Mass transport by non-linear internal waves on the Malin Shelf

Abstract

Packets of non-linear internal waves (NIWs) were observed with a moored ADCP in the seasonal thermocline at the edge of the Malin Shelf, to the west of Scotland, during August 1995. A wide range of wave sizes was encountered including some that were strongly non-linear. The largest waves had a vertical displacement of about 25 m (in 145 m water) and a vertical shear greater than 0.4 m s −1 over a depth of 65 m across the thermocline. In general they propagated toward the ESE, which is south of a line normal to the shelf break, with a phase speed of between 0.54 and 0.60 m s −1. Within each packet the direction of the current flow in successive waves veered at a similar rate to that of waves with a tidal frequency. However, large NIWs were only encountered during neap rather than spring tides, so their relationship with the tide is not entirely clear. The first empirical mode dominated the motion, but its shape was often more convoluted than an equivalent baroclinic mode. A two-layer KdV theory for the mass transport due to the waves, which relates transport to the elevation of the interface and the linear long wave phase speed, is presented. It compares well with the observed transport in the lower layer. None of the waves possessed significant dispersion, so they did not conform to the KdV soliton solution. In a typical NIW packet a lower layer transport of about 5 m 2 s −1 offshore was maintained over a period of about 1.5 h, with a peak of about 20 m 2 s −1. A sustained transport of about 0.3 m 2 s −1 can be attributed to NIWs at the Malin Shelf edge in summer.