Just put your lips together and blow.
Those words spoken by Lauren Bacall’s character Slim Browning in the movie To Have and Have Not are the most famous instructions ever for how to whistle. This is not how the Caribbean Sea whistles due to its obvious lack of lips, but that body of water is making its own kind of whistling noise that can be heard in space.
Researchers at the University of Liverpool were studying the Caribbean Sea currents that feed Gulf Stream when they noticed odd oscillations that didn’t fit their models. Sea Level Science Professor Chris Hughes proposed looking for a cause in historical (1958 to 2013) satellite and oceanic data of tidal movements in the region. That’s when they found the whistle.
According to their report in the journal Geophysical Research Letters, first they found Rossby Waves. Named for meteorologist Carl-Gustaf Rossby who theorized them, Rossby Waves are long, slow waves that travel west across the floor of the Caribbean. As they end in the west, they begin again in the east, causing a so-called Rossby Wormhole. This complete replenishment of the water in the Caribbean basin happens once every 120 days and is great enough to affect the Earth’s gravitational field. And that, according to Hughes, it what makes the Rossby Whistle.
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. Similarly, an ocean current flowing through the Caribbean Sea becomes unstable and excites a resonance of a rather strange kind of ocean wave called a 'Rossby wave'. Because the Caribbean Sea is partly open, this causes an exchange of water with the rest of the ocean which allows us to 'hear' the resonance using gravity measurements.
The whistle tone is an A-flat, but it registers at an octave too low for humans to hear. It's fortunate that satellites can pick it up because this whistle can act as a warning for flooding as the water levels rise along the Caribbean coasts. It’s important for the rest of the Atlantic Ocean as well, since the Gulf Stream has such a major influence on it.
With all due respect to Professor Hughes and Carl-Gustaf Rossby, the Rossby Whistle is nowhere nearly as sexy as the Bacall Whistle.