Emergence and secondary instability of ekman layer rolls

We1 study the non-linear stages of the instability of the neutrally-stratied Ekman ow. We nd that two-dimensional equilibrated rolls exist, and are reached very closely, although not exactly, after a small random initial perturbation to the Ekman ow has evolved and reached nonlinear saturation. We perform in turn a linear stability analysis of these equilibrated rolls and nd that they are subject to an instability of the hyperbolic type. In both stud-ies, we investigate the inuence of the latitude and of the direction of the geostrophic wind on these stability proper-ties.