As I mentioned in one of my previous posts, in climate discussions we deal with a parameter called the Arctic Oscillation (AO), which plays an important role in determining weather conditions at middle and more northerly latitudes. As explained in that post, the Arctic Oscillation has two phases, negative and positive, and fluctuates between them from year to year. In some years the weather of the Northern Hemisphere is influenced by the negative phase of the AO, and in others by the positive phase.
During the positive phase, weather at northern latitudes near the polar regions is colder than normal, while farther south, where more people live, conditions are warmer than normal (Figure 1).
 |
| Image from the original article |
Figure 1. Left: effects of the positive phase on Northern Hemisphere weather. Right: effects of the negative phase on these regions.
For example, 1390 SH was strongly influenced by the positive phase. In Canada, this contributed to weather warmer than normal and a very short, pleasant winter, while severe cold remained confined to the polar regions. This pattern reversed in 1391 SH: during the year just ended, the middle latitudes of the Northern Hemisphere experienced very cold weather under the influence of the negative phase.
That pattern is still continuing. During the past week, for example, temperatures in Canada were 10–15°C colder than during the corresponding period the previous year, and here we celebrated Nowruz amid exceptional snow and blizzards.
Although the negative phase brings cold, snow, and blizzards to regions such as Canada and northern Europe, at middle latitudes such as the Middle East and Iran it is accompanied by abundant precipitation. Because much of Iran has an arid or semi-arid climate, this is a welcome oscillation phase for those regions.
Nevertheless, forecasts indicate that the pressure system over the polar regions will weaken during the next few weeks (Figure 2), so we should soon see warmer weather in more northerly areas. This means relatively greater precipitation for regions such as northern Europe and North America and, unfortunately, warmer and drier weather—relative to the preceding few weeks—for Mediterranean regions and the Middle East.
 |
AO
|
Figure 2. Forecast AO index.