Why Is It Still Rainy in Iran and Snowy in Canada?

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.

Winter Cold and the Arctic Oscillation

If you live almost anywhere in North America, Europe, or Asia, it is probably not news to you that December 2009 and early January 2010 were very cold. The image shows how temperatures during that December differed from the average recorded from 2000 to 2008. Blue indicates areas where temperatures were below the recorded average, while red indicates temperatures above average. Many parts of the Northern Hemisphere were experiencing a cold winter, while polar regions were exceptionally warmer than average.

During winter, the Arctic Oscillation (AO) is an important pattern affecting Northern Hemisphere weather. It is defined by the difference in atmospheric pressure between the middle latitudes—around 45° north, roughly the latitude band between Montreal, Canada, and Bordeaux, France—and the polar regions. [source]

While a low-pressure air mass usually sits over the polar region, higher pressure tends to occur farther south in the middle latitudes. This pressure difference produces winds that help confine intensely cold air to the polar region. At times this pressure pattern weakens, meaning the pressure difference between the polar and middle-latitude regions decreases. This allows cold polar air to move farther south while warmer air from the south moves toward the polar regions. An Arctic Oscillation weaker than average is known as its “negative phase.” When the pressure systems are strong, the Arctic Oscillation is in its “positive” phase.

According to the U.S. weather service, during December 2009 the oscillation over the North Atlantic was strongly negative. The image above shows the effects of this negative phase on surface temperatures across the Northern Hemisphere, observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Terra satellite.

As can be seen, polar air made the land surface at middle latitudes considerably colder, while more northerly areas such as Greenland and Alaska were much warmer than usual. [source] [source]

For more information about the Arctic Oscillation, see:

• Polar regions and the Arctic Oscillation
• Effects of an exceptionally negative Arctic Oscillation phase on warmer polar regions

Additional Original Article Links [source] [source]

• Original source link