Days get longer when the wind blows

(ORDO NEWS) — How long is a day on Earth? The obvious answer – 24 hours – is accurate enough for many applications.

But for those interested in GPS or deep space, understanding day-length fluctuations of around one millisecond can be crucial.

A Met Office team led by Professor Adam Scaife has calculated that these fluctuations in day length can be predicted more than a year in advance, and this is due to forecasting the strength of atmospheric winds.

The figure shows predicted changes in wind strength for examples starting in November 1980. The higher the predicted change, the longer the day.

The harder the winds blow around the Earth, the slower the Earth rotates to compensate for this, and therefore the longer the day.

The results of the study were published yesterday in the journal Nature Geoscience.

Commenting on his research, Professor Skaife said: “The fact that global winds can affect the speed of the Earth’s rotation is a consequence of the laws of Newtonian physics and has been known for a long time.

But what is new here is that we can predict these fluctuations for many months and even for a year or two ahead.”

“While the change in the Earth’s rotation does not have a direct effect on the atmosphere (it is too small), the offsetting change in winds is much larger and strong enough to change regional weather and climate.”

“It turns out that the jet stream at mid-latitudes changes with a delay of about a year after the length of the day first changes in the tropics (often called El Niño or La Niña).

This has applications in long-range forecasting and is another piece in the puzzle long-range weather prediction.

One of the really new things the team has discovered is that these predictable signals lurk in the atmosphere, not in the ocean, where we usually look for long-term weather and climate signals.

This means that there is a long-term memory in the atmosphere, which opens up all sorts of interesting possibilities.”

The work “Long-term Predictability of Extratropical Climate and Daylength” will have many applications, including perhaps even timing when it is necessary to add a leap second to clocks that keep track of world time.

This study was supported by the UK-China Research and Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Foundation.

It was also supported by the Hadley Center Climate Program (HCCP), funded by BEIS and Defra, and the European Commission’s Horizon 2020 EUCP project.

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