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

Packets of non-linear internal waves (NIWs) were observed with a moored ADC P in the seasonal thermocline at the edge of the Malin Shelf, to the west o f Scotland, during August 1995. A wide range of wave sizes was encountered including some that were strongly non-linear. The largest waves had a verti cal displacement of about 25 m (in 145 m water) and a vertical shear greate r 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 shel f break, with a phase speed of between 0.54 and 0.60 m s(-1). Within each p acket the direction of the current flow in successive waves veered at a sim ilar rate to that of waves with a tidal frequency. However, large NIWs were only encountered during neap rather than spring tides, so their relationsh ip with the tide is not entirely clear. The first empirical mode dominated the motion, but its shape was often more convoluted than an equivalent baro clinic 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 tra nsport in the lower layer. None of the waves possessed significant dispersi on, so they did not conform to the KdV soliton solution. In a typical NIW p acket 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 sustai ned transport of about 0.3 m(2) s(-1) can be attributed to NIWs at the Mali n Shelf edge in summer. (C) 2001 Elsevier Science Ltd. All rights reserved.