We present a mechanism to explain the permanent excitation of warps in spiral or barred galaxies. We show that non-linear mode coupling with the spiral wave can be very efficient to drive two warp waves. We derive an expression of the coupling coefficient in the WKB approximation. We show that the coupling is too weak in the stellar disk, except at the Outer Lindblad Resonance where the spiral slows down and is efficiently coupled to warp waves. There, the spiral can almost completely transfer its energy and angular momentum to a ”transmitted“ warp, which we can observe in HI, and a ”reflected“ one, which we can observe as a corrugation. Our mechanism reproduces the observed amplitudes of the warp and of the corrugation.

We first present side results concerning the propagation of warps in moderately thick disks.