Because of the geology of the Isle of Rhé in France, it's perfectly situated for the formation of cross sea waves. Wikimedia Commons
Watching an ocean wave crash against a craggy, rock-strewn coast is beyond magical. But there is one type of wave — the cross sea wave — that is beyond anything you may have ever experienced. Although still mesmerizing and surreal, cross seas are dangerous and have been responsible for many ship accidents.
Before we get into the physics of cross seas, let's review how ocean waves are generated. The wind is mostly responsible for the creation of ocean waves, which shouldn't be confused with waves created by earthquakes, landslides and volcanic eruptions. As the wind blows, it transfers energy to the surface of the water creating a constant disturbance that results in a wave crest.
Waves generally run parallel to the shore, but a cross sea is different. It's created when the wind from two weather systems — one old, the other more recent — collide. When that happens, the waves of the newer weather system run at an oblique angle to the old weather system, whose waves continue on despite the shift in wind. The result looks like a quilt-like pattern of squares on the ocean surface.
Cross seas generally occur along coastal areas, and they are rare. Yet, there is one place along the west coast of France cross seas occur with astonishing regularity. Because of the geology of the Isle of Rhé, it's perfectly situated for their formation. The sight of the waves brings thousands of tourists to the island's lighthouse each year.
When a cross sea blows, the water can be difficult to navigate for boaters, as well as swimmers. Part of the reason cross seas are so dangerous is that they generate powerful rip currents, and powerful waves, which can reach nearly 10 feet (3 meters) high, more than enough to swamp a large boat.
Cross seas are usually spotted in shallow waters, such as those off the Isle of Rhé, and Tel Aviv, Israel. Scientists say cross seas are an example of the Kadomstev-Petviashvili equation at work. The formula describes non-linear wave motion and is often used to explain how weather systems interact with one another.