Change in Atlantic jet stream making for more powerful storms, finds research

The jet stream over the North Atlantic is moving northwards and increasing in speed, which is making storms affecting Ireland more powerful, according to climate research conducted at Maynooth University.

Jet streams are fast bands of air which flow around the globe at about 10 kilometres above the Earth’s surface in the mid- to upper- troposphere – the layer of the planet’s atmosphere where humans live and breathe.

The average winter northern hemisphere jet stream position over the North Atlantic has moved northwards by up to 330 kilometres, and the mean winter jet speed has increased by 8 per cent to 212km/h over a 141-year period, confirmed Dr Samantha Hallam of the ICARUS Climate Research Centre at Maynooth University and the National Oceanography Centre (NOC) in the UK.

Over the past week, it brought storms Dudley, Eunice and Franklin to Ireland and the UK. In addition to having significant influence on storm activity, the changed pattern is also affecting temperature patterns across the Northern Hemisphere, which can impact the weather through strong winds and flooding events.

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“A stronger jet stream makes storms more powerful and our research shows the mean winter jet speed over the North Atlantic, which is relevant for Irish weather, has increased by 8 per cent to 132mph (212km/h) during the period from 1871-2011,” added Dr Hallam who was lead author in the study published this week in the scientific journal Climate Dynamics.

“The jet stream orientation roughly determines the track of the storm and our study finds the average winter jet position over the North Atlantic has moved from 44 degrees North to 47 degrees North, which is a closer latitude to that of Ireland,” she added – it has been broadly located over Ireland recently which has brought the three storms in quick succession.

“Long-term trends in the jet stream are potential indicators of climate change,” she noted.

Jet stream variability is therefore an important component of climate “noise” and understanding seasonal variability and changes over decades can help inform the study of what climate change will look like on of different regions around the world, she explained. The findings also have implications for models used for climate and weather predictions.

The study, which is the longest regional study of the Northern Hemisphere jet stream ever conducted and provides a regional comparison over land and ocean, shows trends vary on a regional and seasonal basis.

“Significant increases in winter jet latitude and speed are observed over the North Atlantic and Eurasia which is consistent with the decreasing temperature and increasing pressure gradients observed between the equator and the Arctic over the period,” she said.

The North Atlantic has significant increasing jet latitude trends in all seasons; by up to 3 degrees in winter. Eurasia has significant increasing trends in winter and summer, however, no increase is seen across the North Pacific or North America.

Jet speed shows significant increases evident in winter (by up to 16km/h), spring and autumn over the North Atlantic, Eurasia and North America, though no increase over the North Pacific is observed.

“The results highlight that Northern Hemisphere jet variability, and trends differ on a regional basis. This is important for making climate predictions and in developing plans to combat climate change,” Dr Hallam said.

The vast amount of data was analysed using what is known as the “Twentieth Century Reanalysis Dataset”, an ensemble of powerful computers analysing global atmospheric circulation and surface pressure observations – and deploying a weather prediction model.

The research was supported by the Marine Institute and funded by the Government – and involved collaboration between ICARUS, the University of Southampton and NOC.