A cross-sea is a strange, yet dangerous phenomenon that results in the creation of a square-shaped wave pattern near the shore.
A cross-sea (also referred to as a squared sea or square waves) is a sea state of wind-generated ocean waves that form nonparallel wave systems. Cross seas have a large amount of directional spreading. This may occur when water waves from one weather system continue despite a shift in the wind. Waves generated by the new wind run at an angle to the old.
Two weather systems that are far from each other may create a cross-sea when the waves from the systems meet at a place far from either weather system. Until the older waves have dissipated, they create a perilous sea hazard.
This sea state is fairly common and a large percentage of ship accidents have been found to occur in this state. Vessels fare better against large waves when perpendicular to the waves. In a cross-sea, vessels are more likely to be stuck in a dangerous way.
A cross swell is generated when the wave systems are longer-period swells, rather than short-period wind-generated waves.