Jet Stream heads south - UK floods

[Jan Klimkowski]

Well well well....

The article is from June 20, but parts of the UK are currently flooded as a month's rain falls in 24 hours. Again.

And the Jet Stream is still way south of where it should be and the key factor in these summer monsoons and periods of prolonged extreme weather (of one type or another)....

Nothing to see here....

Blame the jet stream for return of heavy rain


Paul Hudson | 15:28 UK time, Wednesday, 20 June 2012

After a short spell of fine and warm weather in the last few days, Atlantic weather fronts are once again expected to spread wind and rain across the country tonight and tomorrow, leading us into a fourth successive weekend washout.

According to Philip Eden, writing in the Sunday Telegraph, the first half of June across the UK was the wettest for 150 years - and let's not forget April was the wettest on record too.

The weather pattern is so stuck in a rut that there's every chance that low pressure will dominate our weather for the rest of June and into the first half of July too - meaning more cool and at times wet weather - with a few fine warm days in-between.

And it's all down to the position of the jet stream.

The jet stream is a fast moving zone of winds high up in the atmosphere, caused by the temperature contrast between cold air to the north, and warmer air to the south.
It's along this boundary, where warm and cold air constantly battle each other, that most of our rain bearing weather systems form.

And the jet stream, for some time now, has been further south than normal - hence the inclement weather.

Looking back through the climate records for the last few years it's striking just how polarised our weather has become, with one particular type of weather lasting for weeks on-end.

So far this year March was one of the warmest and sunniest on record; April the wettest on record; now the first half of June is the wettest for 150 years.

This followed the driest 24 months since records began; and let's not forget April last year was the warmest on record which followed the coldest & snowiest December since 1890, during the coldest winter for over 30 years.

This pattern of extreme weather swings in the last few years seems to be getting more common.

Recent research published in Nature offers a possible explanation as to why this seems to be the case, suggesting that the unusual behaviour of the jet stream could be linked to warming that has been observed in the Arctic.

The research shows that because the jet stream is a function of the temperature contrast above the Atlantic, if that contrast is reduced because higher latitudes are warmer than normal, then the jet stream would weaken.

This would effectively slow its eastwards propagation. And one of the consequences would be that a particular type of weather may persist for longer.

Arctic warming may also be causing the jet stream to become more amplified at times, the research claims, causing warmer air to travel further north than normal, and colder air to travel further south than normal - leading to more extreme warmth and cold.

A more amplified jet stream doesn't just mean long periods of poor weather for the UK. At the moment we are stuck in a trough in the jet stream, but we could just as easily be under the influence of a warm ridge, meaning longer periods of dry weather (and in summer, warm weather too).

Recent research has also pointed the finger at weak solar activity as a possible explanation for the cold, dry winters that Europe and the UK has experienced in the last few years.

These were caused by the jet stream being unusually far south, and the research conducted by Reading University concluding that such winters could become more common in the next decade or so as a result of expected weaker solar activity.

Whichever theory is correct - and it's plausible that both are exerting an influence at the same time - experiencing long periods of the same type of weather may be something we should get used to.