Weather Q&A: Are rolling winter storms the shape of things to come?

More extremes may be coming our way from the Atlantic as a result of global warming

In less than a week Ireland and much of northwestern Europe has experienced three storms – Dudley, Eunice and Franklin. This is yet another reminder we live on the doorstep of an often turbulent and unpredictable Atlantic Ocean.

Separately, confirmation by a climate scientist based in Maynooth University that the North Atlantic jet stream over a 141-year period is moving northwards and increasing average speed, especially in winter, has potentially big implications for Ireland.

This is because jet streams can have a significant impact on storm activity. They are fast bands of air which flow around the globe at about 10km above the Earth’s surface within the layer enveloping our atmosphere where humans live and breathe.

The average winter northern hemisphere jet stream position over the North Atlantic has moved northwards by up to 330km and the mean winter jet speed has increased by 8 per cent to 212km/h (132mph) over a 141-year period up to 2011, according to research by Dr Samantha Hallam of the Icarus Climate Research Centre. This shift constitutes subtle, but sustained, change over many decades.

It means Ireland may be more often in the line of fire though there is huge variability in natural winter storm patterns, ie arising as a consequence of normal weather. Some track above us; others go below.

The bottom line is the jet stream was sitting directly over Ireland and the UK in recent stormy days, and that is a perfect illustration of the scenario.

What exactly is driving changes to our jet stream?

As with all things to do with weather and climate, the answer is not in black and white. Long-term trends in the jet stream, nonetheless, are potential indicators of climate change.

First things first, it should be underlined that we cannot say with certainty recent storms were a consequence of climate change being caused by human activities, says storms expert Prof Peter Thorne, who is also with Icarus.

That normal variability in frequency of winter storms has to be factored in, rather than immediately blaming it on human influence and resulting global warming.

Are we inclined to blame climate change when storms hit?

“It’s important not to be seen to proverbially cry wolf and link any and every weather event to climate change,” said Thorne during recent storms.

What is clear is that jet stream orientation roughly determines the track of a storm, he says, which at the very least points to the pressing need for more accurate climate models including ability to track storms.

At present, he notes, too many models are inaccurate; very costly decisions are being made based on poor information.

While jet stream trends have no direct implications for five- to seven-day weather forecasting, "it has implications for climate models we should use," he adds. That is why Icarus is working with Met Éireann on the Translate Project, identifying models focussing on the mid-latitude jet stream, which affects Ireland most. This is to ensure more accuracy on storm movement and, ultimately, to be better positioned to respond appropriately.

About 70 climate models are available to predict future climatic trends but most are of little use to Ireland, he says.

As for recent storms, "we are on scientifically stronger ground around other storm impacts than winds. Coastal storm surge and extreme rainfall impacts have been made considerably more likely . . . already serious and self-evident impacts of climate change," says Thorne.

So the science on likelihood of more frequent and severe storms due to climate change is not conclusive, yet other consequences – such as flood risk and the threat to coastlines – are clearcut.

In addition to having significant influence on storm activity, the changed jet stream pattern is also affecting temperature patterns across the northern hemisphere, which can affect the weather through strong winds and flooding events.

Do  ocean-current changes risk compounding our difficulties?

There is another issue that may exacerbate Ireland’s vulnerability in coming decades. And that is to do with ocean currents. The Gulf Stream in the North Atlantic, which bestows benign effects on Ireland’s climate by increasing winter temperatures, is at its weakest in over a millennium. Human-caused global warming is the most likely cause, scientists have found.

Further slowdown in the larger system of which the Gulf Stream is part, known as the Atlantic Meridional Overturning Circulation, is approaching a tipping point. This could cause dramatic shifts in global weather patterns, it was predicted last year.

It could result in more extreme weather events in Europe. And this could manifest as winter storms coming off the Atlantic, possibly intensifying them. Other studies found the possibility increases potential for intense cold in winter, heat waves or reductions in summer rainfall as a consequence.

The immediate impact of this shift in increased rainfall and meltwater from ice sheets, and reduced ocean salinity, could cause significant sea rise on the east of the United States. It could disrupt the flow of vital nutrients that phytoplankton, marine algae that make up the foundation of the aquatic food web, need to grow in the North Atlantic.

The Atlantic not only drives “so much of the variability of our weather across seasons, years and decades” – as Thorne puts it – it is a vital sustainer of marine life in our midst