A step forward in forecasting cold winters?

Sudden stratospheric warming (SSW) has been the 'in' topic in meteorological circles in the last couple of years, ever since the severe winter of 2009/2010 in which December was the coldest since the late 19th century.

SSW is linked to sudden large increases in temperature over a few days in the stratosphere over the Arctic.

This temperature change cause winds to reverse their normal direction.

For some time, forecasters have noted that a sudden weakening in high altitude winds in the stratosphere was often followed in winter by blocking surface weather systems.

These blocking weather systems tend to bring much colder conditions across Europe and the UK from the east, stopping milder air pushing in from the Atlantic.

There have been notable successes from observing this phenomenon on shorter time scales.

A week before the onset of severe cold that begun at the end of November 2010, stratospheric warming was observed, which led to a forecast which successfully included a risk of cold conditions developing across the UK.

The cold weather which occurred in 2006 and 2010 also coincided with sudden stratospheric warming.

But it would be much more helpful if the onset of such severe weather could be forecast further ahead, and that is what researchers at the Met Office have been working on, publishing research in Environmental Research Letters last month.

A breakthrough came last year when scientists at the Met Office demonstrated a clear link between stratospheric influence on climate during a sudden stratospheric warming, with easterly winds burrowing down through the atmosphere to affect the jet stream.

Following on from this, researchers at the Met Office have produced a model that is better at simulating stratospheric warming, which may give forecasters a better chance of signalling cold winters in future.

By using this new model with data available from autumn 2009, the Met Office showed that they could have seen the cold coming well in advance.

But blasts of cold weather are not always due to SSW.

There are several competing influences each winter, such as Atlantic sea surface temperatures, volcanic eruptions, snow cover and solar forcing.

For example, the research highlights the deep solar minimum as a contributory factor to the observed severe weather conditions in 2009/2010.

But separating their effects, and establishing which has the largest impact, remains a big headache for forecasters.

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