Giant ocean vortex linked to monsoon

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

From OurAmazingPlanet

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.

"[Oceanic] 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.

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.

No predictable pattern

The researchers analyzed the Great Whirl's habits by combining 18 years of satellite records with data collected from a research cruise in 1995.
There have been very few research cruises through the Arabian Sea since 1995 because of Somali piracy, Beal said.

The vortex lasts for roughly 166 days each year, but the team found no predictable pattern to its location and orientation.

Over the years, the Great Whirl's wanderings were caused by its own mini-cyclones, the researchers discovered.
As the current spins, it creates two to three flanking cyclones along its edge.
(Cyclones rotate counter-clockwise, opposite to the Great Whirl.)
The interaction between the smaller cyclones and the "mother current" makes the Great Whirl move and shift around in response.

"The Great Whirls spins up these flanking cyclones because it has such high velocity shear along its edge. The water is basically rotating these cyclones clockwise around its flank, and it's causing a kind of turbulence. It's like a mutual eddy advection," Beal said.

Flip-flopping currents

The Great Whirl is not the only strange phenomenon in the Arabian Sea.
The basin is the only place in the world where the ocean's currents reverse direction every year.

"The entire circulation of the basin switches direction from summer to winter, which is really crazy.
It doesn't happen anywhere else in the world's oceans," Beal said.
Understanding how the region's currents respond to the monsoon winds is important because the circulation is directly linked to sea surface temperature, Beal said.
As with the Pacific Ocean's El Niño, sea surface temperature is the No. 1 effect on rainfall, she said.

Because the Great Whirl brings up warm water in its core, but cold water in its smaller flanking cyclones, the current has a complex effect on climate and moisture content in the monsoon winds.

Beal plans to further explore the link between the Great Whirl and Rossby waves.
"If there is some feedback between the previous monsoon and how the Great Whirl is initiated, that could give us some predictability on what the strength of the monsoon will be, and also some predictability about the rainfall, which will be important to people who live in southern Asia."