The Coastal Calypso

The Caribbean Sea is clad,

Between Belize and Trinidad;

And underneath an azure sky,

The ocean waves blow loudly by.

 

Four months to cross from east to west,

Then at the boundary they congest;

Escaping with a wet reply,

The ocean waves blow loudly by.

 

And somewhere up in outer space,

We measure and detect their trace;

An A-flat let out like a sigh,

The ocean waves blow loudly by.

 

The whistle makes the waters rise,

And signals the port town’s demise;

We cannot hear them laugh or cry,

The ocean waves blow loudly by.

 

This is a Kyrielle, written about recent research which found that the Caribbean Sea acts like a whistle and can be ‘heard’ from space.  By analysing sea levels and pressure readings for over 50 years, researchers have noticed a phenomenon which they have called a ‘Rossby whistle.’ This happens when giant waves (called Rossby waves) move slowly from east to west across the ocean’s basin, interacting with the seafloor, and causing water to slosh in and out of the basin. But how are they like a whistle?

When you blow into a whistle, air enters the end that you blow into. As the air reaches the other, closed end of the whistle, all of the air molecules pile up and compress, causing a high-pressure region. This air then escapes out of the small hole in the whistle, and makes a sound, with the frequency (or pitch) of the sound dependent on the length of the whistle. In the case of these oceanic waves, the waves enter from the east and then ‘pile up’ when they reach the coastal boundary in the west. The Caribbean Sea is a partly open system, meaning that some of these waves can ‘escape,’ causing oscillations in the Earth’s gravity field, with a frequency that is measurable from space using very sensitive instruments. The frequency that is observed corresponds to the 120-day time period of the Rossby waves (i.e. it takes these waves 120 days to travel from the east of the Caribbean Sea to the west), and results in an A-flat sound, but at a much lower frequency than that which can be detected by the human ear.

The sloshing of water caused by these Rossby whistles can alter sea levels by up to 10 cm along the Colombian and Venezuelan coast, where cities such as Barranquilla, have been identified as areas where severe floods can be caused by an increase in sea level of just 20 cm. With global sea level rising due to Global Warming, these Rossby whistles could have a potentially detrimental effect on such coastal areas.

An audio version of this poem can be heard here.