If you have had it up to here with floods in England, if you are left cold by the snow in the United States or mystified by the unseasonably mild temperatures in Scandinavia, blame it on Santa Claus!
That’s the message at the AAAS Annual Meeting 2014
coming from atmospheric scientist Jennifer Francis whose “Santa’s revenge” hypothesis suggests that the global weather weirdness that we’re currently seeing at middle latitudes could be linked to recent warming in the Arctic.
The jet stream that circles Earth’s north pole travels West to East. That polar jet stream can travel at speeds greater than 100 mph. Here in the picture, the fastest winds are coloured red. The slower winds are in blue.
But when the jet stream interacts with a Rossby wave, as shown here, the winds can wander far North and South, bringing frigid air to normally mild southern states. The slowing of the West to East flow of the jet stream produces large meandering lobes that can stall, resulting in long periods of unchanging weather.
What are the Jet Streams?
Jet streams are fast flowing, narrow air currents in the atmosphere of Earth. The main jet streams are located near the tropopause, the transition zone (approximately 13 kilometres high) between the troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude.
Jet streams are caused by a combination of the Earth’s rotation on its axis and atmospheric heating by solar radiation (and for some planets of the Solar System, by internal heat). Jet streams form near boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air towards the equator. Both the Northern and Southern hemispheres, each have a polar jet and a subtropical jet.
The major jet streams on Earth are westerly winds (flowing West to East). Their paths typically have a meandering shape. Jet streams start, stop, split into two or more parts, or combine into just one stream, or flow in various directions, including the opposite direction of most of the jet.
The strongest jet streams are the polar jets, at around 7-12 kilometres (23,000-39,000 ft) above sea level. The Northern Hemisphere Polar Jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the Southern Hemisphere Polar Jet mostly circles Antarctica all year round.
The second strongest are the higher and somewhat weaker subtropical jets at around 10-16 kilometres above sea level (33,000-52,000 ft).
Other jet streams also exist. During the Northern Hemisphere summer, easterly jets can form in tropical regions, typically in a region where dry air encounters more humid air at high altitudes. Low-level jets are typical of various regions of the globe, such as the central United States.
Their main commercial relevance of the jet streams is in air travel, as flight time can be dramatically affected by either flying with or against the flow of a jet stream. A potential hazard to aircraft passenger safety, clear-air turbulence, is often found in a jet stream’s vicinity, but it does not create a substantial alteration on flight times.
Meteorologists also use the location of some of the jet streams, as an aid in weather forecasting.
Francis’ theory begins with the polar jet stream, the high-altitude “river of air” that flows over parts of the Northern Hemisphere. This particular jet stream owes its existence to the temperature differential between the Arctic region and middle latitudes.
As the warm air expands, that temperature differential produces a “hill” of air with England at the top and Greenland at the bottom. The Earth’s rotation means that air doesn’t flow straight down this hill; instead, it curves around and meanders, producing the West-East flow seen in animations like the one in this video from the NASA Goddard Science Visualization Studio.
As the Arctic has warmed, the slope of the “hill” has decreased. The result, Francis argues, is that the jet stream is no longer behaving like a river rushing down a mountain. Instead, it’s become wavier and slower, like a river meandering through a delta. Her research also suggests that the jet stream is increasingly getting stuck in a particular pattern – such as the one that has been sending storm after storm across the Atlantic Ocean to the United Kingdom ever since mid-December 2013.
Atmospheric scientist Francis’s theory explains how the United Kingdom’s recent bad weather fits into her theory of Arctic warming and the jet stream. But Francis’ theory is considered controversial among atmospheric scientists, including some who have questioned the evidence in favour of a wavier jet stream. Not especially new, this research is however relevant in the context of a spate of weird weather creating headaches for people and governments across the Northern Hemisphere.