Internal Wave Generation in the Lee of Periodic Topography

Laboratory experiments are performed to examine the generation of internal waves in uniformly salt-stratified fluid by towed two-dimensional periodic topography of finite extent. Linear theory predicts that vertically propagating internal waves are generated if the Froude number Fr= U/N λ < 1/2 π . (Here U is the towing speed, N is the buoyancy frequency, and λ is the distance between successive hill crests.) As found by Baines and Hoinka (1985), when Fr is moderately smaller than this value, a large amplitude lee wave is generated downstream of the topography. This horizontally propagating wave is characterised by spatially decaying, periodic vertical displacements of near-surface isopycnals on the order of the hill amplitude immediately in the lee of the topography. The flow is laminar. The lee wave itself excites vertically propagating internal waves which have amplitude approximately twice that of internal waves generated directly above the topography. That is, larger waves are excited after the topographic forcing is turned off! This occurs because the internal waves resonantly interact with the lee waves acting both to establish the pattern of lee waves and to extract energy efficiently away from them.