We investigate experimentally the dynamics of pattern excitation in presence of an advection-like effect in a nonlinear optical system. This advection effect, created when the angle of incidence of the pump beam is not zero, causes the existence of: (i) a transition from convective to absolute instabilities, (ii) the formation of new pattern geometries, (iii) the seeding of noise-sustained structures. All these phenomena, previously observed in nonlinear Kerr-type systems, are demonstrated in our setup composed by a photorefractive nonlinear medium in a tilted feedback configuration.