As we officially enter "meteorological winter" (December, January, and February)...something you may hear us refer to from time to time is an index called the North Atlantic Oscillation, or the NAO. It is one of several different climate "signals" that can strongly influence our weather during the winter months. But what exactly is it, and what does it mean in terms of sensible weather? The following has been paraphrased from "Northeast Snowstorms" by Paul J. Kocin and Louis W. Uccellini:
According to research conducted in the late 1990s, there are several climatic "signals" that strongly influence North American climate. One of these signals is the El Nino Southern Oscillation, or ENSO. A second signal is the North Atlantic Oscillation, or NAO. The NAO is a measure of pressure anomalies across the Atlantic Ocean, measured between the Azores and Iceland, that represent circulation changes over the Atlantic and surrounding land areas. The positive phase of the NAO is accompanied by lower pressures over the Arctic and higher pressures over much of the Atlantic, resulting in stronger than normal westerlies over the Atlantic into Europe with southerly flow common across the eastern United States, a pattern that would allow relatively mild conditions to occur. The negative mode of the NAO is accompanied by an anomalously weak subtropical high and Icelandic low, with the Icelandic low displaced to near Newfoundland or eastern Canada. Winters are comparatively mild in Greenland and colder in the northeastern United States.
The value of the NAO is calculated by the Climate Prediction Center (CPC) on a daily basis. While an NAO that averages in the negative phase over the course of December, January, and February typically correlates with a cold northeastern winter, the daily trends indicate that as a negative NAO pattern weakens or shifts, a storm system nearing the southeast U.S. coast has a better chance of moving up the east coast as a major snowstorm, rather than eastward over the Atlantic Ocean. This offers a strong suggestion that the onset of negative NAO values, whether it is expected to last for days, weeks, or months, may be indicative of a cold period for the Northeast and an increased potential for significant Northeast snowstorms.
Feel free to email us at stormtrackerweather@wbngtv if you have any questions regarding the NAO! It will be interesting to pay attention to this index in the months ahead.