Giant Ocean Vortex Linked to Monsoon

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The edge of the Great Whirl, shown by high chlorophyll concentrations along its flank.
IOCCG

One of the ocean's weirdest currents is the Great Whirl, a giant clockwise eddy that emerges every summer off the coast of Somalia. The swirling waters shift sea-surface temperatures, influencing moisture carried to Asia by monsoon winds.

For more than 100 years, sailors have known the Great Whirl arrived with the onset of monsoon winds in early June and disappeared about one month after the winds died down in August. Monsoon winds are some of the strongest on the planet, blowing at a constant 30 mph (48 km/h).

Because the massive vortex has a powerful impact on local climate, including the monsoon winds, scientists are studying how and why the Great Whirl appears.

It turns out the Great Whirl is even more closely linked to the monsoon than previously thought, but through the ocean, not through the atmosphere. A new study reveals the clockwise current spins up nearly two months before the winds arrive.

" Rossby waves are bringing in energy well before the wind forcing sets in," said Lisa Beal, an oceanographer at the University of Miami in Florida. "We've got this precursor even before the monsoon hits it. That was rather surprising," she told OurAmazingPlanet.

The results were published online the week of Jan. 28 in the Journal of Geophysical Research.

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Arabian sea's massive eddy

The Great Whirl is a humongous anti-cyclone: 185 miles (300 kilometers) across and about 3,280 feet (1,000 meters) deep. Its waters move clockwise, with the surface current (the fastest part) clipping along at a speedy 4.5 mph (7 km/h).

The annual arrival of oceanic Rossby waves in April triggers the clockwise circulation, nearly two months before the monsoon winds start, found Beal and co-author Kathleen Donahue of the University of Rhode Island. Rossby waves are slow-moving ocean waves, only 2 inches (5 cm) high, that travel from east to west. In the Indian Ocean, these waves are linked to the previous year's monsoon, Beal said.

"The waves themselves are disturbances caused by the previous monsoon winds, which is really neat. It's kind of a feedback from one monsoon to the next via these planetary wave processes," Beal said. However, the current relies on the monsoon winds for its power. "The monsoon winds don't initiate it, but it wouldn't be there if there wasn't monsoon winds," she said.