What type of Polar vortex configuration can we expect for this winter? (by Diego Fdez-Sevilla)
Considering that at this part of the year the upper atmosphere in the North Hemisphere starts to describe patterns similar to those found in the past winter 2013-2014 I wonder: What type of Polar vortex configuration can we expect for this winter?
Last year the winter’s atmospheric circulation in the Northern Hemisphere was defined by a broken Polar vortex and a very wet and cold winter. The recent forecast released by the Met Office indicating Strong winds and heavy rain affecting parts of the UK (6th Oct 2014) resembles similar climatic events (milder this time) as those described in the Winter of 2014 under a broken polar vortex. For this reason I am sure that many eyes are already set on the current variations in the atmospheric circulation described by the Jet stream and the Polar Vortex in the North Hemisphere.
We are entering the previous phase to Winter in the North Hemisphere (Autumn) and actual events might give us clues pointing to what we can expect in the development of the atmospheric conditions ahead. In this post I look into the actual state of the atmospheric circulation trying to identify similarities with those conditions found in the winter of 2014 under a broken polar vortex. Even though the comparison is between present Fall and past Winter, I want to test the strength of Inertial momentum in Hemispheric Circulation.
Strong winds and heavy rain affect parts of the UK (6th Oct 2014)
6 10 2014. Source Met Office News Blog Official blog of the Met Office news team.
After an exceptionally dry September, the UK has seen its first bout of widespread heavy rain and strong winds so far this autumn. An area of low pressure centred close to Iceland has driven a cold front eastwards across Britain, bringing unsettled weather, particularly in the west.
Highest rainfall totals
Some of the highest rainfall totals are shown below (between 10pm (5th October) to 10am (6th October):
|SITE NAME||AREA||RAINFALL (mm)|
|SOUTH UIST||WESTERN ISLES||41.4|
Strongest wind gusts
There have been some strong wind gusts in parts, particularly across exposed western areas. The highest gusts are below:
|DATE/TIME||SITE NAME||AREA||WIND GUST (MPH)|
|06/10/2014 03:00||SOUTH UIST RANGE||WESTERN ISLES||84|
|06/10/2014 05:00||ALTNAHARRA NO 2||SUTHERLAND||78|
|06/10/2014 01:00||MAGILLIGAN NO 2||LONDONDERRY||70|
Earlier rainfall image across the UK:
The rain will continue to move east across the UK during the rest of today with drier conditions following for a time, before showers follow in places overnight. Although winds will tend to ease for most areas, further gales are expected across northeast Scotland through tomorrow. During this unsettled weather we’d encourage everyone to keep up to date with the latest forecasts and national severe weather warnings.
North and South Polar vortexes
An interesting phenomenon occurs near the geographic poles of the Earth, namely the polar vortex, which is a massive cyclone forming near the vertical edges of the planet. Polar vortexes are located in the middle and upper parts of the troposphere and the stratosphere.
They surround the polar areas and are part of the so-called “polar front” – the climate boundary between the polar area air masses and the neighboring warmer areas from both hemispheres. The Antarctic polar vortex is stronger and lasts longer than the Arctic one. This is due to the large land regions concentrated in the high latitude areas of the northern hemisphere, which create the Rossby atmospheric waves, thus diminishing the intensity of the vortex. In contrast, there are not too many factors able to influence the vortex from the South Pole.
Jet streams in the upper troposphere usually reach their maximum on the western side of the oceanic basins where the land-sea thermal contrasts are the strongest. East of the same basins, synoptic eddies get larger amplitude and feedback onto the jet streams. This feedback is characterized by the breaking of Rossby waves in the upper troposphere and plays a crucial role in the low-frequency atmospheric variability like the North Atlantic Oscillation (NAO). Two kinds of wave breaking exist; cyclonic wave-breaking pushing the jet southward and closely linked with the negative phase of the NAO and anticyclonic wave-breaking shifting the jet poleward and favoring the positive phase.
