The focusing of surface waves by internal waves

The surface current generated by internal waves in the ocean affects surfac e gravity waves. The propagation of short surface waves is studied using bo th simple ray theory for linear waves and a fully nonlinear numerical poten tial solver. Attention is directed to the case of short waves with initiall y uniform wavenumber, as may be generated by a strong gust of wind. In gene ral, some of the waves are focused by the surface current and in these regi ons the waves steepen and may break, Comparisons are made between ray theor y and the more accurate solutions, For ray theory, the occurrence of focusi ng is examined in some detail and exact analytic solutions are found for ra ys on currents with linear and quadratic spatial variation - only the latte r giving focusing for our initial conditions. With regard to interpretation of remote sensing of the sea surface, we find that enhanced wave steepness is not necessarily associated with a particular phase of the internal wave , and simplistic interpretations may sometimes be misleading.