Waving weather

Small children can be very annoying at times. They have a curiosity about them that often results in a string of ‘Why?’ questions.

For many of us the only way to bring them to an end is to wearily reply, “Go ask your mother!”

It can be the same in meteorology. People want to know why the weather has been particularly good, bad or just extreme. ‘What caused this flooding?” Why is there a heatwave there?”

The immediate, easy answer is to explain the situation in terms of ‘high’ and ‘low’ pressure systems. Generally speaking, high pressure equates to fine weather, low pressure to bad weather.

It is when the enquiring mind comes back with a ‘Why?’ that we then have look at the bigger weather pattern and the answer can often be traced to the work of Carl-Gustav Rossby.

Many of our weather patterns are the result of the actions of the jet stream in the upper atmosphere – bands of wind, blowing at up to 400kph which mark the boundary between warm air towards the tropics and cold air towards the poles.

The interesting thing about the winds in the upper atmosphere is that they do not simply blow in straight lines around the globe. Instead they meander like a river.

The meandering is a result of the conservation of absolute vorticity (CAV) which results from the rotation of the Earth and the action of obstacles to the movement of air.

As air is forced to rise over mountain ranges, which disrupt the general east to west airflow, it tends to turn to the right (left in the southern hemisphere) and to the left when it descends (right in the southern hemisphere).

The result is a series of long waves which traverse the globe – usually between three and six of them. These are Rossby waves first identified by the Swedish-US meteorologist.

Two of these waves, at 70 degrees west and 150 degrees east, are semi-permanent features caused by the blocking effects of the Rockies and the Tibetan plateau.

The rest ebb and flow but the more of them there are, the greater their amplitude tends to be. In other words, the troughs of these waves push further south and their crests further north than at other times.

This is the pattern we have seen for much of this year and the results have been spectacular in some regions.

Take North America, for instance. Here, the jet stream winds have been pushed well to the north. This has allowed heatwave conditions to develop across much of central and eastern North America.

Until the heat broke earlier this week, temperatures were the highest in recorded history in some states, with consecutive days of high temperatures not seen since the Dust Bowl of the 1930s.

On the other side of the Atlantic, those same winds have been driven well to the south of the UK and Western Europe. Being on the cold side of a jet stream is never a good place to be and the UK has been battered by the worst June and early July rainfall since records began back in 1910.

For Eastern Europe it has been a different story as the Rossby waves have curved northwards allowing another heatwave to develop. That has produced daytime temperatures between 5 and 10 degrees Celsius above average and those high temperatures have also spawned massive thunderstorms around the Black Sea.

The flash flooding which hit Samsun in Turkey and Krymsk in southern Russia, producing a combined death toll of around 180, was the result of supercells which fed off this heat and were able to develop because of the unusual wind profile in the upper atmosphere.

Of course, not all extreme weather can be directly linked to Rossby waves. The severe thunderstorms which have hit the southern Japanese island of Kyushu are a feature of the meiyu-baiu frontal zone which is often located in the region during July.

Flooding rain across many parts of China is also linked to this frontal boundary.

The devastating monsoon rains in Bangladesh and Assam were also a feature of the regularly occurring monsoon rains.

Whether any of these monsoon rain events were influenced or enhanced by the bigger atmospheric waves, only detailed research will be able to reveal.

Let us not also forget the El Nino-Southern Oscillation which is tied to wave patterns in the deep oceans. Both El Nino and its counterpart, La Nina, greatly influence weather patterns over periods of many months.

So whilst waves in the atmosphere do not provide the complete answer to the complexities of our weather, they can often be found to be playing a part in some of the more extreme events.