The Polar Vortex

[Koppite]

Could someone please explain what the Polar Vortex is and its effects on our winter if any.

Edited November 16, 2008 by RUSS L.F.C.

[chionomaniac]

The polar vortex is a persistent large-scale anti clockwise cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America and over NE Siberia. The polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometers). When the polar vortex is strong, deep purples can be seen on the 500hPa charts as shown below:

When these deep purples are situated over Greenland and Iceland then lower pressure dominates in the upper atmosphere which has a knock on effect to the lower levels. This tends to drive Atlantic based weather to these shores, giving us mild, wet weather. The stronger the polar vortex over the Arctic the Arctic Oscillation (known as the AO) will be more positive. When the AO is negative the polar vortex tends to be weaker and disrupted with areas of upper high pressure forming. These can 'block' the normal path of westerly surface low pressure systems and are known as blocking highs, and in the right position can lead to colder polar air heading southwards towards us.

[sundog]

The polar vortex is a persistent large-scale anti clockwise cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America and over NE Siberia. The polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometers). When the polar vortex is strong, deep purples can be seen on the 500hPa charts as shown below:

When these deep purples are situated over Greenland and Iceland then lower pressure dominates in the upper atmosphere which has a knock on effect to the lower levels. This tends to drive Atlantic based weather to these shores, giving us mild, wet weather. The stronger the polar vortex over the Arctic the Arctic Oscillation (known as the AO) will be more positive. When the AO is negative the polar vortex tends to be weaker and disrupted with areas of upper high pressure forming. These can 'block' the normal path of westerly surface low pressure systems and are known as blocking highs, and in the right position can lead to colder polar air heading southwards towards us.

How does it affect the BI if the pv is near NE Siberia? I presume its better to have it there then near Greenland for a chance of polar air?

[Koppite]

The polar vortex is a persistent large-scale anti clockwise cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America and over NE Siberia. The polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometers). When the polar vortex is strong, deep purples can be seen on the 500hPa charts as shown below:

When these deep purples are situated over Greenland and Iceland then lower pressure dominates in the upper atmosphere which has a knock on effect to the lower levels. This tends to drive Atlantic based weather to these shores, giving us mild, wet weather. The stronger the polar vortex over the Arctic the Arctic Oscillation (known as the AO) will be more positive. When the AO is negative the polar vortex tends to be weaker and disrupted with areas of upper high pressure forming. These can 'block' the normal path of westerly surface low pressure systems and are known as blocking highs, and in the right position can lead to colder polar air heading southwards towards us.

Thanks very much for that Chionomaniac, what is the AO at the moment positive or negative? I presume the blocking high you mentioned is the elusive Greenland high that most people on here wish for.

[chionomaniac]

Sorry, I may not have been clear. The Arctic polar vortex tends to have two centres and is elongated in shape due to the disruption of the vortex due to the presence of land masses at high latitudes. The polar vortex occurs above the core of the coldest polar air. Since frigid air is dense, heights are lower aloft (blues and purples on the 500hPa charts) because cold air has a lower thickness than warmer air. At the surface of the polar air mass will be high pressure, but low heights will occur aloft at 500 mb since the air is compacted due to high density air near the surface.

The polar vortex can often be located over Canada and NE Siberia simultaniously since the coldest surface air is often found over high latitude icy/land locations. The polar vortex aloft propagates toward where the polar air mass moves.

So it is better not to have it around Greenland / Iceland as that will drive surface low pressure systems from the Atlantic right at us. Even if the polar vortex can be shifted over to Scandinavia it can have a positive effect for us here as this chart from 1995 shows:

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Thanks very much for that Chionomaniac, what is the AO at the moment positive or negative? I presume the blocking high you mentioned is the elusive Greenland high that most people on here wish for.

Correct or Scandi High.

AO is positive but forecast to drop - http://www.newx-forecasts.com/ao_2.html