Airborne lidar sensing of internal waves in a shallow fjord

A dual polarization lidar was used to sense internal waves from a small aircraft. Internal waves are gravity waves that are formed by a vertical displacement of a density gradient in the ocean. If the perturbation is great enough, a nonlinear wave is produced and the balance between nonlinearity and dispersion can create a soliton-like wave packet. We observed nonlinear wave packets in West Sound, Orcas Island, Washington. In this region, a density gradient is formed in the summer by solar heating of the surface water. The perturbation is produced by strong tidal flow through a narrow, shallow channel at the mouth of the sound. Plankton layers form in association with the density gradients, and these layers produce an enhanced lidar return that moves up and down with the wave. We observed these internal waves with a lidar operating at 532 nm. They were much more visible when the receiver was polarized orthogonal to the transmitted laser pulse. This was the case whether linear or circular polarization was used, with no significant difference between the two cases. These internal waves were also visible to the naked eye, when the surface currents produced by the waves modulated the small surface waves that produce the apparent texture of the ocean surface.