When you think “tranquility,” what comes to mind? If you’re like many people, you envision crystal-clear waves lapping gently at the shores of the Caribbean Sea. Yet that sparkling cerulean surface conceals a massive ruckus. Researchers say the movements of the sea itself produce a whistling-type resonance. They published their findings in the journal Geophysical Research Letters.

Our oceans and seas are no stranger to weird noises (remember The Bloop?), but historically those sounds have been made by animals, vehicles, and natural phenomena in the water—not by the water itself.

The research team hadn’t set out to find strange sounds. Earlier studies had shown that the sea’s structure and flow create a meteorological formation known as a Rossby wave. Named for Swedish-American meteorologist Carl-Gustaf Arvid Rossby, the waves are swirls in the atmosphere produced by the meeting of cold polar air moving toward the equator and warmer equatorial air moving poleward. The topography of the Caribbean seafloor, along with the exchange of water between the sea and the ocean, produces a Rossby wave.

The Rossby wave model explains some of the activity within the Caribbean basin and at its edges, where the sea spills into the ocean. But it doesn’t explain all of it.

“We were looking at ocean pressure through models for quite different reasons, and this region just didn’t work,” University of Liverpool sea level scientist Chris Hughes of the University of Liverpool said in an interview with Gizmodo. “It felt like a sore thumb.”

To get to the sandy bottom of the issue, the team looked at pressure readings and sea level measurements over the six decades from 1958 to 2013, as well as data from tide gauges and NASA satellite readings of the Earth's gravity.

What they found was a very large whistle. When waves crash against the western side of the basin, their collision and rebound creates a reverberation, much like the way that blowing air across an opening creates a whistling noise. The researchers say the note is approximately an A flat.

The sea is so big that this Rossby Whistle, as the researchers named it, is pitched way too low for us to hear. But that doesn’t mean we can’t detect it. The whistle’s resonance is so strong that it affects our planet’s gravitational field, as the researchers learned from the satellite data.

This is a pretty neat discovery, but it also has practical implications. In addition to messing with Earth’s gravity, the whistle’s waves affect sea levels. "This phenomenon can vary sea level by as much as 10 cm along the Colombian and Venezuelan coast,” Hughes said in a press statement, “so understanding it can help predict the likelihood of coastal flooding."

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