When the Deep Freeze Warms Up

People in Fairbanks enjoyed a relatively warm December again this year, with a record high of 40 degrees Fahrenheit on December 16. In effect, this heat wave arrived as the baggage of a meteorological tourist, a mass of subtropical air that unexpectedly blew into interior Alaska.

A wind from the south sounds warm by itself, but its heating effect on the Interior is enhanced by the Alaska Range. Air that flows from the south up over the mountain peaks dries out and is compressed as it sweeps down their north sides. When air is compressed, it heats up. That's why refrigerator coils are hot and why warm winds, or chinooks, are produced by the snow-capped Alaska Range.

On a grand scale or local one, warm winds in December are not what one associates with interior Alaska. How these winds originate requires some explanation of global weather patterns. Then we can ask what was unusual in those patterns during our comparatively torrid month.

Over the north polar region lies the coldest air in the northern hemisphere; it's a region of low pressure. Along the tropical belt lies a ring of high-pressure, high-temperature air. The earth's rotation, plus the thermal contrast between pole and equator, powers the westerly jet stream at mid-latitude.

Meanders get built into the jet stream because of the earth's topographic differences and the differing thermal properties of ocean and land. Generally, the jet wanders north over oceans and drifts south over continents. Thus, the climate along western coasts in the northern hemisphere is generally warmer than along eastern coasts, even in the high latitudes. Scandinavia enjoys a more moderate climate than does eastern Siberia, and Alaska does better than Greenland, despite their comparable latitudes.

Technically, the upper-air jet is pushing a warm air mass northward at the west coast, creating a stationary ridge of high pressure. We may not be able to see it, but it is as substantial as a rocky ridge for deflecting moving air. At the same time, the jet pushes a mass of chilly arctic air south along the east coast, creating a stationary trough of low pressure. Usually, the jet stream begins crossing North America, blowing west to east, fairly close to the U.S.-Canada border.

In mid-December, the westerly jet ran into an obstacle. A large cell of high-pressure air hovered near Seattle. The normal high-pressure ridge on the west coast had become, so to speak, a real high-pressure mountain range. The obstructing air mass was stable and stubborn. It deflected the jet stream offshore and northward, right over Alaska. While the Interior enjoyed the influx of subtropical warm air, the stream blew on into the Yukon, picking up cold air to threaten midwestern states and even mid-Atlantic regions with a very white Christmas indeed.

This relatively unusual high-pressure air mass is called a blocking high because of its effect on the westerly jet. (Rigorously defined, a blocking high requires the westerly jet to be split into northern and southern branches surrounding the high pressure cell.)

The new high-temperature record in Fairbanks can be credited to the presence of the blocking high lying off Seattle, but we cannot yet explain confidently why it built up when it did. Theories involving atmospheric wave dynamics or resonant mechanisms abound, but we don't yet have enough information to sort out which is the best. Meteorologists have been arguing the point since the 1950s, and the end is not yet in sight.

Whatever the real mechanism may be, sometimes the blocking high does form, and it does shove the westerly jet into a meander that brings a warm air mass north to surprise Fairbanks.

It may also confuse our ability to detect signs of global warming. It has been hard not to associate Alaska's significantly warmer winters over the past decade with the carbon-dioxide propelled greenhouse effect, but---in part because we have seen greater change in winter than in summer---it may be that a bloom of blocking highs has changed our average temperature.

Of course, some clever meteorologist may yet find a way to link increased carbon dioxide with the formation of blocking highs. In this field of science, one of the greatest sources of heat may be the heated disagreements among colleagues!