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Why Did This Winter Start So Cold ?
Our weather during the recent cold spell from roughly December 25 through January 10 reminded me of winters in Madison, Wisconsin. Quite remarkably, it was actually colder and snowier here than it was there during much of that stretch. Some of the meteorological statistics for Northeast Kansas during the first month and a half of winter 09-10 are stunning when placed in a historical perspective. According to the National Weather Service, the period form Dec 1 through January 11 ranked as the 5th coldest and 2nd snowiest on record in Kansas City. Similar rankings were noted across many locations in Northeast Kansas, including Lawrence.
I should point out that cold periods are often accompanied by snowy periods. So it’s not surprising that we observed top-ten rankings in both of those categories simultaneously. This is because a deep snowcover tends to cool the local climate, which in turn tends to favor snowfall rather than rainfall during precipitation events. Nonetheless, it’s safe to say that such unusually brutal conditions over a similar multi-week duration is not likely to happen again for a long time.
But what caused them? I’ve seen many reports that have simply attributed the nation’s lengthy chill to an unusually negative “AO”. Like El Nino, the “AO”, which stands for Arctic Oscillation, is a hemispheric-scale pattern of climate variability. When the AO index is negative, the most notable atmospheric characteristic is that the really cold air typically found at far northern latitudes has likely spilled southward, while at the same time warm air has likely surged northward toward the pole. As schematically shown in the figure below, the negative phase (on the right) is associated with a southward displacement of cold and snowy air over the central United States. The AO index measured during the last half of December and the first part of January was “off the charts” low. In fact, it was the lowest ever recorded since record keeping began in the middle of the last century. http://worldonline.media.clients.ellingtoncms.com/img/blogs/entry_img/2010/Jan/27/AO1.jpg
One problem with this line of thinking is that the atmosphere has so many moving parts that no single phenomenon ever causes another. There are always multiple intertwined reasons why we get the weather we do. A tropical cyclone near Japan one day, aside from all the other reasons, may very well have something to do with a cold spell in Kansas City the following week. Another issue I have with the notion that the AO caused our cold is that the AO is not likely a fundamental physical process by itself. It really owes its existence to changes in weather patterns that are themselves related to other physical processes. The state of the AO tells us how the atmosphere is behaving rather than telling the atmosphere how to behave.
I find it more useful to look at real physical processes that act like external forcing mechanisms to our weekly weather here in the Central Plains. Ones that are associated with changes in the oceanic and/or atmospheric conditions over Earth’s tropical regions are well known to impact weather patterns over the United States. ENSO, which stands for El-Nino/Southern Oscillation and owes its existence to coupled ocean/atmosphere interactions in the tropical Pacific, is one of them. Unlike the AO, ENSO is a mechanism that tries to drive the climate system in a particular direction. There is plenty of literature that links El Nino to enhanced probabilities of certain seasonal averages. For example, El Nino conditions often, but not always, lead to warmer than average winters over western North America and cooler than average temperatures across the eastern U.S.
Another tropical phenomenon that science has clearly identified as a culprit in changing our weather patterns is the MJO, or Madden-Julian Oscillation. As with ENSO, the MJO directly modifies the temperature distribution over the tropics at levels from the surface to 10 miles altitude. Research has clearly demonstrated that nudging the atmosphere over the tropics in the ways that the MJO and ENSO do can directly change the jet stream patterns worldwide.
In an effort to conjure up an analogy for how such remote actions can affect our local weather, imagine throwing a rock in a fast moving stream and picture the wave patterns that are generated from it. If you throw the rock at a different spot in the water, you will get a different set of waves at your location. In this example, changing the rock’s entry point is like changing the temperature patterns in the tropics, and the waves that bypass your location are like the weather systems that affect us on a weekly basis. Ping the atmosphere in one place, and you might get a warm spell. Ping it in another, and you might be shoveling snow.
In our recent cold spell, ENSO and the MJO were indeed configured in a way that favored a cold and snowy pattern for the eastern two thirds of the United States. These systems were acting to align the atmosphere in a way that allowed relatively mild air to surge toward the North Pole and frigid polar air to surge southward across the United States. I would argue that ENSO and the MJO, and their interaction with the natural evolution of the day-to-day weather systems traversing the globe, played a big role in giving us our cold blast, and that the negative AO we recorded was just a measure of how the atmosphere was driven by physical mechanisms such as these. There’s certainly more to the puzzle, but these are my top perpetrators.