A shift could be underway by the last week to ten days of November as both the NAO and AO look headed for a dip.
Low pressure over the pole results in a strengthening of the zonal flow (westerlies) usually meaning milder conditions for the mid-latitudes. Conversely, when the Arctic Oscillation turns negative high pressure forms above the poles (blocking) and typically you’ll see a weakening of the zonal flow and instead draw cold air out of the pole and down into mid-altitudes.
Looking at the chart below, you’ll see we’ve had a prolonged run of positive NAO and AO this Summer and Autumn to date, but things look about to change.
The ensembles are very scattered, but there are certainly more predicting a fall come the latter half of November. Once you get the NAO and AO turning negative, cold weather descending into the mid-latitudes becomes extremely likely.
Note the dips during late October and recall the corresponding cold spell Europe experienced, when heavy snow fell in all four nations of the UK along with record early accumulations in the Alps resulting in the slopes opening a month ahead of schedule.
If you see a dip in the NAO or AO during the winter months then you’re probably in for a tough time of it. And when you couple that with solar activity falling off a cliff and you have the potential for a extremely frigid NH winter.
Get the feeling *something* could be starting to happen for the last week to ten days of November.
After the northerly/Greenland High on the 00z 06z builds a monster high pressure of 1050mbrs towards northern Russia around 300hrs. pic.twitter.com/fkczIXZZqQ
— GavsWeatherVids (@GavinPartridge) November 7, 2018
Climate Variability: Arctic Oscillation
The Arctic Oscillation (AO) refers to an atmospheric circulation pattern over the mid-to-high latitudes of the Northern Hemisphere. The most obvious reflection of the phase of this oscillation is the north-to-south location of the storm-steering, mid-latitude jet stream. Thus, the AO can have a strong influence on weather and climate in major population centers in North America, Europe, and Asia, especially during winter.
The AO’s positive phase is characterized by lower-than-average air pressure over the Arctic paired with higher-than-average pressure over the northern Pacific and Atlantic Oceans. The jet stream is farther north than average under these conditions, and storms can be shifted northward of their usual paths. Thus, the mid-latitudes of North America, Europe, Siberia, and East Asia generally see fewer cold air outbreaks than usual during the positive phase of the AO.
Conversely, AO’s negative phase has higher-than-average air pressure over the Arctic region and lower-than-average pressure over the northern Pacific and Atlantic Oceans. The jet stream shifts toward the equator under these conditions, so the globe-encircling river of air is south of its average position. Consequently, locations in the mid-latitudes are more likely to experience outbreaks of frigid, polar air during winters when the AO is negative.
More on that here.
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