It’s midwinter at the North Pole and midsummer in the south. I want to save a record of how they compare on the weather maps, hoping to do so again in July when the seasons have reversed. The shape of the upper level air pressure configuration is the key determinant of all activity in the upper atmosphere, with an endless number of potential options, so that is the image we’ll start with. The south comes first because everything about it is nice and orderly:
Notice the compact shape of the blue zone and the tight fit of the green zone around it. This enables the formation of a single, strong jetstream wind tracking most portions of the closely aligned outer borders of the two zones:
A number of atmospheric rivers (ARs) are roaming around in the upper atmosphere, all of them having a common propensity to flow straight into the polar zone along with the precipitable water (PW) they are made of. They have a problem doing so because of the strong jet streams that are effectively blocking their way, so only a few fragments actually break through. Most of the polar zone atmosphere therefore stays relatively dry:
As long as the polar zone air stays dry most of the surface will stay cold. Everything within the light blue shading is below freezing, even though midsummer is the peak of the heating season. The cold surface helps to sustain the current shape of the upper level air pressure gradients, which may be able to stay in place about like they are now for the rest of the summer, and then start to expand outward next winter.
In the north we have a very different story. The blue and green zones of air pressure gradients are large in total area, but far from compact:
As a result the jet winds that follow the borders of each of these zones tend to be irregular in positioning and inconsistent in strength:
ARs, for the most part, are currently not very successful in their efforts to break through this complex pattern of jet winds, with one notable exception. The strongest jet stream of all, moving from south to north on the western side of the Atlantic, is not in the kind of position that normally will block AR movement. Instead, this stream is literally transporting a large AR over a very long distance, from its warm water origin as far as Scandinavia, where the combination breaks down and sends some of its remnants over the Svalbard region:
The track followed by this combination is vividly expressed on a map showing a long and unbroken stream of rainfall:
Temperatures have stayed below freezing for practically all land surfaces contained within the gradients marked off by the borders of the air-pressure green zone:
Carl