It’s called a Rossby whistle.
You may think you are a good whistler, but it likely doesn’t compare to the whistling abilities of the Caribbean Sea. A new study, published in the journal Geophysical Research Letters and conducted by ocean scientists at the University of Liverpool (UL), has shown that the Caribbean Sea, which is a basin in the Gulf of Mexico, can whistle so loudly that it can be ‘heard’ from space.
The phenomenon, dubbed a “Rossby whistle,” happens when a Rossby oceanic wave — a large wave that slowly travels west in the ocean between the warm upper layer and the cold deeper part of the ocean — interacts with the seafloor.
The interaction causes the wave to die out at the western boundary of the basin and reappear back on the eastern side, creating what has been nicknamed a “Rossby wormhole.” As a result, water moves in and out of the basin every 120 days. This change in the amount of water in the basin is enough to oscillate Earth’s gravity field, creating a whistle that can be measured by satellites.
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Image of the Caribbean Sea basin bathymetry (underwater topography). Photo credit: GLOBE and ETOPO1/wikipedia (CC BY-SA 4.0)
Since the phenomenon has a 120 day period, the whistle plays a note of A-flat. It is several octaves below the audible range, so unfortunately, we can’t hear it.
Professor Chris Hughes, an expert in sea level science at UL, explained in a press release, "We can compare the ocean activity in the Caribbean Sea to that of a 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," Hughes continued.
The phenomenon can actually result in sea level changes of 10 centimeters (4 inches) along Caribbean Sea coast, so understanding the process can help predict the chances of coastal flooding. The Rossby whistle may also have an impact on the entire ocean climate. The North Atlantic regulates the flow in the Caribbean Current, which leads to the Gulf Stream — the engine responsible for the ocean climate, and therefore, the distribution of heat across the planet.
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