Cool Temps for the East: Teleconnection Influences

     Despite the official meteorological start of Autumn still being a few days away, the majority of the East Coast has already been feeling the cool temperatures this week. The passage of two strong cold fronts left overnight temperatures in the mid to upper 30s on Tuesday evening for much of the Great Lakes region. In Northern Minnesota, overnight temperatures dropped below 30 degrees, prompting freeze warnings for much of the area. It's quite an amazing flip from the unbearable heat we all experience this summer. Now you might be wondering what is causing this swift change in weather; the answer is lies within shifts in teleconnection patterns

     A teleconnection is a statistical linkage between climate anomalies over a large distance. These linkages interact with each other in various ways, shifting between positive and negative phases, and ultimately affecting weather and climate patterns for certain parts of the world. Phase shifts for most of these teleconnection patterns can be observed in changing sea surface temperatures for lengths of time. The two teleconnection patterns we will be focusing on with regards to the Eastern U.S. cool down will be the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO).


The North Atlantic Oscillation


     The NAO is a fluctuation in differences between the semi-permanent pressure features in the Atlantic known as Azores High and the Icelandic Low. In the positive phase of the NAO, the two pressures strengthen, increasing the strength of the jet stream. The strong jet stream prevents cold air from the Arctic intruding southward into lower latitudes, keeping the Eastern U.S. warm and wet. In the negative phase, the Azores High and Icelandic Low weaken, in turn weakening the jet stream and increasing the amplitude of troughs and ridges. The increased meridional flow can promote a blocking pattern, keeping a trough located over the eastern half of the country.

     We are currently in a negative phase of the NAO, resulting in the intrusion of Arctic air into the Great Lakes and Northeast regions of the United States. This negative phase has also resulted in a well-established a blocking pattern in the jet stream, keeping this cold air in the region. Below is a look at the current 500 millibar heights showing this pattern:

The Arctic Oscillation



     The AO is related to the pressure differences in the Arctic and midlatitudes. During the positive phase (pictured on the left in the above graphic), strong low pressure in the polar region strengthens the east-west flow of the jet stream, locking cold air in the Arctic regions. When the AO shifts into a negative phase, zonal flow of the jet stream weakens and allows for cold air to move into the midlatitude regions.

    Above is the current AO index. Currently, the AO is also in a negative phase, indicating strong polar air infiltration into the U.S. This, in combination with the negative NAO phase has resulted in the prolonged below normal temperatures for the Great Lakes, Upper Midwest, and Northeast. The blocking pattern appears to persist for the next week, bringing waves of Arctic air into these regions. Below is the five day temperature anomaly including this coming weekend.





However, forecast models are indicating a shift in these indices, leading to the return of slightly warmer air in the later half of the 0-16 forecast period. Below is showing the five day temperature anomaly two weeks out.



Graphics courtesy of WeatherBELL Models, the Climate Prediction Center, and NSIDC.