Don't blame the Arctic oscillation
We've all experienced prolonged stretches of unusual weather. Multi-week cold snaps and oppressive summer heat waves are part of the natural variability in our climate system. They plague populated regions of the globe, and for now, challenge the skill of our weather prediction systems.
One of the more notable stretches of anomalous winter weather in the modern meteorological record occurred just one year ago. For much of the 2009-2010 winter, the eastern half of the lower 48 endured one of the most brutal combinations of cold and snow I've seen in my lifetime. And to a somewhat lesser degree (less snow in the mid-Atlantic), the combination has repeated itself this winter.
To explain long-lasting weather extremes like the ones we've seen the last two winters, meteorological studies often invoke a phenomenon called the Arctic Oscillation (AO), or its close relative the North Atlantic Oscillation (NAO). While there is undeniably some utility in using the AO index to conceptualize the evolution of highly complex weather regimes, and to package bizarre weather patterns into a single number, we need to use caution in doing so and not blame the AO for the unusual weather.
The AO can be thought of as a fluctuation in the shape of the global circulation. The strength of the oscillation, as defined by the AO index,measures the awkwardness of the circulation's geometry by contrasting the weather in polar regions from that observed in more temperate latitudes like ours. Often times when the AO index is negative, frigid air typically found at far northern latitudes has been displaced southward, while at the same time warm air has been moved northward toward the pole.
As schematically shown in the figure below, the negative phase (on the right) is typically associated with a southward penetration of cold and snowy air over the Central and Northeastern United States as well as in Europe. Perhaps not surprisingly, the AO index measured during much of last winter was "off the charts" low. In fact, it bottomed out at values not reached since record keeping began in the middle of the last century.
This winter, the AO plunged again. During one stretch of remarkably low AO values that began last November and continued through December and January (see image below), much of eastern half of the United States was unusually cold while much of eastern Canada was strikingly warm.
In the picture below, which is a temperature map averaged over the 3-week period from Dec 5-25 2010, areas in blue denote regions of colder than average temperatures and areas in yellow/red likewise indicate warm conditions. During that time, temperatures reached the 40s across Northern Quebec and the shores of Greenland, while the central Florida orange crop was iced over (many cities in Florida had their coldest December on record.).
With the AO now reverting to its positive phase and predicted to remain so by models through late February, temperature forecasts from these same models are simultaneously predicting both the northward penetration of subtropical air into the Lower 48 and the retreat of cold air back to the Arctic.
Of course they are. In this example of circular reasoning, the positive AO and the temperature pattern are manifestations of each other. But the fact that they are identified with one another does not imply causality.
This is because the AO really isn't a mechanism that brings us these weird weather patterns. It's a symptom of them. Common wisdom has latched on to the idea that the AO is a dynamical structure- one that moves around the world and occasionally creates meteorological havoc
by producing lengthy spells of uncommon weather. It is not.
Rather, the AO is a response to, or a byproduct of, the interaction among a group of fundamental atmospheric mechanisms. On more technical terms, it can be understood without appealing to fluid dynamics, and it is not a solution to any governing mathematical equation.
The real question is what causes the atmosphere to slosh back and forth in this way that removes cold air from the poles. No one knows for sure. But when weather systems capable of long-range air displacements eject tropical warmth into polar territory and dislodge cold air from it, the effects tend to last more than a few days.
It often takes the atmosphere a while to "fix" itself because the typically observed westerly jet stream is reversed in this situation and instead blows from the east. As smaller weather systems pass through this deformed jet stream, they often just reinforce the flipped pattern.
Scientists are trying to better understand the complicated processes that create these unusual, long-lasting flow geometries, otherwise known as AO phases.
Some believe that the forcing comes from tropical weather phenomena, like El Nino or the Madden-Julian Oscillation. Matt Rogers' recently wrote about a mechanism proposed by researcher Judah Cohen linked to fall snow cover over Siberia. Andrew Freedman has written about possible connections to Arctic ice extent and global warming. Others believe the cause can be linked to the natural life cycle of really big storm systems, such as the blizzard that parts of the Northeast experienced the day after Christmas. Maybe it's all of them and while it is true that the AO index is a very useful statistical concept that gives a name to uncommon weather regimes, it does not explain how or why we get them.
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