Winter '11 / '12 - Arctic Oscillation - Day 20

The persistantly positive state of the Arctic Oscillation (AO) may be starting to lean toward a long awaited change of sign.

This three day Height-Latitude Cross Section of Zonally Averaged Zonal Wind loop shows deep-layer easterly winds blowing over the pole...a weakening polar vortex (PV)...and a strengthening sub-tropical jet (STJ)...all of which are favorable ingredients for the Arctic Oscillation to finally go negative.

As a result...the AO has ducked below zero for the first time in what seems like ages.

12/18	-0.024
12/19	-0.882
12/20	-0.438

Whether this trend continues remains to be seen.
Recent daily AO data here.

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Contours show zonal wind speed with an interval of 2.5 m/s.
Easterly winds are shaded in warm colors.
Graphics courtesy the Japanese Meteorological Agency (JMA).