Recent laboratory experiments show that a stable wave packet propagating into a region characterized by an opposite current may become modulationally unstable. Observations have been gathered in two independent wave tank facilities: a wave flume at Plymouth University (UK) and a narrow directional wave tank at the University of Tokyo (Japan). Both facilities are equipped with a wavemaker and a pump for generating a current propagating in the opposite direction with respect to the waves.
Figure 1: Deformation of the wave profile due to an opposite background current
Apart from refracting the wave field, the current is responsible for a steepening of the wave profile. If the current speed is sufficiently high, this trigger the instability of an otherwise stable wave group, leading to the formation of rogue waves (Fig. 2). The experimental results support a recent conjecture based on a current-modified nonlinear Schrodinger equation which establishes that rogue waves can be triggered by a non-homogeneous current characterized by a negative horizontal velocity gradient.
Figure 2: Evolution of the surface elevation along the flume: with no current the wave packet is stable (left panel); when propagating against an opposite current, the same wave packet gets unstable, resulting in the generation of a rogue waves (right panel)
Details can be find in
Toffoli, A., Waseda, T., Houtani, H., Kinoshita, T.,
Collins, K., Proment, D., Onorato, M., 2013. Excitation of rogue waves in a
variable medium: An experimental study on the interaction of water waves and
currents. Phys. Rev. E, 87, 051201(R),
4 pages ( http://pre.aps.org/abstract/PRE/v87/i5/e051201 )
