The winter was mild across much of Eurasia and North America.
Meanwhile, the cold air that remained in the polar circle helped the Arctic Sea to reach its largest February ice cover in the past 11 years.
Sea ice analysis indicated that during this rather mild winter in the middle latitudes, trapped colder air in the polar circle had a positive impact on the rapid growth and recovery of the polar ice cap, Severe Weather Europe meteorologists said. The Arctic ocean freezes during winter, reaching a maximum sea ice extent around March.
In two animations produced by SWE, the rapid growth of ice cap can be observed, as well as the ice getting thicker over time.
The second animation there is a Arctic rapid sea ice growth between September 2019 and February 2020:
Here the polar sea ice extent diagramm
The Polar vortex and the Arctic Oscillation
The Arctic Oscillation (AO) is an atmospheris circulation pattern characterized by winds circulating counterclockwise around the Arctic at around 55°N latitude.
The most obvious reflection of the phase of this oscillation is the north-to-south location of the storm-steering, mid-latitude jet stream.
Positive Arctic Oscillation
When the AO is ‘positive’, a ring of strong winds circulating around the North Pole acts to confine colder air across polar regions.
During a positive AO, lower-than-average air pressure over the Arctic are paired with higher-than-average pressure over the northern Pacific and Atlantic Oceans.
The jet stream is farther north than average under these conditions.
Thus, the mid-latitudes of North America, Europe, Siberia, and East Asia generally see fewer cold air outbreaks than usual.
Negative Arctic Oscillation
In its negative phase, the AO is weaker and more distorted, allowing southward penetration of colder, arctic airmasses and increased storminess into the mid-latitudes.
In this case, the AO 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, mid-latitudes are more likely to experience outbreaks of frigid, polar air during winters.
Positive Arctic Oscillation Right Now!
We are currently in a positive state, with a very strong jet stream, farther north than average and cold air concentrated in the polar regions.
That ‘positive’ Arctic Oscillation is responsible for:
- the mild winter in the United States and parts of Europe,
- turning the North Atlantic into a virtual bomb cyclone express lane,
- the ice recovery currently taking place around the North Pole.
Meanwhile, on the other side of the world, in Antarctica, scientists have recorded the warmest temperature ever measured on the continent. But again it’s summer over there!