Winds that slow down the Earth’s rotation unravel mystery of long-range weather forecasts

Faster atmospheric winds extend the length of day on Earth and can trigger long-term changes in weather and climate, scientists have found, in a breakthrough that could improve forecasts.

Strong winds in the atmosphere are known to slow down the Earth’s 24-hour rotation by around one millisecond a day, as the planet compensates for the extra momentum.

But a team at the Met Office has found that when the phenomenon occurs, it sets off a wave of atmospheric fluctuations that migrate slowly towards the poles, impacting important air currents, such as the jet stream.

Researchers found that the day slows down around a year before the atmospheric winds reach the jet stream - giving an early warning system of what to expect.

The finding could help forecasters make predictions about cold winters or flooding far earlier than they can currently achieve using traditional methods.

Professor Adam Scaife, head of long-range prediction at the Met Office, said: “The interesting bit for climate prediction is that these signatures in the atmosphere are very long-lived as they move outwards towards the mid-latitudes on the timescale of about a year, which gives us some long-range predictability.

“If you talk to anyone in this field, they will say the atmosphere only has a short memory and if you want to do a long-term forecast you have to do it using the ocean.

“But It turns out that the jet stream in the mid-latitudes is affected with a lag of about a year after the length of day first changes in the tropics.

“This suggests there is long-term memory in the atmosphere, and you can use it to give an indication what the jet stream is going to be doing a year ahead.”

Regulating energy supplies

Forecasting bad weather early is important for regulating energy supply as cold winters increase fuel demands, while strong westerlies bring winter storms and flooding. Knowing that storms or chills were on the way could allow governments to prepare in advance.

Although the change in the rotation of the Earth has no direct effect on the atmosphere, the compensating change in the winds is much bigger and is strong enough to change regional weather and climate, Prof Scaife said.

The team has also shown is it possible to predict when the day lengthening will occur, based on other predictable climate phenomena, such as El Nino or La Nina, which could give an even earlier indication of jet stream changes.

“This has applications in long-range forecasting and is another piece in the puzzle of long-range weather prediction,” added Prof Scaife.

The findings were published in the journal Nature Geoscience.