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January-February 2009

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Alaska's Climate and Weather


Mt. McKinleyThis is the second article in a series focusing on the climate and weather of the 50 states.

Many superlatives can be used to describe Alaska: it easily ranks as America’s largest state in area (more than twice the size of Texas); it is home to America’s tallest peak (Mt. McKinley, 20,320 feet); and its glaciers take up more area (about 29,000 square miles) than many contiguous U.S. states combined. With its geography ranging from turquoise-blue fjords in the south to the windswept, open tundra in the north, it’s no wonder Alaska is affectionately nicknamed “the last frontier.” The range of weather that one can experience here varies just as widely—from the mild and wet maritime climate of the far Southeast Panhandle, to the cold and dry continental climate of the Interior.

Alaska’s northerly global position (51°N-71°N latitude) and widely varying amount of solar radiation throughout the year play significant roles in controlling its climate. For example, locations north of the Arctic Circle have periods of 24-hour darkness and 24-hour sunlight during certain times of the year. And the higher the latitude, the longer these periods can extend. In Barrow (71.30°N), the Sun remains below the horizon from about November 20-January 22. In Anchorage (61.22°N), the Sun’s motion toward the horizon is limited to about 6° in elevation in winter. North of the Arctic Circle, even during summer, the midday Sun might circle for nearly 24 hours but never gets higher than about 50° from the horizontal (or about halfway up in the sky). Not surprisingly, terms such as polar night and midnight Sun are typically used to describe mid-winter and mid-summer skies, respectively.

This astronomical setting (and associated reflection from snow and ice cover under low Sun angles) contributes to an overall deficit in solar energy and leads to low average annual temperatures. For much of the state, average annual temperature values are below freezing and, typically, the farther north you go, the lower the average. In winter, radiational heat loss from snow cover during long nights can dramatically lower temperatures. During summer, with clear skies, even a relatively low Sun angle can heat interior locations, with Fairbanks reaching the upper 80s to near 90°F on occasion. You might be surprised to know that the all-time high temperature in Alaska is 100°F—the same as that of Hawaii.

Since Alaska is essentially a large peninsula on the North American continent, the ocean has a significant influence on its weather and climate. Water heats up and cools down much more slowly than land does, and the open ocean provides a considerable atmospheric moisture source. Thus, coastal areas experience a milder, less variable, and wetter climate than inland locations. When sea ice is present, the maritime influence is tempered. The maximum extent of Arctic sea ice occurs from mid-February to early April and reaches well into the Bering Sea, though not quite down to the Aleutian Islands; the minimum of sea ice occurs in late summer and features a shrunken ice pack north of Alaska’s Arctic coast.

A Land of Storms    
Ask any mariner what it is like to navigate the Bering Sea or any region offshore of Alaska, and he will probably use adjectives such as treacherous, rough, or dangerous. Hurricane-like storms with strong winds and high seas are the norm here, particularly for the southwest coast. This is because the so-called Aleutian Low parks itself here. The significance for Alaska’s weather and climate is that from about October through April, low pressure systems tend to intensify in this region, track eastward across the North Pacific, and move into the state, bringing with them wet and windy weather. In fact, according to NOAA, 6 of the top 25 windiest U.S. cities are in Alaska. St. Paul Island, Alaska, ranks as the second-windiest location, behind Mt. Washington, New Hampshire, with an average annual wind speed of 16.9 mph. Coming in at numbers 3, 7, 12, 13, and 24, respectively, are Cold Bay (16.8 mph), Barter Island (13.2 mph), Kotzebue (12.7 mph), Bethel (12.5 mph), and Barrow (12.0 mph).

If you were to take a drive along the entire trans-Alaska pipeline from Valdez, its southern terminus in the Prince William Sound, to the oil fields of the Arctic coast near Prudhoe Bay, you would travel more than 800 miles and cross the continental divide, the Arctic Circle (66°33”N latitude), and the mighty Yukon River. In the process, you would also crest three significant mountain ranges going from east to west: the Chugach Range, the Alaska Range (home to Mt. McKinley), and the Brooks Range, along with broad river valleys of the Interior and the Arctic  coastal plain.

These mountains—along with the Wrangell-St. Elias and Coastal ranges located in the southeast and bordering Canada’s Yukon Territory and British Columbia—have a tremendous influence on weather and climate. Low pressure systems moving across the Gulf of Alaska reach the southern coast and encounter mountains rising 5,000 feet or more from sea level. As the air is forced over these mountains, the additional windward side uplift adds to the system-induced precipitation. As a result, precipitation here is the highest in the state and leads to the large number of glaciers and icefields. Two of these, the Bering and Malaspina glaciers near the south-central coast, each cover an area of about 2,000 square miles. (For comparison, the area of Delaware is slightly less than 2,500 square miles.) However, once you traverse to the north (leeward) side of these ranges, you enter a region of sinking air that marks the mountain’s rain-shadow, with associated drier and sunnier weather.

