Arctic Weather Patterns
Factors affecting weather and climate interact with each other to produce the major weather patterns or geophysical processes of the Arctic. Most patterns described here can be observed globally, but some are specific to the Arctic.
A cyclone, also called a low or depression, is a weather pattern consisting of a region of low air pressure some 1000 kilometers to 2000 kilometers in diameter around which air circulates counterclockwise in the Northern Hemisphere. This counterclockwise motion is due to the Coriolis force: air moving toward the low-pressure center of the depression is deflected to the right in the Northern Hemisphere. In a cyclone, air ascends near the center of the low, and the weather tends to be stormy, with precipitation.
Cyclones in the Arctic
Winter cyclones in the Eurasian Arctic occur most frequently in the Barents and Kara Seas region. An average of six cyclones pass through the southern Barents Sea, and five through the southern Kara Sea in any winter. On average, four cyclones pass through the southern East Siberian Sea and the lower course of Kolyma river in each of the winter months. Cyclones can bring in warm air, causing rapid warming and melting even in the middle of winter. Over the North American Arctic, the highest frequency of cyclones occurs east of Greenland in association with the Icelandic Low. Cyclones are also common in Baffin Bay.
The summer distribution of air pressure and cyclones is different from that of winter. With more uniform temperatures over the northern parts of the Atlantic and Pacific oceans, summer cyclones tend to be weaker than their winter counterparts. The semipermanent Icelandic and Aleutian Lows weaken. In July and August, only one or two cyclones move to the arctic seas from the northern Atlantic. On the other hand, more cyclones move towards the pole from the midlatitudes of Siberia and Canada. The number of cyclones in the Chukchi Sea in July can be as high as six, and in the East Siberian Sea up to four or five.
The opposite of a cyclone, an anticyclone, is a weather pattern consisting of a broad region of high air pressure around which air circulates clockwise in the Northern Hemisphere. In an anticyclone, air descends near the center of the high, and the weather tends to be fair.
Anticyclones in the Arctic
A persistent anticyclone or high pressure ridge called the Arctic High, also known as the Beaufort High, sits over the Beaufort Sea and the Canadian Archipelago in winter and spring.
Eastern Siberia is a center of frequent winter anticyclones, and these are known climatically as the Siberian High. Strong cooling in this region results in the lowest air temperatures in the Northern Hemisphere. In the winter and spring, anticyclones in the Russian Arctic move mainly from the circumpolar regions through the eastern parts of the Barents and Kara seas. Some also move into the Barents Sea from the northern coast of Greenland. Across the Arctic, anticyclones are less common and generally weaker in summer.
The Polar Vortex
The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered generally in the polar regions of each hemisphere. In the Arctic, the vortex is asymmetric and typically features a trough (an elongated area of low pressure) over eastern North America. It is important to note that the polar vortex is not a surface pattern. It tends to be well expressed at upper levels of the atmosphere (that is, above about five kilometers).
Semipermanent Highs and Lows
The Arctic is characterized by "semipermanent" patterns of high and low pressure. These patterns are semipermanent because they appear in charts of long-term average surface pressure. They can be considered to largely represent the statistical signature of where transitory high and low systems that appear on synoptic charts tend to be most common.
This semipermanent low pressure center is located near the Aleutian Islands. Most intense in winter, the Aleutian Low is characterized by many strong cyclones. Traveling cyclones formed in the subpolar latitudes in the North Pacific usually slow down and reach maximum intensity in the area of the Aleutian Low.
This low pressure center is located near Iceland, usually between Iceland and southern Greenland. Most intense during winter, in summer, it weakens and splits into two centers, one near Davis Strait and the other west of Iceland. Like its counterpart the Aleutian Low, it reflects the high frequency of cyclones and the tendency for these systems to be strong. In general, migratory lows slow down and intensify in the vicinity of the Icelandic Low.
The Siberian High is an intense, cold anticyclone that forms over eastern Siberia in winter. Prevailing from late November to early March, it is associated with frequent cold air outbreaks over east Asia.
The Beaufort High is a high pressure center or ridge over the Beaufort Sea present mainly in winter.
North American High
The North American High is a relatively weak area of high pressure that covers most of North America during winter. This pressure system tends to be centered over the Yukon, but is not as well-defined as its continental counterpart, the Siberian High.
Small cyclones forming over open sea during the cold season within polar or arctic air masses are called "polar lows." Typically several hundred kilometers in diameter, and often possessing strong winds, polar lows tend to form beneath cold upper-level troughs or lows when frigid arctic air flows southward over a warm body of water.
Polar lows last on average only a day or two. They can develop rapidly, reaching maximum strength within 12 to 24 hours of the time of formation. They often dissipate just as quickly, especially upon making landfall. In some instances several may exist in a region at the same time or develop in rapid succession.
In satellite imagery polar lows show characteristic spiral or comma shaped patterns of deep clouds, sometimes with an inner "eye" similar to those seen in tropical cyclones. Convective cloud bands occupy the surroundings. Analysis of aircraft and radiosonde data collected during field experiments reveals that polar lows may possess warm cores. This finding, coupled with their appearance in satellite imagery, has prompted some investigators to refer to polar lows as "arctic hurricanes," although they seldom, if ever, possess hurricane strength winds.
Polar lows are difficult to predict even with current high resolution and high performing operational numerical models, because they usually occur in remote oceanic regions where data are too sparse to define the model initial state on a sufficiently fine scale. However, present-day models can depict synoptic-scale patterns favorable to the development of the smaller scale systems, allowing forecasters to use the predictions in conjunction with satellite imagery and conventional observations to make subjective forecasts of their occurrence.
