Another Life: Behind the jet stream’s weird and wavy weather

Between the slashing hail squalls borrowed from Canada’s winter, I kept an eye on the sky above the mountain, looking for contrails speeding far too fast against the sunrise.

In January, after all, a plane flying New York to London reached near-supersonic ground speeds, pushed along by the wind at its tail. I didn’t catch any of these, as it happens, but I need to get my head around the new quirks of the jet stream. It powers so much of the great cloudscape we enjoy, and is bringing the new bouts of extreme weather to both sides of the ocean.

The sky, like the sea, has its separate layers and masses, differing in temperature and flow. It’s not quite a century since weather balloons, sent up near Mount Fuji in Japan, encountered the uttermost winds of the atmosphere, first termed the “jet stream” by a German meteorologist in 1939.

In the second World War, pilots flying the Atlantic sometimes noted a helping hand that made such a big difference between their ground and air speed. It depended, of course, on their direction, since the spin of Earth keeps the jet streams whirling west to east – “the Coriolis effect”.

There are several jet streams wrapped around the planet, broad rivers of wind often thousands of kilometres long and shifting between north and south as seasons change. Ours is the strongest and most northerly stream, born of the meeting of cold polar air and warmer air from the south. The pressure created by colliding differences in density speeds up the wind along their boundary, dragging weather systems with it in the variable flow of the North Atlantic Oscillation.

The jet stream is notoriously wavy or wiggly, pulling cold Arctic air towards the tropics and warmer air to the north. These waves, I read, are caused by “blobs” of air spinning west to east and deflected north or south above the planet’s rough surface. The Earth spins faster at the poles than at the Equator (think about it), a difference that sends the blobs bouncing chaotically north or south to create the so-called Rossby waves.

Abnormal waves in the jet stream can suck unusual extremes of weather north and south, as in the exceptional snow storms of eastern North America, the long droughts of the southwestern states, the heat waves of Russia and some uncommonly warm winters in Scandinavia (a reader, just returned from northern Sweden where he was meant to test the winter performance of vans, reported the trials cut short for want of cold).

The Arctic has been warming more than twice as fast as the rest of the northern hemisphere and the melting of its sea ice may be causing the jet stream's weird waviness. The main proponent of this link is Prof Jennifer Francis, an ocean climate scientist at Rutgers University in the US, who will lecture in Dublin this month.

Dr Francis has published some 40 peer-reviewed papers on the link between rapid Arctic warming and weird extremes of weather in North America, Europe and Russia. The Arctic has warmed more than twice as fast as the rest of the northern hemisphere, the result of feedback loops that amplify the process.

As white sea ice retreats in summer, for example, (by an extra 40 per cent in three decades) some 2 million sq km of dark ocean absorbs more of the sun’s heat. Released back into the atmosphere in autumn, this has raised temperatures by 2-5 degrees C.

As the high latitudes warm more than mid-latitudes, Dr Francis argues, the slackening difference between cold polar air and that from the south makes the waves of the jet stream not only deeper but moving eastwards more slowly, so that weather gets “stuck” for longer periods, especially in autumn before Arctic sea ice freezes again.

In her latest paper, published online last month in Environmental Research Letters, she warns that as the Arctic continues to warm faster than elsewhere in response to rising greenhouse gas, "the frequency of extreme weather events caused by persistent jet-stream patterns will increase".

Dr Francis is lead speaker at a one-day conference of the Irish Meteorological Society on March 28th at the National Botanic Gardens. Its title is "The Perfect Storm", which may fit oddly with such a tranquil setting. The gardens' director, Dr Matthew Jebb, however, will describe the impact of recent destructive storms, other speakers will tell of the new extreme waves and how they batter the west coast, and an Aer Lingus pilot will describe "the good, the bad and the ugly" of the new aviation weather.

Talking of which, if weather goes on getting stuck, we may get a great “blocking high” to give us the sunniest of springs.