Nonlinear Water Waves and Wave–Current Interactions at Arbitrary Latitude

We survey some exact solutions in the Lagrangian framework, representing waves at arbitrary latitude that propagate eastward or westward above a flow which accommodates a constant underlying background current, waves that can be both in the direction of the current and in the opposite direction. These waves are linearly unstable to short-wavelength perturbations, if their steepness exceeds a specific threshold. This threshold depends on the latitude and the strength of the underlying current.