Snowfall data along the Alaska pipeline provides further evidence of topographic and ocean forcing. Valdez, at the base of the Chugach Mountain Range, gets more than 320 inches of snow per year. By comparison, Fairbanks (in the Interior) gets 68 inches, and Prudhoe Bay (on the Arctic Coast) gets only about 30 inches. However, in the far north it can snow any day of the year, while in Valdez the snow is on the ground from mid-October to mid-May.

Taking all these factors into consideration—latitude, Sun angle, snow and ice cover, proximity to water, the Aleutian Low, and topography—one can divide Alaska into a number of distinct climate regions: arctic (in the far north), continental (encompassing a large region of the Interior), maritime (along the southern and western coasts), and transitional (areas experiencing both continental and maritime characteristics depending on time of year and weather situation). These represent the broader climate and weather categories. However, given the sheer size and varied topography of Alaska, there is one type of climate that can rule above all—the microclimate.

The local environment and surrounding terrain characteristics often exert significant control over the weather in an area at any given time. Even the long-term average of basic weather parameters can vary significantly among areas in close proximity. Juneau, the state’s capital city, provides a prime example. Juneau is the only capital in the nation that requires a ferry (known locally as the “Alaska Marine Highway”) for access. The reason is that Juneau lies in the southeast panhandle—an intricate collection of fjords and islands (many of them glaciated) connected by waterways. The airport, located at the same elevation about eight short air miles northwest of town and near the base of Mendenhall Glacier, has an average annual temperature that is 2°F colder than downtown Juneau. The airport also receives about 20 inches less precipitation on average than downtown due to local topographic influences. In addition, local winds can be drastically different here than what you might expect from looking at a daily weather map of this area. Air can, and often does, rush down off the glacier in a type of gravity-driven flow of cold air (known as a katabatic wind). Similar types of microclimate effects on weather and climate can be found throughout southern Alaska where the terrain is complex.

Microclimates dominate the intermountain Interior regions, too. The most extreme example of this comes during winter under conditions of clear skies and calm winds, especially in sheltered valley locations. Intense radiational heat loss during long nighttime periods favors formation of low-level temperature inversions (vertical temperature profiles in which temperature increases with height). Because cold air sinks (and light winds under the inversion inhibit mixing of the air), the lower you are in elevation, the colder you are. Almost nowhere on Earth are surface-based temperature inversions stronger than in Interior Alaska.

The Fairbanks area (central Alaska) represents the consummate example of this wintertime phenomenon. That’s because the town is sheltered on three sides by uplands. Cold air from higher elevations drains down to the valley floor, where it is trapped underneath the inversion. Once formed, this inversion can keep residents in a deep freeze with temperatures plummeting to -40 (Fahrenheit or Celsius; the scales meet at this value) for a week at a time. On average, Fairbanks drops below -40 about 10 times each winter. Underneath the inversion, emissions from cars, homes, and industries become trapped, and the result is Alaska’s dreaded ice fog. The dense layer of suspended ice crystals brings very low visibility and extremely poor air quality to the valley and other low-lying locations. Meanwhile, people living in the surrounding uplands less than 10 miles away from downtown (though a few hundred feet higher in elevation and above the inversion layer) are basking in higher temperatures and clearer skies.

Temperatures in the uplands can be a balmy -10°F while the valley bottom is -40°F. It is no wonder that the more sought-after places to live are above the valley floor.
Regardless of where one resides, nearly all Alaskans need special heaters to warm the oil pan, battery, and engine block of their vehicles before they head out on the road. There are even electrical outlets in parking lots at schools and businesses to help Alaskans cope with this situation.

Lightning, Forest Fires    
Even though Alaska is a long way from central Florida—the lightning capital of the United States—it does have its fair share of lightning activity. Lightning strikes have been monitored since 1986, and more than 30,000 strikes occur per year, primarily during June and July, the warmest time of year. There is a clear tendency for strikes to occur during the late afternoon hours—just after the warmest part of the day—and the seasonal total is often determined by a few days of intense lightning outbreaks, rather than many strikes scattered throughout the season.

A natural process in the boreal (or northern) forests that dominate a large portion of Alaska is wildfire. Although the Interior region is most susceptible, records show a high variability in acres burned from year to year. Lightning-ignited fires often burn the largest area of the state (compared to human-caused fires). Two of the 3 worst wildfire years have occurred recently, with 6.5 million acres burned in 2004 and nearly 4.5 in 2005. Although there is not a clear-cut annual trend, the five worst fire years were all marked by persistent summer warmth and dryness. In 2007 there was a fire near Anaktuvuk River, north of the Brooks Mountain Range in northern Alaska. A total of 256,734 acres burned in that fire, more than all other fires in the historical record above 68°N latitude combined.   
A Shifting Climate
Data show that significant changes and variability have occurred in Alaska’s weather and climate in recent time. These can be linked to environmental changes such as glacier retreat, permafrost thaw, increased erosion, and vegetation shifts. In fact, systematic weather observations began only about 180 years ago in Alaska. Sitka, which is in the southeast panhandle, has weather records dating back to 1828, and its weather observation site is still in operation today. Most data records for the state are not as lengthy, however. Two aspects of a changing climate in Alaska are clear from these observations: temperatures have increased over the last century, and the rise has been greatest for the continental and arctic climate regions. But the change has not been linear; rather, data show variability not only from year to year but on a multi-decadal scale. This variability is much stronger for the colder seasons than for the summer.

