Friday, February 22, 2013

Patterns in Arctic Weather and Climate

The unique geography of the Arctic leads to unique weather patterns that reappear in the region year after year. Some weather patterns, such as cyclones or anticyclones, occur in the Arctic as well as other regions. Other weather patterns, like the Arctic Oscillation, are specific to the Arctic. Weather patterns that recur or persist over multiple seasons are called semipermanent highs and lows, because these patterns show up in long-term averages of the regional weather.

Cyclones and Anticyclones

Cyclones, or low-pressure systems, are circular weather patterns that rotate in a counter clockwise direction. In a cyclone, air moves upwards in the center of the pattern, bringing stormy wet weather. In the Arctic, cyclones occur year round, but they tend to happen more in certain places depending on the time of year. Semipermanent lows in the Artic include the Aleutian Low, a low-pressure center that experiences many cyclones and storms in the winter, and the Icelandic Low, a low-pressure center located near Iceland.

Anticyclones are the opposite of cyclones, high-pressure systems that rotate in a clockwise direction. An anticyclone known as the Beaufort High recurs year after year, sitting over the Beaufort Sea and Canadian Archipelago in winter and spring. An anticyclone also frequently appears over Siberia, known as the Siberian High.

Polar Lows

Cyclone over the Arctic Ocean.

Polar lows are small, intense cyclones that form over open ocean during the cold season. From satellite imagery, polar lows can look much like a hurricane, with a large spiral of clouds centered around an eye—for this reason they are sometimes called Arctic hurricanes. Polar lows range in size from around 100 to 500 kilometers (62 to 310 miles) in diameter. Wind speeds average around 50 miles per hour, although they can occasionally reach hurricane strength (64 miles per hour).

Polar lows tend to form when cold Arctic air flows over relatively warm open water. The storms can develop rapidly, reaching their maximum strength within 12 to 24 hours of formation, but they dissipate just as quickly, lasting on average only one or two days.

Semipermanent Highs and Lows

Weather maps show the circulation and pressure patterns over one or several days. But maps of sea level pressure can also be averaged over several months or years, to show the average circulation patterns in the atmosphere. These averaged maps remove some of the variability caused by day-to-day weather changes, instead showing longer-term patterns that can affect weather and climate both within and outside of the Arctic.

Researchers compare the relative strengths of semipermanent highs and lows, and report these comparisons in indices such as the North Atlantic oscillation and the Arctic oscillation. These indices have been linked to variability in temperatures and to sea ice conditions in the Arctic.

Arctic Oscillation

The Arctic Oscillation refers to an opposing pattern of pressure between the Arctic and the northern middle latitudes. Overall, if the atmospheric pressure is high in the Arctic, it tends to be low in the northern middle latitudes, such as northern Europe and North America. If atmospheric pressure is low in the middle latitudes it is often high in the Arctic. When pressure is high in the Arctic and low in mid-latitudes, the Arctic Oscillation is in its negative phase. In the positive phase, the pattern is reversed.

Meteorologists and climatologists who study the Arctic pay attention to the Arctic Oscillation, because its phase has an important effect on weather in northern locations. The positive phase of the Arctic Oscillation brings ocean storms farther north, making the weather wetter in Alaska, Scotland, and Scandinavia and drier in the western United States and the Mediterranean. The positive phase also keeps weather warmer than normal in the eastern United States, but makes Greenland colder than normal.

In the negative phase of the Arctic Oscillation the patterns are reversed. A strongly negative phase of the Arctic Oscillation brings warm weather to high latitudes, and cold, stormy weather to the more temperate regions where people live. Over most of the past century, the Arctic Oscillation alternated between its positive and negative phase. For a period during the 1970s to mid-1990s, the Arctic Oscillation tended to stay in its positive phase. However, since then it has again alternated between positive and negative, with a record negative phase in the winter of 2009-2010.

Left: Effects of the Positive Phase of the Arctic Oscillation. Right:Effects of the Negative Phase of the Arctic Oscillation. —Credit: J. Wallace, University of Washington.
Climate Change in the Arctic

Arctic sea ice extent for September 2012 was 3.61 million square kilometers (1.39 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole.
—Credit: National Snow and Ice Data Center

The Arctic region is warmer than it used to be and it continues to get warmer. Over the past 30 years, it has warmed more than any other region on earth. Most scientists agree that Arctic weather and climate are changing because of human-caused climate change.

Arctic warming is causing changes to sea ice, snow cover, and the extent of permafrost in the Arctic. In the first half of 2010, air temperatures in the Arctic were 4° Celsius (7° Fahrenheit) warmer than the 1968 to 1996 reference period, according to NOAA. Satellite data show that over the past 30 years, Arctic sea ice cover has declined by 30 percent in September, the month that marks the end of the summer melt season. Satellite data also show that snow cover over land in the Arctic has decreased, and glaciers in Greenland and northern Canada are retreating. In addition, frozen ground in the Arctic has started to thaw out. Scientists first started to see changes in the Arctic climate in the 1970s and 1980s.

Changes in the Arctic climate are important because the Arctic acts as a refrigerator for the rest of the world. The Arctic region gives off more heat to space than it absorbs from outside, which helps cool the planet. So changes in the Arctic climate could affect the climate in the rest of the world.
Arctic amplification

Researchers say that the changes in the Arctic are worrisome, because they could lead to feedback effects that spur further warming. For instance, when the white sea ice melts in summer, areas of dark open water are exposed which can absorb more heat from the sun. That extra heat then helps melt even more ice. Permafrost may also be involved in feedbacks. As permafrost thaws, plants and animals that were frozen in the ground begin to decay. When they decay, they release carbon dioxide and methane back to the atmosphere that contributes to further warming.

Scientists have already seen evidence that positive feedbacks are occurring in the Arctic. They call this Arctic amplification. Predicting the Arctic climate is difficult. Some of the changes in the Arctic could also have negative feedback effects, or effects that reduce the amount of warming. For example, if warm temperatures make the Arctic growing season longer, more plants can survive and take up more carbon from the air. However, most evidence suggests that the positive feedback effects outweigh the negative effects. A recent report by NOAA concluded that Arctic climate is unlikely to return to previous conditions.

Scientists are studying the many factors that influence Arctic climate to help figure out how feedbacks work and what will happen in the future. Researchers are also investigating how the changes in the Arctic climate will affect climate in other parts of the world. Scientists study data collected by satellites and at ground stations and also used use sophisticated computer models.

This image shows trends in mean surface air temperature over the period 1960 to 2011. Notice that the Arctic is red, indicating that the trend over this 50 year period is for an increase in air temperature of more that 2° C (3.6° F) across much of the Arctic, which is larger than for other parts of the globe. The inset shows linear trends over the period by latitude.
—Credit: NASA GISS

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