9:15 AM | “Teleconnections” provide additional support for upcoming significant temperature pattern change
The month of December has been much warmer-than-normal in the Mid-Atlantic region (PHL +13.8°, DCA +11.3°,NYC +13.8°), but there are numerous signs that a significant pattern change is coming which will allow for much more in the way of sustained cold air outbreaks in the area. The first two signals suggesting a pattern change – stratospheric warming and a tropical disturbance known as the Madden Julian Oscillation (MJO) – have been discussed already in recent postings on our “Climate Info” page: http://www.vencoreweather.com/blog?category=Climate%20Info ). Now there are other “teleconnection” signals that also suggest a pattern change is coming to the Mid-Atlantic region for the month of January and perhaps even beyond which will bring far different weather than what we experienced during this potentially record-breaking warm month of December.
“Teleconnection” in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of miles). All teleconnection patterns are a naturally occurring aspect of our chaotic atmospheric system and can arise primarily as a reflection of internal atmospheric dynamics. Meteorologists can track changes in teleconnection patterns through index values and when important phase changes take place during the winter season, significant pattern changes are often the result.
Arctic Oscillation (AO) teleconnection pattern
The Arctic Oscillation (AO) is a large scale mode of climate variability, also referred to as the Northern Hemisphere annular mode. The AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55°N latitude. When the AO is in its positive phase, a ring of strong winds circulating around the North Pole acts to confine colder air across polar regions. This belt of winds becomes weaker and more distorted in the negative phase of the AO, which allows an easier southward penetration of colder, arctic air masses and increased storminess into the mid-latitudes.
The AO index has been largely positive during the month of December (black line) and, in fact, in recent days very largely positive. Cold air masses have indeed been generally confined to northern latitudes in recent weeks perhaps as a direct result of this atmospheric pattern. There are, however, signs that a sharp drop in the AO index is coming soon (red line) and it may stay in a negative phase for a sustained period of time. During the month of January, negative AO index values are typically correlated with high-latitude blocking and colder-than-normal conditions in the Mid-Atlantic region. In fact, the temperature composite map for the month of January depicts virtually a coast-to-coast colder-than-normal pattern in the US when the AO index is consistently in a negative phase (composite map courtesy "MAD US Weather"). The “negative AO” teleconnection pattern was a key part to the overall “2015-2016 Winter Outlook” issued by Vencore Weather in mid-October that suggested there would be multiple cold air outbreaks: (http://www.vencoreweather.com/2015-2016-winter-outlook ).
A closely-related cousin to the Arctic Oscillation is known as the North Atlantic Oscillation (NAO) and it too will be undergoing a shift although not as dramatic as with the AO. During much of December the NAO has been on the positive sign (black line), but it is forecasted to drop to neutral or perhaps even slightly on the negative side in the near term (red line). A neutral signal in January for the NAO index suggests relatively normal temperatures across much of the nation in the temperature composite map and this would be a far cry from the warmth of December (composite map courtesy "MAD US Weather"). Should the NAO index actually fall into negative territory for a sustained period of time - much like the AO index is expected to do - then the analog temperature composite map strongly suggests colder-than-normal weather for much of the nation (not shown).
Pacific/North American (PNA) teleconnection pattern
The Pacific/ North American teleconnection pattern (PNA) is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extra-tropics. The positive phase of the PNA pattern features above-average heights in the vicinity of Hawaii and over the intermountain region of North America, and below-average heights located south of the Aleutian Islands and over the southeastern United States. The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. The positive phase is associated with an enhanced East Asian jet stream and with an eastward shift in the jet exit region toward the western United States. The positive phase of the PNA pattern is associated with above-average temperatures over western Canada and the extreme western United States, and below-average temperatures across the south-central and southeastern U.S. The negative phase is associated with a westward retraction of that jet stream toward eastern Asia, blocking activity over the high latitudes of the North Pacific, and a strong split-flow configuration over the central North Pacific.
The PNA index value has generally been on the negative side of normal during recent weeks in the month of December. Similar to the AO, this index appears headed towards an important phase change (to positive) for the foreseeable future (red line). The composite map depicts the January temperature pattern in the US when the PNA is in a positive phase and near-normal to below-normal temperatures can be expected in the Mid-Atlantic region (composite map courtesy "MAD US Weather").
The combination of recent signals from the stratospheric temperature pattern over the North Pole, a tropical disturbance known as the MJO, and AO/NAO and PNA “teleconnection” patterns support the idea that January and perhaps even February are quite likely to be rather different in the Mid-Atlantic region for temperatures compared to the month of December. It appears this significant temperature pattern change will get underway in earnest as soon as we reach the beginning of the New Year. Stay tuned – we’ll continue to monitor these far-reaching signals over the next several weeks.
Meteorologist Paul Dorian