DISCUSSION: For much of the US east of the Rocky Mountain Front Range, the winter of 2019-2020 has not shown much in the way of persistent chilly Arctic air infiltration. Instead, many areas east of the Rockies have witnessed weather patterns found with large-scale ridging (general poleward motion of air associated with high-pressure systems) while the Intermountain West and Pacific coast have remained fairly cool under the influence of large-scale troughing. So what is a potential cause for the unusually warm winter season? One hypothesized answer lies in the unfavorable combination of teleconnections that are currently being observed. An article previously published here goes into more detail on how these teleconnections work so for brevity, the teleconnection patterns responsible will be mentioned in the light of influencing multi-day/multi-week weather patterns as of late.
Now, all teleconnections are important in some degree since they help modulate, or control, the flow patterns across the world. In other words, a certain pattern that favors milder, more stable weather patterns over one part of the world may be countered by cooler, more progressive (stormier) weather patterns downstream, and several of these large-scale patterns are observed at the same time at different parts of the world. But, these teleconnections can undergo changes throughout the course of a year, more so on an inter-monthly time frame. Therefore, the essence of understanding large-scale patterns is to chain information from multiple teleconnections in order to gain a clearer depiction of what is occurring. The Arctic Oscillation, or AO for short (pictured above), has remained predominantly positive. Recall that a +AO pattern yields a tendency to retain colder Arctic air north of the midlatitudes while keeping a zonal flow pattern. In essence, there is limited to no buckling in the mean flow that will help displace cold Arctic air equatorward. The North Atlantic Oscillation (NAO), which corresponds to the phenomena located between Icelandic Low and Azores High, shows as being predominantly positive as well. The East Pacific Oscillation (EPO), after being mostly positive through much of winter, is trending towards neutral and negative values. Normally, the EPO can play a key role in modulating the flow pattern across North America such that -EPO leads to large-scale ridging over Alaska and the Pacific Northwest with subsequent troughing over the northern half of the US. However, this is countered by a negative Pacific/North American (PNA), which encourages advection of polar air over the Pacific coast and intermountain west with subsequent large-scale ridging over the eastern two-thirds of the US. Storm tracks are also favored out west due to the strong grip of a typically dominant high-pressure system over the mid-latitude Pacific Ocean. When putting all of the pieces together, the result favors colder, more progressive patterns to the west and milder, subtler conditions over the east. To see all of the teleconnections described in the article, be sure to visit the Climate Prediction Center’s teleconnection page located here. The science behind the teleconnections and is still a dominant topic in the world of climate science as researchers continue to investigate the impact that different teleconnections have as a unit. There is much to learn about these teleconnections and the interplay between them as certain patterns, such as that of recent, are not always an end-all for distinct weather patterns over the US and the rest of the world. So next time that a weather report indicates longer periods of mild and stable weather or colder and stormier weather patterns, teleconnections may provide clues into the true state of the atmosphere. To find more articles on interesting winter weather topics, be sure to click here! © 2020 Meteorologist Brian Matilla
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