How long is a day on earth? The obvious answer of 24 hours is accurate enough for many applications. But for those interested in GPS or deep space, then understanding the fluctuations of about one millisecond in the length of a day can be fundamentally important.

A team at the Met Office, led by Professor Adam Scaife, has calculated that these length of day fluctuations are predictable out to more than one year ahead and this is all to do with predicting the strength of atmospheric winds.
Length of Day
© Met Office UK
The figure shows predicted changes in the wind strength for example cases starting in November 1980. The higher the predicted changes, the longer the length of day.
The stronger the winds blow around the Earth, the slower the Earth rotates to compensate and hence the longer the length of day. The findings were published yesterday in Nature Geoscience.

Commenting on his research, Professor Scaife said: "The fact that global winds can affect the speed of the Earth's rotation is a consequence of Newton's laws of physics and has been known for a long time. What's new here is that we can predict these fluctuations many months and even a year or two ahead.

"Although the change in the rotation of the Earth has no direct effect on the atmosphere (it's just too small), the compensating change in the winds is much bigger and is strong enough to change regional weather and climate.

"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 (often triggered by El Niño or La Niña). This has applications in long-range forecasting and is another piece in the puzzle of long-range weather prediction."

One of the really novel things the team discovered is that these predictable signals are lurking in the atmosphere and are not in the ocean where we normally look for long-range weather and climate signals. "This means that there is a long-term memory within the atmosphere - opening all sorts of interesting possibilities.

The paper Long-range predictability of extratropical climate and the length of day will have many applications including potentially even calculating the timing of when a leap second needs to be added to the clocks keeping track of global time.

This research was supported by the UK-China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund. It was also supported by the Met Office Hadley Centre Climate Programme (HCCP) funded by BEIS and Defra and by the European Commission Horizon 2020 EUCP project.