Why does the Northern polar vortex usually have two centers?

Question

From http://glossary.ametsoc.org/wiki/Polar_vortex

polar vortex

A planetary-scale mid- to high-latitude circumpolar cyclonic circulation, extending from the middle troposphere to the stratosphere. The Northern Hemisphere vortex often features two centers—one near Baffin Island and the other over northeast Siberia—with analogous circumpolar asymmetry atypical in the Southern Hemisphere. (emphasis mine)

Why does the Northern Hemisphere polar vortex often have two centers, as opposed to one for the Southern? I gather that this may have something to do with the Arctic vortex being elongated in shape, but this leads to the question of why it is elongated? Does it have something to do with the terrain features below?

Answer 1

Northern Hemisphere

The spatial structure of the polar vortex is determined by the stationnary waves. Stationary waves in the northern hemisphere are primarily modulated by stationary (surprise!) forcings of the northern hemisphere :

• Topography : Rockies and Himalayan mountains
• Land-sea ocean heating contrast

This spatial pattern is mostly locked but they are slightly modulated by the seasonaly changing meridional temperature gradient and from internal chaotic processes (i.e. turbulence).

This article is a must-read and not too technical for the northern hemisphere circulation:

http://onlinelibrary.wiley.com/doi/10.1029/134GM01/summary

vs Southern Hemisphere

When you compare such forcings between the northern and southern hemisphere, there is a big difference: much less land-sea contrast and less topographic forcing in the southern ocean. Hence, a "wavenumber one" configuration, as compared to a "wavenumber two" for the northern hemisphere.