Q: Where do ocean waves come from and why are they bigger in winter?

A: Arctic and antarctic storms which rage for long periods of time produce most ocean swell, while smaller storms, hurricanes, and local winds produce smaller waves. Except for tsunami, which are generated by earthquakes or landslides, the wind is the source of all ocean waves. Wind on the ocean surface generates a spectrum of waves. The range of wavelengths and heights depends on the speed and duration of the wind and the distance over which it blows. In the center of the storm the ocean surface is choppy and chaotic. As the waves spread outwards like ripples, the long waves travel faster than the short waves so they sort themselves by length. Gradually the sea surface disperses into regular and rhythmic swell. As the waves spread out their height decreases in proportion to the square of the distance traveled. Eventually the smaller waves disappear leaving only the larger ones. At any given place, the ocean surface consists of trains of waves of different heights and lengths moving away from storms in various parts of the ocean. Since there are always storms somewhere and it takes time for waves to arrive, there are always waves coming from different directions. Most ocean swell is generated in the far northern and southern oceans where conditions favor the development of cyclonic storms that are more intense in winter. The seasons are reversed in the northern and southern hemispheres so there is a seasonal shift in the direction of the dominant waves. Big waves come from the north in winter because the arctic storms are relatively close to us. The waves are not well organized and tend to arrive in sets which are not completely sorted by dispersion. In the summer the south swell is generated by storms in the antarctic where it is winter. Having traveled further the waves are smaller and more regular than the northerly waves of winter. On any give day the biggest waves may come from any direction, but there are seasonal patterns which are generally predictable.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send question to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email o rickb@hcc.hawaii.edu

Waves and Wind ©1995 Richard C. Brill