The vertical propagation of Coastal Trapped Waves (CTWs) due to subsurface ridges is explored with the help of linear numerical and analytical models. Results show that submerged ridges projecting from the shoreline can scatter a horizontally propagating single baroclinic mode Kelvin wave into both upward and downward propagating Kelvin wave (KW) beams, emanating from the ridge top. The semi-infinite shelf response to an incident single mode KW reveals alongshore dependence of the vertical structure in the form of multi-modal ridge-top KWs, suggesting that the width (alongshore extent) of the ridge is an important factor in determining the basin response past the ridge. We hypothesize that over narrow ridges (less than twice the Rossby radius of deformation on ridge-top), the trapped solutions at the edges of the ridges overlap and interact to transmit horizontally propagating energy into the surface layer of the downstream basin. At the same time narrow ridges result in a weaker subsurface peak in velocity next to the ridge top in the downstream basin. This decreases the amplitude of vertically propagating KW beams.
The relative strengths of horizontally and vertically propagating KW modes, is explored by imposing the alongshore KW velocity profiles as a boundary condition next to edge of the ridge in the downstream basin. When the strength of the subsurface peaks in the vertical profile of alongshore currents is greater the basin response includes relatively stronger amplitudes of vertically propagating KW beams. On the other hand, when the strength of the broader surface maximum is greater, the strength of the horizontal propagation of energy at the surface is higher relative to the amplitudes of upward propagating KW beams in the downstream basin.
The west coast of India near Kollam has a shelf-slope width comparable to that of the first mode Rossby radius of deformation, and vertical propagation of intraseasonal coastal trapped waves (CTWs) has been observed downstream of this ridge. To check the applicability of the KW experiments to such a situation, the numerical model was configured with a sloping coastal bottom similar to that found along the west coast of India, and an offshore ridge similar to the one used in the KW experiments. A first-mode CTW was imposed as the incident wave. The generation by the ridge of upward propagation was similar to that seen in the KW models. Close to the coast, however, the downward propagating beam quickly reflects from the sloping bottom into an upward propagating beam.
The effects demonstrated in these idealized experiments are likely to be found wherever KW or CTW propagation is important and the alongshore coastal bathymetry contains large changes in depth over short distances.