Australasian Science: Australia's authority on science since 1938

Sea Swell Decay Deduced by Satellite

By Stephen Luntz

The factors that determine the decay of ocean swells have been resolved from satellite data, allowing better forecasts of wave conditions.

Prof Ian Young of the Research School of Earth Sciences at the Australian National University used decades of satellite data to analyse which waves decay quickly in deep water and which survive.

Surfers have long known that steeply sloping waves will break earlier than more rounded ones. While Young says that wave behaviour in deep water is quite different, it is also true that the steeper the wave, the more quickly energy is dissipated. “In deep water, swells decay because of interaction with background turbulence, although we don’t know if this is with the air, with turbulence in the water below or some of both.”

Young says there has only been one previous study looking at the question, and while it relied on a much smaller data set it produced broadly similar results. The question is important, he says, because “the Southern Ocean is dominated by big low pressure systems that move across it year-round. These systems generate waves that then grow and can travel tens of thousands of kilometres from where they were actually formed, to crash on a beach in Australia.”

The results may allow better forecasts of wave conditions 2–4 days ahead for beachgoers, but could also give warning to oil and gas drilling platforms to shut down before the largest waves arrive, or for ships to avoid unloading in uncomfortably heavy swells. Structures could also be designed to avoid resonance at common wave frequencies.

“At the moment wave energy is such a minute portion of the grid that prior information is not needed to allow power authorities to balance the load, but if it becomes larger this work could be helpful,” Young says.