As more and more of our little blue home is explored, it reveals even more layers from the endless depths of the secrets it holds, and the songs it sings. Or as it is in the case of the Caribbean sea, it whistles a tune that only a giant can hear, as an ocean modeling study has recently learned.

While modeling ocean currents in the Caribbean Sea, researchers from the University of Liverpool found that something didn’t add up: their models kept revealing pressure oscillations across the basin that they couldn’t explain, results that stood out in stark contrast to what was originally expected.

To verify that this was indeed happening in the real world, the team employed historical water pressure and tide gauge records and found that these anomalous oscillations were indeed happening, producing an auditory effect not unlike that of a giant whistle, albeit one that is too low to be heard by the human ear.

“You have a current that flows east to west through the Caribbean Sea,” explains team lead Chris Hughes. “It’s very narrow and quite strong. Just like a narrow jet of air, it becomes unstable and creates eddies.” They dubbed the phenomenon the “Rossby Whistle”, after the Rossby waves propagating through the Caribbean that produce this effect. Rossby waves themselves are very shallow, long-term waves that are only an inch or two in height, but can be hundreds of miles across horizontally, and are an effect generated by the rotation of the planet. These waves, combined with the pressure at the sea bottom, produce the low-frequency sound in question.

"We can compare the ocean activity in the Caribbean Sea to that of a whistle," continues Hughes. "When you blow into a whistle, the jet of air becomes unstable and excites the resonant sound wave which fits into the whistle cavity. Because the whistle is open, the sound radiates out so you can hear it."

It takes these pressure waves 120 days to cross the Caribbean basin, and the tone that is produced is an A-flat, although it is 30 octaves below that of what a piano can produce. The waves can also be recorded from space, using the minute gravitational changes that they produce. Further study is planned for the Rossby Whistle, in the hopes that it might yield a new method of predicting coastal flooding patterns.