The weakening of the polar vortex in the Arctic

First, what is the polar vortex?  I will have to post what others have written:

From the National Snow and Ice Data Center:

The Polar Vortex

The polar vortex is a persistent large-scale 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. It is important to note that 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).

And, what is the AO or Arctic oscillation?

The Arctic Oscillation

The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and high latitudes.

The oscillation exhibits a "negative phase" with relatively high pressure over the polar region and low pressure at midlatitudes (about 45° North), and a "positive phase" in which the pattern is reversed. In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase, as illustrated below.

Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.

Effects of the Positive Phase      |     Effects of the Negative Phase
of the Arctic Oscillation                    of the Arctic Oscillation

(Figures courtesy of J. Wallace, University of Washington)

Semipermanent Highs and Lows

The Arctic is characterized by "semipermanent" patterns of high and low pressure. These patterns are semipermanent because they appear in charts of long-term average surface pressure. They can be considered to largely represent the statistical signature of where transitory high and low systems that appear on synoptic charts tend to be most common.

Aleutian Low

This semipermanent low pressure center is located near the Aleutian Islands. Most intense in winter, the Aleutian Low is characterized by many strong cyclones. Traveling cyclones formed in the subpolar latitudes in the North Pacific usually slow down and reach maximum intensity in the area of the Aleutian Low.

Icelandic Low

This low pressure center is located near Iceland, usually between Iceland and southern Greenland. Most intense during winter, in summer, it weakens and splits into two centers, one near Davis Strait and the other west of Iceland. Like its counterpart the Aleutian Low, it reflects the high frequency of cyclones and the tendency for these systems to be strong. In general, migratory lows slow down and intensify in the vicinity of the Icelandic Low.

Siberian High

The Siberian High is an intense, cold anticyclone that forms over eastern Siberia in winter. Prevailing from late November to early March, it is associated with frequent cold air outbreaks over east Asia.

Beaufort High

The Beaufort High is a high pressure center or ridge over the Beaufort Sea present mainly in winter.

North American High (not the one in California on the beach, ok)

The North American High is a relatively weak area of high pressure that covers most of North America during winter. This pressure system tends to be centered over the Yukon, but is not as well-defined as its continental counterpart, the Siberian High.

From the National Weather Service's Climate Prediction Center:

The Arctic Oscillation Index

(Click on image to enlarge)

The daily geopotential height anomalies at 17 pressure levels are shown for the previous 120 days as indicated, and they are normalized by standard deviation using 1979-2000 base period. The anomalies are calculated by subtracting 1979-2000 daily climatology, and then averaged over the polar cap poleward of 65°N. The blue (red) colors represent a strong (weak) polar vortex. The black solid lines show the zero anomalies. The daily AO indices are shown for the previous 120 days, and the ensemble forecasts of the daily AO index at selected lead times are appended onto the time series. The indices are standardized by standard deviation of the observed monthly AO index from 1979-2000. A 3-day running mean is applied to the forecast time series. The values at the upper left and right corners of each figure indicate the mean value of the AO index and the correlation coefficients between the observations and the forecasts, respectively. The first panel shows the observed AO index (black line) plus forecasted AO indices from each of the 11 GFS ensemble members starting from the last day of the observations (red lines). The ensemble mean forecasts of the AO index are obtained by averaging the 11 GFS ensemble members (blue lines), and the observed AO index (black line) is superimposed on each panel for comparison. For the forecasted indices (lower 3 panels), the yellow shading shows the ensemble mean plus and minus one standard deviation among the ensemble members, while the upper and lower red lines show the range of the forecasted indices, respectively.   Link:  http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index_ensm.shtml

From NCAR (for those with a more high-tech bend than I have:

The breakdown of the Stratospheric Polar Night Vortex is an atmospheric event that occurs once or twice each year in the polar wintertime stratosphere. As the polar vortex is formed, sharp gradients of potential vorticity at the vortex edge isolate polar air from the air at lower latitudes, producing conditions favorable for wintertime polar ozone depletion. Rossby waves propagating upward from the troposphere along the edge of the Polar Vortex grows exponentially in amplitude, eventually tearing the vortex apart.

	Related Links
	High Resolution Simulation

The animations depict the flow of the Polar Vortex by visualizing Potential Vorticity (a variable that acts as a tracer) over the 16-day simulation. In the second and succeeding images, the height of the data has been greatly exaggerated to better show the rich vertical structure contained in the vortex. In reality, the vortex is only a few tens of kilometers thick, a pancake-thin region that can extend over much of the Northern Hemisphere.

QuickTime | Real | MPEG

This image shows a satellite view of the earth from space with three isosurfaces of Potential Vorticity mapped over the Northern Hemisphere from day 16 of the simulation. Brighter colors correspond to increasing Potential Vorticity. The animations at right, show the entire 16-day evolution