Rogue Waves

For centuries, sailors described waves that rose without warning from ordinary seas, walls of water two or three times the height of everything around them, strong enough to break a ship's hull. Oceanographers largely dismissed these accounts, since standard wave models predicted that a wave so far outside the surrounding sea state should be vanishingly rare. That assumption became untenable on January 1, 1995, when an instrument on the Draupner platform in the North Sea recorded a single wave 25.6 meters high, more than twice the 11.92-meter significant wave height measured around it at that same moment. Unlike a sailor's account, this reading came from a laser sensor, giving researchers an exact record of something previously only reported. Since then, oceanographers have shifted their attention from whether rogue waves exist to what produces them. The explanation favored through the 1990s and 2000s, a nonlinear effect called modulational instability, treats a rogue wave as several ordinary waves briefly reinforcing one another until one shoots far above the rest. More recent field data instead points to a simpler cause: constructive interference, amplified by the fact that real ocean waves rise into sharper crests than the troughs they fall into. Modulational instability still holds up in the laboratory, but real seas rarely match those conditions closely enough to make it dominant.