What causes waves in the ocean?

There are many types of waves flowing across the oceans, from small ripples to huge walls of water, 30 metres high peak-to-trough. They are mainly caused by winds, and, if unimpeded, can travel for thousands of kilometres.

Waves occur when the force of the wind blows over surface water, transferring its energy and  causing the water to move in a circular motion. The rise and fall of water molecules creates a wave that moves in the direction of the wind. The wave transports energy, not water. An analogy is a crowd of people doing a Mexican wave in a stadium: they stay in the same location, but the wave they create travels around the arena.

As waves reach the shore, the shallow floor begins to run into their base, slowing their deeper parts. This causes the wave to stand up then pitch forward before eventually breaking.

The height of a wave is determined by the wind strength, the length of time it blows and its fetch – the distance it travels over the water. Oceanographers measure wave height using a term called “significant wave height”, which is the average height of the largest 33 per cent of waves in that region of ocean at the time.

Wind blowing for an hour at a speed of 55.6 kilometres per hour over a stretch of water 1000km wide would generate waves with a significant wave height of 0.7 metres. But if it blew for 48 hours, the wave’s crest would increase to 10.5 metres, about twice the height of a double-decker bus.

Occasionally waves merge to form one that is far higher than those surrounding it. These rogue waves were once thought to be fictional, or vanishingly rare. However, through better monitoring of the seas, we now know these freak waves are surprisingly common. The complex forces that give rise to them are now better understood, raising the possibility of rogue wave forecasts in the future.

Some research suggests that extreme waves are becoming more likely as a result of climate change, due to an increase in storms and melting of polar ice.

Not all waves are caused by the wind though. Seismic activity can cause a series of long-wavelength waves known as a tsunami, due to the displacement of large amounts of water. The gravitational pull of the sun and moon also cause waves to form – these are known as tides.