Major wind shifts emerge from smaller changes
Upper atmosphere winds change direction every 14 months or so, but until now no-one knew why. Phil Dooley reports.
Scientists have solved the riddle of strong winds that circle the planet in the upper atmosphere, and why they reverse their direction seemingly at random.
The new model could not only help atmospheric scientists understand the weather on Earth, but could apply to fluid flow in the planet’s interior, and in the atmospheres of stars and gas giants such as Jupiter, and the magnetic fields they generate.
Above the storms and changeable winds we live in, there is a much more stable layer known as the stratosphere. In this zone, more than 10 kilometres up, there are strong, steady winds that blow in the same direction for months on end before mysteriously reversing direction. The change is unrelated to any seasonal effects.
The wind is known as the Quasi-Biennial Oscillation because it switches direction and then returns to its original direction on average every 26 months. The individual reversals occur anywhere between 11 months and 17 months apart – behaviour that a team led by Louis-Alexandre Couston from Aix Marseille University in France has now managed to simulate in computer models.
The surprise is that the long-term behaviour is the result of energy injected from storms in the troposphere, the turbulent zone below the stratosphere, in short and seasonal bursts.
“You have turbulence in the troposphere, cyclones and storms that try to push up into the stratosphere, and they create waves there,” explains Couston.
The group set up their model, reported in the journal Physical Review Letters, to explore the effect of the waves, using 10 minute increments.
With a good stretch of supercomputer time at his disposal, Couston and colleagues decided to run the simulation for a long time to see what happened.
To his surprise the waves didn’t just pass through the stratosphere, but, over a period of many months, began to build up a constant flow direction.
If easterly winds happened to initially dominate the troposphere, Couston found an easterly flow began in the stratosphere. Once moving, the flow picked up energy more efficiently from extra easterly waves than from westerly ones, building the overall flow direction.
The reversal came when the energy in the easterly waves had been exhausted, and westward flows began to take over again.
“The stratosphere is working in layers – not all of it is moving to the east,” Couston says.
“The change of direction comes from the top of the stratosphere, even though the waves are coming from the bottom. The top layer starts changing direction, and then the bottom layer follows.”
The interaction between the two atmospheric layers appears to be mutual. The troposphere seems to be affected by the stratospheric flows, too. Work at the UK meteorological office linked the Quasi-Biennial Oscillation to irregular weather patterns in UK.
Couston hopes the new model could also help understand the dynamics of the sun, in which the set up is reversed, with a turbulent layer on the outside and a more stable one inside it.
As to whether the turbulence in the Earth’s liquid interior could generate reversing flows linked to reversals in the Earth’s magnetic field, Couston is cautious.
“It’s an open question. This is very preliminary, but it is not out of the question that it is influencing it in some way,” he says.