A team of scientists led by Professor Chris Hughes (University of Liverpool, NOC) analyzed the Caribbean Sea levels and pressure readings taken from the bottom of the sea and noticed a new phenomenon which they have called a 'Rossby whistle.'
The team has found the Caribbean Current flow is unstable, which causes it to shed eddies, or swirling currents of water hundreds of kilometers in diameter. This is similar to the way in which a jet of air sheds eddies when it hits the lip of a whistle.
The 'Rossby whistle' phenomenon happens when a Rossby wave – a large wave which propagates slowly to the west in the ocean – interacts with the seafloor. This causes the wave to die out at the western boundary and reappear on the eastern side of the basin, an interaction which has been described as a 'Rossby wormhole'. Only waves of particular lengths can survive this process without canceling themselves out, but these particular waves reinforce themselves, producing an oscillation with a sharply-defined period.
As a result, water sloshes in and out of the basin every 120 days. This mass change is sufficient to make changes to the Earth's gravity field which can be measured from satellites. The 120 day period means this whistle plays a note of A-flat, although it is many octaves below the audible range.
Advanced computer models of the ocean, run at the UK's National Oceanography Centre (NOC), predicted this should happen. This prediction was later confirmed using a range of observations, including satellite gravity, satellite sea level measurements, coastal tide gauges and a bottom pressure recorder which is part of the global tsunami warning network.
“It was a real surprise to find this oscillation. We were looking at ocean bottom pressure data from round the world as part of an NOC contribution to the global sea level database, which we host, and found this region. It behaved quite differently from the rest of the tropics, which are typically very quiet. With hindsight, we found theoreticians had predicted this kind of behavior but had never thought to apply their models to the Caribbean Sea – ironically this seems to be the only place where conditions are suitable,” Hughes said.
The oscillation is always present, sometimes with higher and sometimes with lower amplitude. Since the waves van be seen as they propagate across the Caribbean Sea, scientists can predict when the wave will arrive at the coast and cause the sea level to rise or fall at least 120 days in advance.
This phenomenon can vary sea level by as much as 10 cm along the Colombian and Venezuelan coast, so understanding it can help predict the likelihood of coastal flooding. Small sea level changes can greatly increase the probability of flood losses and Barranquilla, in Colombia, has been identified as a city in which flood losses will sharply increase with as little as 20 cm of sea level rise.
Scientists believe that the 'Rossby whistle' may also have an impact on the entire North Atlantic as it regulates the flow in the Caribbean Current, which is the precursor to the Gulf Stream, an important cog in the ocean's climate engine.
This is a video of how the Rossby wave propagates, together with the matching ocean bottom pressure time series. The sound is the actual note played by the ocean, sped up to the audible range, and repeated several times.
- "A Rossby Whistle: A resonant basin mode observed in the Caribbean Sea" – Chris W. Hughes, Joanne Williams, Angela Hibbert, Carmen Boening, James Oram – June 2016 – Geophysical Research Letters – doi: 10.1002/2016GL069573
Featured image: GRACE Global Gravity: North and South America. Credit: NASA/JPL/University of Texas Center for Space Research
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