Graphic recreation of Wind velocity at 250 hPa (Jet Stream) on 6th Oct 2014
During December and January 2013/14 the pattern of winds over the North East Pacific and North America was very disturbed. The North Pacific jet was deflected a long way north, with a secondary branch extending southwards into the tropical Pacific accentuating the separation of the Pacific and Atlantic jet streams. The effects of this over North America and into the North Atlantic were profound. The deflection of the jet to the north led to colder air being carried south over Canada and the northern US to enter the North Atlantic jet and establish a stronger than normal temperature gradient at the entrance of the North Atlantic Jet. This acted to strengthen the jet and provide the conditions for active cyclogenesis, which in turn led to a sequence of strong storms across the UK throughout December and January.
Graphic recreation of Wind velocity at 10 hPa (Polar Vortex) on 8th Oct 2014
Above the troposphere, westerly winter winds in the polar night jet stream were very strong during December 20013 and January 2014. The polar night jet exceeded twice its normal strength at times during the winter, reaching speeds in excess of 100ms-1 in the upper stratosphere. A strengthening of the polar night jet often precedes periods of a strong Atlantic jet stream below and a positive North Atlantic Oscillation pattern, as was seen during the whole December to January period and consistent with the increased winter storminess this year.
Although internal fluctuations in the strength of the polar night jet cannot be excluded, there has also been an external factor in the current winter, again in the tropics, that has helped to precondition the system for a strong polar night jet. In the tropical stratosphere the winds circulate around the globe from west to east in some years and from east to west in others. This cycling of the tropical winds occurs roughly every two years – hence its name, the Quasi-Biennial Oscillation (QBO). Although it may seem remote from the North Atlantic, historical records show that when the QBO winds are westerly, this increases the chance of the positive phase of the North Atlantic Oscillation and a strong jet stream. The QBO has been in an unusually strong westerly phase throughout the winter 2013/14, and this factor was cited in the Met Office October long-range outlook for the November to January period, which pointed out the risk of increased storminess in early winter.
Conditions warming the Northern Polar Vortex induce colder winters in Eastern US and Europe.
“Some scientists are beginning to suspect that the lack of sea ice allows the oceans to pump heat into the atmosphere in the Arctic in a way that could impact weather patterns such as the North Atlantic Oscillation,” said Mark Serreze, director of the National Snow and Ice Data Center. “The idea is still very much in its infancy, but it’s worth looking into. If it turns out to be right, it could help to explain the frigid winters the eastern United States and Europe have experienced in the past years.”
“When the Polar Vortex — a ring of winds circling the Arctic — breaks down, this allows cold air to spill south, affecting the eastern United States and other regions,” says Dr. Overland. “This can result in a warmer-than-average Arctic region and colder temperatures that may include severe winter weather events on the North American and European continents.”
Similarities in Arctic Sea Ice Extent
Similarities between Wind & Temp patterns at 700 hPa
Winter Feb14 Fall Oct/14 Winter 14/15??
There is a strong association between the stormy weather experienced in the UK during December and January 2013/14 and the up-stream perturbations to the jet stream over North America and the North Pacific.
In the lower troposphere, at 850mb, the winds over the North Atlantic were much stronger than normal during December and January 2013/14. Likewise the perturbed flow over the North Pacific was also very clear with an anomalous anticyclonic system off the west coast of North America, which was a persistent feature of this winter’s weather. At these lower levels in the atmosphere, the clash between the cold northerly airstream from North America with the warm, moist airstream from the tropical Atlantic was notable. Not only this acted to invigorate storms forming on the jet stream, but the inflow of warm, moist air from the tropics enhanced the moisture being carried by the storm systems and led to higher rainfall downstream over the UK.
(If you feel like sharing this post I would appreciate to have a reference about the place or platform, by private or public message, in order for me to have the opportunity to join the debate and be aware of the repercussion which might generate d.fdezsevilla(at)gmail.com)
- Met Office. The Recent Storms and Floods in the UK (Feb 2014) (by Diego Fdez-Sevilla)
- The breakdown of the Polar Vortex. It happened before so, What would follow? (by Diego Fdez-Sevilla)
- (Updated) A Groundhog forecast on climate at the North Hemisphere. (by Diego Fdez-Sevilla)
- Why there is no need for the Polar Vortex to break in order to have a wobbling Jet Stream and polar weather? (by Diego Fdez-Sevilla)