling shoreward under wind relaxation. The finite amplitude amplification of the disturbances alter the alongshore mean flow state potentially promoting growth at new scales. Wave-wave interactions between finite amplitude instabilities may lead to growth at different scales as well. To examine these possibilities, tangent linear ROMS simulations about basic states that represent different approximations to the unstable upwelling evolution are performed. Under relaxing wind conditions it is found that a significant fraction of the scale change is associated with the alteration of the mean flow state by the finite amplitude instabilites. The scale change observed under sustained upwelling winds however results primarily from wave-wave interactions.