Climate and Weather: Information and Education

Arctic Oscillation

Figure 1

Figure 1, The positive and negative phase of Arctic Oscillation

You have probably noticed that some of the winters are more frigid than the other; that happens sometimes due to the changes in phases of Arctic Oscillation. This is one of the important factors which bring climate or weather variability in North America, Europe and even Asia is Arctic Oscillation (AO).

Arctic Oscillation refers to a pattern of atmospheric circulation mainly over mid to high latitudes of northern hemisphere. AO reflects as north to south ward severe storms and mid-latitude jet streams. AO occurs in positive and negative phases.

The positive phase of AO happens when air pressure is lower than average in the Arctic and higher than average in Pacific and Atlantic Oceans. The jet stream shifts to further north from its usual path. As a results Siberia, North America, Europe and eastern Asia face less cold weather in winter. However, these regions face more frigid weather if the AO is in its negative phase. The negative phase of AO is denoted by higher than average air pressure over Arctic but lower than average pressure in the Pacific and Atlantic Oceans (Figure 1).

The positive phase of AO brings wetter weather to Scotland, Scandinavia and drier weather to Spain and Middle East. On the other hand, a higher number of coastal storms known as 'Nor-easter' strikes New England during the negative phase of AO.

Figure 2
Figure 2, Arctic Oscillation Indexes during last five decades

The graph clearly shows that the AO indexes were strongly positive during early 1990's compared to the previous 40 years but the values of AO indexes have been variable or low during last 9 years.