The Arctic as a "Heat Sink"
The Arctic plays a key role in the earth's heat balance by acting as a "heat sink." The global earth-atmosphere system gains heat from incoming solar radiation, and returns heat to space by thermal radiation. Most of the heat gain occurs in low latitudes, and this gain is balanced (on average) by heat loss that takes place at latitudes north and south of about 40 degrees. Therefore the Arctic is said to act as a "heat sink" for energy that is transported from lower latitudes by ocean currents and by atmospheric circulation systems.
Heat is transported to the Arctic primarily in the following ways:
- Sensible heat is transported poleward during the exchange of air masses from the tropics to the middle and high latitudes. This transfer of heat is largely accomplished by cyclones.
- As storms travel poleward, some of the water vapor condenses as clouds, thereby releasing latent heat.
- Ocean currents bring heat from the tropics to the the northern part of the Atlantic Ocean and into the Arctic.
The Arctic Oscillation
The Arctic Oscillation refers to opposing atmospheric pressure patterns in northern middle and high latitudes.
The oscillation exhibits a "negative phase" with relatively high pressure over the polar region and low pressure at midlatitudes (about 45 degrees North), and a "positive phase" in which the pattern is reversed. In the positive phase, higher pressure at midlatitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska, Scotland and Scandinavia, as well as drier conditions to the western United States and the Mediterranean. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal, but leaves Greenland and Newfoundland colder than usual. Weather patterns in the negative phase are in general "opposite" to those of the positive phase, as illustrated in the figure to the right.
Over most of the past century, the Arctic Oscillation alternated between its positive and negative phases. Starting in the 1970s, however, the oscillation has tended to stay in the positive phase, causing lower than normal arctic air pressure and higher than normal temperatures in much of the United States and northern Eurasia.
Feedback Loops: Interactions that Influence Arctic Climate
In the climate system a "feedback loop" refers to a pattern of interacting processes where a change in one variable, through interaction with other variables in the system, either reinforces the original process (positive feedback) or suppresses the process (negative feedback). In order to model and predict arctic (and global) climate variability correctly, feedback loops must be understood. Two major feedback processes that scientists consider in studies of arctic and global climate change are described below in simple terms. In nature, the processes are considerably more complicated.
Rising temperatures increase melting of snow and sea ice, reducing surface reflectance, thereby increasing solar absorption, which raises temperatures, and so on. The feedback loop can also work in reverse. For instance, if climate cools, less snow and ice melts in summer, raising the albedo and causing further cooling as more solar radiation is reflected rather than absorbed. The temperature—albedo feedback is positive because the initial temperature change is amplified.
Temperature—Cloud Cover—Radiation Feedbacks
It is thought that if climate warms, evaporation will also increase, in turn increasing cloud cover. Because clouds have high albedo, more cloud cover will increase the earth's albedo and reduce the amount of solar radiation absorbed at the surface. Clouds should therefore inhibit further rises in temperature. This temperature—cloud cover—radiation feedback is negative as the initial temperature change is dampened.
However, cloud cover also acts as a blanket to inhibit loss of longwave radiation from the earth's atmosphere. By this process, an increase in temperature leading to an increase in cloud cover could lead to a further increase in temperature – a positive feedback.
Knowing which process dominates is a complex issue. Cloud type plays a strong role, as do cloud water content and particle size. Another factor is whether the cloud albedo is higher or lower than that of the surface. Research indicates that the effect of this feedback in the Arctic may be different than in other latitudes. Except in summer, arctic clouds seem to have a warming effect. This is because the blanket effect of clouds tends to dominate over reductions in shortwave radiation to the surface caused by the high cloud albedo.
Climate Change and the Arctic
Arctic climate is showing signs of rapid change, but extensive study is needed to fully understand the changes and what they may mean to the Arctic and to global climate systems.
Changes beginning in the 1970s and 1980s include:
- warmer winters and springs over North America and Eurasia, partially compensated by cooling over the northern North Atlantic,
- warmer air over the central Arctic Ocean,
- warming in the Arctic Ocean at 200 to 900 meters depth,
- reductions in sea ice and snow extent,
- increases in terrestrial precipitation,
- warming permafrost in Alaska and Russia, and cooling permafrost in Canada,
- increased plant growth and northward migration of the treeline.
Most of these trends derive from relatively short environmental records, and the magnitude of the high-latitude temperature increase is no larger than the interdecadal temperature range for the last century. Since climate is naturally variable, the occurrence of an exceptionally warm period may not be abnormal because it may fall well within the expected range of temperature variability for a specific area. However, identified patterns of arctic change agree generally with those predicted by current climate models under scenarios of enhanced greenhouse warming.
The temperature increase at high latitudes partly reflects recent (1990s) atmospheric circulation shifts. In particular, the polar vortex has strengthened and surface pressures in the central Arctic are lower as a result. In turn the normal clockwise circulation of sea ice in the Beaufort Sea (the Beaufort Gyre) has weakened. Other changes linked to observed shifts in atmospheric circulation are:
- generally positive phases of the North Atlantic Oscillation and the Arctic Oscillation, implying stronger westerlies in midlatitudes,
- increased heat and moisture advection into the Greenland and Barents Sea regions,
- an increase in the temperature of the Atlantic water flowing into the Arctic Ocean and therefore warmer Arctic water,
- decreased sea-ice cover,
- thinning of multiyear arctic sea ice,
- negative mass balance of glaciers.
Material for this section was drawn from Serreze et al. (2000); Rothrock et al. (1999); and the National Science Foundation's Arctic Systems Science program's Report on the Arctic Change Workshop, (1998).)
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