There have been periods in Alaska’s recent climate history in which temperatures have been cooler than average and also warmer than average for extended periods of time. The culprit is the variability in air circulation patterns. Patterns of temperature variability are strongly correlated to an oceanic and atmospheric feature in the north Pacific termed the Pacific Decadal Oscillation (PDO). Depending on the phase of the PDO, air circulation over Alaska is enhanced either from a southerly direction (positive phase) or northerly direction (negative phase) along with a strengthening or weakening, respectively, of the aforementioned Aleutian Low. In addition to the overall PDO effect, the prevailing wind flow over coastal mountain ranges enhances the PDO pattern. A positive PDO (southerly winds) adds to precipitation on southward facing slopes but lessens it on the leeward side.

Although the PDO is more strongly linked to Alaska’s climate than to its weather (though climate and weather are certainly related), shifts in weather patterns can create striking swings in high and low temperatures across the state. For example, if an upper-level ridge and associated surface high are present across the state, the source region of the air and further modifications due to snow cover and clear skies can lead to very low temperatures, sometimes as low as -60°F. The approach of a low pressure system with stronger southerly winds, clouds, and precipitation can raise the temperatures to 40°F. As the air masses associated with these temperatures replace one another, swings of 100°F over a period of a few days are possible, especially in inland locations. The bottom line is that whatever the “average” temperature might be, extremes will dominate in this most northern state.

In fact, if one had to characterize Alaska’s weather and climate in just a few words, the words: extreme (see the table to the right) and variable (both in a spatial and a temporal sense) would dominate. Whether you prefer a wet and mild coastal environment, a dry and sunny continental climate, a windy and frigid Arctic plain, or anything in between, you’ll find it in Alaska.

Effects of Climate Change

There are many physical changes taking place in Alaska that are either a direct or indirect result of the changing and variable climate. These include, but are not limited to, glacier retreat, permafrost thaw, lake drying, greening of the Arctic, a longer growing season, sea ice reduction, and shifts in animal habitats. One of the more dramatic changes involves coastal erosion (most significant to communities on Alaska’s northwest coast). Sea ice extent in the Arctic has shown significant reductions in recent years. The year 2007 showed a record low extent in September (based on satellite data going back to 1972), with the most significant reductions in the western Arctic near Alaska and eastern Russia. Yet in March 2008, St. Paul Island remained in sea ice through the month. St. George, the southernmost Pribilof Island, was in ice for a total of 18 days during March. This might be a recent record for duration so late in the winter season.

When there is no ice at the shore to protect the coastline in fall, and a storm comes, then wind and wave action lead to significant erosion problems. Some coastal communities are facing serious survival challenges involving relocation—how to do it when there are no evacuation routes and how to pay for it. One such storm (with a central pressure of 941 millibars—27.79 inches of mercury) came barreling across the Bering Sea in October of 2004, flooding Front Street in Nome (where the Iditarod sled dog race finishes each year) and causing erosion of the unstable coastline. Sustained winds reached 80 mph (stronger than hurricane strength) and contributed to some $20 million in storm damages.

Other changes are not quite as dramatic and include changes in permafrost (permanently frozen ground) distribution and depth of the active layer (the layer of frozen soil that melts every summer). Ground temperature measurements taken from boreholes in Alaska show an overall warming of permafrost temperatures and deepening of the active layer. The reason this is of concern is that when permafrost melts, the below-surface hydrology changes. This adds soil water that is available to vegetation and changes the structural integrity of soil on which structures were built. The latter can even affect oil-drilling operations and power transmission lines.

Many of these physical changes are being studied by scientists in Alaska, the United States, and elsewhere. During the past two years, global efforts linked to the International Polar Year have been under way to research polar regions. This is the fourth extensive study period of its kind and is designed to bring together researchers studying changes and impacts taking place at high latitude areas. And Alaska, representing the only Arctic location in the United States, is a prime study site.                                

MARTHA SHULSKI is a research and service climatologist with the Alaska Climate Research Center at the Geophysical Insitute and teaches in the University of Alaska Geography Program. H. MICHAEL MOGIL is a Certified Consulting Meteorologist and science writer who lives in Naples, Florida.

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