  Ocean waves are complex and often turbulent. While most ocean wave
interactions are essentially linear, sometimes two or more waves interact in a
nonlinear way. For example, two or more waves can interact and yield waves that
are much taller than the sum of the original wave heights. Most of these
nonlinear interactions look like an X or a Y or two connected Ys; at other
times, several lines appear on each side of the interaction region. It was
thought that such nonlinear interactions are rare events: they are not. Here we
report that such nonlinear interactions occur every day, close to low tide, on
two flat beaches that are about 2,000 km apart. These interactions are closely
related to the analytic, soliton solutions of a widely studied
multi-dimensional nonlinear wave equation. On a much larger scale, tsunami
waves can merge in similar ways.
