COASTAL-TRAPPED WAVES ON THE WEST-COAST OF SOUTH-ISLAND, NEW-ZEALAND

The coastal ocean response to wind forcing on the west coast continental shelf of South Island, New Zealand is examined using current, sea level and wind observations. Weather band motions over the northern region of the shelf appear to be dominated by the response to wind-force flux through Cook Strait. Through-strait forcing is still evident in the currents of the southern region but west coast alongshore wind-forcing is also important. Most of the variance of nearshore weather band currents in the northern region of the shelf can be described in terms of coastal-trapped waves (CTWs). A fit of CTW modes 1 and 2 to the observations in that region indicates the equal importance of both modes, in agreement with the theories of strait-generated CTWs. Farther south, the across-shelf depth profile alters dramatically such that CTW structures change from being almost barotropic in the northern region to highly baroclinic. This rapid change in topography suggests that considerable scattering of CTWs might occur, a hypothesis which is supported by current observations in the southern region of the shelf.