Cause for Recent Cold Snap
Arctic Oscillation Keeps Us in the Ice Box
2010 has gotten off to a frigid start....with 8 consecutive days and counting of freezing temperatures, the 4th longest streak on record in Paducah. In fact, temperatures over most of this fall and winter have been running below average, which begs the question "what's causing this?". You may remember back in November, I reported that a moderate El Nino phase was underway, where warm water in the Pacific Ocean travels farther east than usual. In most cases, this results in our area experiencing wintertime temperatures right around average, about 41 or 42 for early to mid January, a far cry from recent readings. So what else is going on? A similar climate variable called the Arctic Oscillation is to blame. The Arctic Oscillation (AO) is a natural pattern of climate variability. It consists of opposing patterns of atmospheric pressure between the polar regions and middle latitudes (where we are). The positive phase (warm phase) of the AO exists when pressures are lower than normal over the Arctic, and higher than normal in middle latitudes. In the negative phase (cold phase), the opposite is true; pressures are higher than normal over the Arctic and lower than normal in middle latitudes. The negative and positive phases of the AO set up opposing temperature patterns.
When the AO goes into a negative phase, higher pressures over the Arctic weaken westerly winds over the mid latitudes, allowing frigid air to plunge much farther south than normal. This winter, the AO has tanked into one of its strongest negative phases in decades, leaving much of the U.S. shivering. But the other side of the coin is that during a negative AO, temperatures over the Arctic are much warmer than normal...and that is the case this winter as well. The image below illustrates that, with temps over the Arctic and Alaska way above normal, and most of the U.S. below average.
Unfortunately, long-range forecast models show this trend continuing through the winter. Not good news for folks looking for warmer weather!