Weather patterns in the tropical Pacific can help to determine the kind of winter the UK will face a year later, in 鈥渞adical鈥� new findings set to improve the accuracy of long-range forecasting.
Ocean temperatures in the tropical Pacific can trigger climate patterns with global effects. Every three to five years or so, the Pacific flips from La Ni帽a conditions, where surface water temperatures in its equatorial region are relatively cool, to El Ni帽o conditions, where these waters become warmer than average.
This cycle, known as the El Ni帽o-Southern Oscillation, or ENSO, is caused by changes in winds blowing across the ocean and the movement of water between the cooler depths and the warmer surface.
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that ENSO patterns influence the climate in parts of the North Atlantic in real time, bringing more rainfall to North America or influencing the chances of a cold winter in the UK, for example.
But at the Met Office, the UK鈥檚 weather service, and his colleagues have now uncovered a new impact that takes effect in the North Atlantic a full year after an El Ni帽o or La Ni帽a has begun.
The arrival of El Ni帽o or La Ni帽a conditions influences the rotation of the atmosphere around Earth鈥檚 axis, an influence that slowly spreads into the northern and southern hemispheres over the course of months, says Scaife. 鈥淲hen I see something like that, it catches my eye immediately, because my job is head of long-range forecasting at the Met Office,鈥� says Scaife. 鈥淪o my next question is very simple: what is the impact on the weather?鈥�
His research suggests that these 鈥渕igrating signals鈥� influence the kind of winter experienced in the North Atlantic a full year later. For example, when an El Ni帽o develops, the rotation of the atmosphere speeds up in places, increasing the strength of westerly winds in the North Atlantic the following winter. That brings mild, wet weather to much of the North Atlantic. When La Ni帽a conditions emerge, the opposite happens, bringing easterly winds and an increasing likelihood of a cold, dry winter the following year in northern Europe.
鈥淚t鈥檚 something we didn鈥檛 know about before,鈥� says Scaife. 鈥淚t鈥檚 quite a radical suggestion.鈥�
This new understanding helps to explain extreme winter weather seen in the past. The winter of 2009/2010 in the UK was one of the coldest on record, with freezing temperatures and widespread snowfall across the country. At that time, El Ni帽o was under way in the tropical Pacific, following a strong La Ni帽a phase the previous year. The delayed and simultaneous effects of these patterns combined to deliver a very weak jet stream with easterly winds, magnifying the likelihood of a cold, dry winter for the UK, says Scaife.
Uncovering this link between recent ENSO cycles and future weather patterns in the Atlantic will inform long-range forecasting, says Scaife. 鈥淚f we want to make long-range forecasts, we鈥檇 better not just think about the El Ni帽o or La Ni帽a that鈥檚 coming. We need to think about the one that鈥檚 already been 鈥� which is not something that we normally do.鈥�
For example, this winter comes as the world emerges from a powerful El Ni帽o pattern that dominated 2023 and early 2024, and a La Ni帽a is expected to develop by the end of the year. Based on the new research, this increases the likelihood that the UK will face a mild, wet and stormy winter this year, says Scaife.
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