Even by Mainers' standards, this winter has been a bitter one with sub-zero temperatures by night and single digits by day. Indeed, most of the eastern half of the country has been bit by the brutal cold. But what is driving the Polar Vortex south so much this year?
The Arctic Oscillation (AO) quantifies the magnitude of atmospheric pressure over the North Pole. AO has a positive phase and a negative phase:
Positive Phase (AO+)
The positive phase of AO is categorized by relatively low pressure toward the North Pole. THis lower pressure allows jet stream winds to remain strong and mostly zonal; that is, no big ridges or troughs in the jet stream, overall. This pattern, in turn, prevents frigid air from making much southward progress into the mid latitudes.
Negative Phase (AO-)
The negative phase of the AO is the exact opposite of AO+. Pressure near the North Pole tends to be relatively high with weaker jet stream winds, accompanied by large ridges and troughs in the jet stream (amplified pattern). These weaker winds allow the frigid air to penetrate southward in conjunction with troughs in the jet.
Much of this winter has been spent under AO-, resulting in a southward migration of the Polar Vortex. It just so happens that one of the troughs in the jet stream set up over the eastern half of the country, bringing the Plains and Northeast to the Gulf Coast a healthy dose of cold temperatures.
The forecast keeps AO in a negative phase for the next week or two (beyond that, the AO can be very difficult to project). However, with other oscillations in play, like the NAO (North Atlantic Oscillation), the next batch of frigid air appears as though it will primarily affect the central Plains and leave the Northeast in a sort of "transition zone," where we may get a cold day or two, followed by moderating temperatures.