Scientists follow a giant oceanic vortex from space

The researchers discovered a new way of using satellites to monitor the Great Whirl, a gigantic swirl of the size of Colorado that forms each year off the coast of East Africa, report a new study.

Using 23 years of satellite data, new discoveries show that the Big Whirlwind is bigger and lasts longer than scientists thought. At its peak, the giant vortex covers an average 275,000 square kilometers (106,000 square miles) and lasts about 200 days a year. Watch an animation of the evolution of the Grand Tourbillon here.

More than just a curiosity, the Grand Tourbillon is closely linked to the monsoon that drives the rainy season in India. The monsoon rains are fueling the Indian agricultural economy, valued at $ 2 trillion, but it is notoriously difficult to predict the amount of rainfall each year. If researchers could use their new method to discern a trend in the formation of the Great Whirlpool, they could perhaps better predict when India will experience a very dry or very wet season compared to the average.

"If we are about to tie these two projects together, we could have an advantage in predicting the strength of the monsoon, which has huge socio-economic impacts," said Bryce Melzer, satellite oceanographer at Stennis Space Center. Mississippi and senior author of the new study in the AGU Journal Geophysical Research Letters.

A swirling sea

The Great Whirlpool is a huge whirlpool that forms every spring off the coast of Somalia, when winds blowing over the Indian Ocean change direction from west to east. The English geographer Alexander Findlay for the first time described the Great Tourbillon in his navigation directory for the Indian Ocean in 1866.

According to Findlay, Lieutenant Taylor of the British Royal Navy described a "great whirlpool of current" flowing clockwise at roughly the same latitude in Xaafuun, Somalia. "This whirlpool creates a very confusing sea," wrote Findlay. The phenomenon took the name of Grand Tourbillon and sailors have long been wary of its strong waves and its intense currents.

The Grand Tourbillon begins to form in April, but its currents are the deepest and strongest from June to September, during the official Indian monsoon season. A study conducted in 2013 using satellite data revealed that at its peak, the Whirl could reach more than 500 km wide, making it wider than the Grand Canyon.

The circular currents of the Grand Tourbillon extend for hundreds of meters down and can reach more than one kilometer deep in some areas. The inertia it generates keeps the whirlwind well beyond the end of the monsoon season in September, until it usually disappears by the end of the fall.

Study the whirlwind from afar

Scientists have been interested in the Grand Tourbillon for years but have trouble studying it directly. Whirlpool monitoring requires many repeated observations over a long period, but widespread piracy off the coast of Somalia has prevented researchers from venturing near or installing instruments in the ocean. to observe it.

And since the Whirl is so big, it does not behave in the same way as smaller whirlpools, and scientists find it difficult to define its limits. As a result, scientists do not really understand how the vortex varies from year to year or exactly when it is formed and if it disappears.

Researchers have recently turned to satellites to see if they could monitor the Whirl from afar. In the new study, Melzer and his colleagues developed a new way to use sea level satellite measurements to better define the boundaries of the Great Whirlpool and track them over time. The center of the Grand Tourbillon actually rises higher than the sea level and currents revolve around this "hill" of water.

Researchers analyzed sea level satellite data from 1993 to 2015 to understand Whirl's year-to-year evolution and its appearance under different climatic conditions.

They found that the Grand Tourbillon is bigger than expected. The Whirl's average size over these 23 years was 275,000 square kilometers (106,000 square miles), making it larger than the state of Colorado.

They also found that there was a lot of variability in when the Great Whirlwind was formed and its duration. But on average, it lasts 198 days – six and a half months – considerably longer than the previous estimates, ie 166 and 140 days.

The great inertia it generates generates a good rotation of the Whirl after the official end of the monsoon in September. The researchers found that the Whirl persisted until November and even December, and that there were three years (2000, 2005, and 2010) during which it persisted in the new year. The longest duration lasted 256 days – more than eight months – in 1997.

Researchers have not yet found a model in the Great Whirlpool that could help them predict the Indian monsoon. But they also hope to apply their method to tracking vortices in other areas. Tourbillons in the Gulf of Mexico, for example, have very strong currents that could affect oil drilling operations in the region.