BLANKETED IN SNOW: People busy shovelling the entrance to Belfonte’s Car Wash in Raymore, Missouri, during a snowstorm earlier this month.

By Steff Gaulter

 To many people, it seems as though the weather has gone mad. Over the last few weeks, Western Europe has been submerged and North America has been frozen to the core, but the strange weather goes back further than that.

In the UK, March 2013 was the coldest March for more than 50 years, and before that June 2012 was the wettest June in over a century. In California, 2013 was the driest year on record. People are now questioning whether there is a link between all this extreme weather and if it will continue.

Certainly it does look as if this winter’s crazy weather in Western Europe and the USA was linked. The trigger appears to have been the unusually warm waters around Indonesia. Warm waters encourage showers, and much of the region has been completely water-logged. In cities like Jakarta, this has led to flooding and in the surrounding countryside, landslides have torn strips out of the mountainsides.

An increase in rainfall across Indonesia at this time of year is usually attributed to a condition known as La Nina. This is the opposite of the more famous El Nino, and is simply a natural change in the temperatures of the Pacific Ocean.

However, although the temperatures of the waters around Indonesia were above average this year, as they would be during a La Nina event, the rest of the Pacific was unaffected. The conditions were therefore not classed as La Nina, but the resulting weather was no less extraordinary.

The heavy rain continued over Indonesia and the rest of the western Pacific throughout December and January. The warm seas ensured that vast amounts of water were pumped into the atmosphere, some of which was picked up by the winds which flow high up in the atmosphere towards the west coast of North America. These winds form a jet stream, and this is just one of many which hurtle around the globe.

Usually we are oblivious to jet streams, but there are a few occasions that we are forced to notice them. Firstly, flights from the USA to Europe are far quicker than flying from Europe to the USA, because pilots often ‘hitch a ride’ on the jet stream, which flows from west to east; we also notice them when the jet stream ‘gets stuck’.

Normally a jet stream will meander north and south, taking the weather systems with it. However, if it remains over one place for a prolonged period, then the area below the jet stream will end up far wetter than usual, but another region to the north or south will remain far drier.

This is what happened to the jet stream over the Pacific this winter. It usually flows from the western Pacific to the northeast of the USA, but in December and January, it was deflected a long way further north than usual. This was bad news for North America.

For the past two years, a drought has gripped California. The northward shift of the jet stream meant that the rain it normally depends on in winter failed to materialise. It is now the worst drought for a century and things are looking very precarious. More than a dozen communities remain at risk of running out of water within a couple of months.

The shift in the jet stream was also bad news for many other parts of North America. It forced colder air south across Canada and the eastern US, giving periods of intense cold and snow, including the much-hyped polar vortex. As the cold air dug south, it clashed with the warm air over the Gulf of Mexico.

Another jet stream starts across the east coast of the US and heads towards northwest Europe. Unfortunately for Europe, a strong clash of temperatures at the beginning of the jet stream resulted in a stronger jet stream, which in turn meant stronger storms with heavier rain and stronger winds.

Another problem for Europe was the fact that a jet stream feeds off the storms that move along it. This meant as the storms from North America moved across the Atlantic in quick succession, they intensified the jet stream, which in turn reintensified the storms, and the result, as we all saw, was flooding over western Europe.

All of this makes logical sense, but it doesn’t answer the questions of what triggered it all in the first place, and whether it will happen again. Certainly over the last few years, we’ve been hearing a lot more about the jet stream and how it has caused widespread droughts and floods, including the flooding in Pakistan in 2010 and the heatwave in Russia.

There is no definitive answer at the moment, but the most likely theory is that the jet stream is being affected by the changing temperature on earth. The real problem seems to be that the Arctic is warming over twice as fast as the rest of the planet. This means there is less of a difference between the temperature in the Arctic and in the lower latitudes. The jet stream tends to meander when the temperature difference is small, and the new theory is that the lack of temperature contrast could also encourage the jet stream to get stuck.

This would certainly explain why the winters of 2010/2011 and 2012/2013 were unusually cold across the UK, but this winter has been exceptionally wet.

Both are very different types of weather, but both are extreme and caused by a change in path of the jet stream. If the latest theory is correct then anyone in the northern latitudes can expect to find the same spell of weather lasting for months on end. Droughts and floods will become more common.