WeatherBlog 10 2019/20 | Is this climate change now?

The basics

In winter, the polar vortex forms over the polar region. The polar vortex is a thermal low pressure area. In contrast to the dynamic lows that travel in our latitudes, the polar vortex is formed purely thermally, i.e. by the temperature. This drops rapidly at the North Pole in winter due to the lack of solar radiation. Cold air is denser and therefore heavier than warm air and sinks downwards. In winter, a lot of air flows from higher layers towards the ground at the North Pole. This causes the air pressure on the ground to rise, while it sinks at altitude. This creates an altitude low, also known as a polar vortex.

The polar vortex, like all other lows, rotates anti-clockwise in the northern hemisphere. At the transition between the cold, polar air masses in the north and the milder air masses in the mid-latitudes, the strong pressure differences create a strong high-altitude current, the so-called jet stream.

Over the Arctic, the polar vortex often has a structure with two or more centers that waft around in a loose network due to the uneven land/water distribution. There is often one center over Baffin Island and one over northeast Siberia. The closer the pressure centers are to each other, or the rounder the polar vortex is, the straighter or more zonal the west-east flow is. If the centers are further apart or irregularly shifted, pronounced waves form in the jet stream. Instead of running directly from west to east, the upper-level current meanders and loops - this is also referred to as a meridional flow.

The latter situation often involves blocking layers, in which a wedge or trough settles somewhere more or less stationary and does not move away so quickly because the driving west-east current is missing.

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500hPa geopotential and ground pressure, exemplary map for Tue. next week. Current still predominantly zonal, frontal zone over ME a little further south again. — 500hPa geopotential and ground pressure, exemplary map for Tue. next week. Current still predominantly zonal, frontal zone over ME again somewhat further south.

What is changing?

With rising global temperatures, the sea ice in the Arctic Ocean is changing. There is less and less perennial ice that survives the summer and it often takes a long time in the fall, depending on the region, for the seasonal sea ice cover to form. "Sea ice present - yes or no?" This is a question that has a very, very strong influence on the local weather and climate of the coastal areas concerned and is also extremely important on a regional and ultimately global level. If seawater is not frozen, it has a temperature of at least just above 0°C. It simply won't get any colder. It simply doesn't get any colder. But when the water turns to ice, the temperature can suddenly drop much lower.

If the sea does not freeze at all, later than usual, or not everywhere, the large-scale temperature gradient between the mid-latitudes and the Arctic, which is warmer overall, decreases on the one hand, and on the other, new temperature patterns emerge: where there is water, it stays warmer. As before, the land masses and sea ice areas are comparatively much colder in winter.

If it no longer becomes uniformly cold everywhere where the polar vortex is located in winter, the conclusion is that the polar vortex could also develop new patterns and that it becomes weaker overall due to the lower overall temperature gradients between mid-latitudes and the polar region.

There is now relatively strong evidence that the position of the usual centers of the polar vortex is shifting (see, for example, here - if you can't get behind the paywall, visit the SciHub platform, which is wrong from a legal perspective but fundamentally correct from a moral perspective) and that the jet is meandering more on average. The latter would - in a further, obvious conclusion - lead to more frequent blocking situations, however pronounced they may be exactly.

So is climate change to blame for the current weather?

Summary: This is not so easy to answer with yes or no and if anyone does, it should be taken with a grain of salt. Weather events can be symptomatic of long-term changes in the climate system, but by no means have to be.

Also:

Not every blocking situation from now on is caused by climate change! Climate change causes a shift in the statistical probabilities of occurrence of certain events, or makes them stronger, it does not fundamentally reinvent events. It is still extremely difficult and often simply not possible to attribute certain weather events directly and causally to climate change. Investigations in this direction are becoming better and more detailed, but this is still a very complex topic and the subject of current research.
Complicated questions rarely have simple answers. It is quite reasonable to speculate how the polar vortex, the position of the winter polar front and the wobble of the jet stream will develop in the long term. However, recognizing the complexity of the interrelationships and the various uncertainties seems important if we want to meet the challenge of climate change with any kind of solution. This applies to this specific issue and in general.
The current situation is not a blocking situation in the sense of a strongly meandering jet stream. On the contrary, the flow is actually quite zonal. The fact that the polar front is currently comparatively far north does not mean that this will always be the case from now on. Nor is it like this everywhere in the world at the moment.

This brings us to:

Some fresh snow from the last small cold front. Glistens nicely but not much underneath...

LH

Current situation and outlook

Large-scale, low pressure development in the NW Atlantic is still being fueled by the inflow of cold polar air in the Davis Strait. The westerly drift over the Atlantic is correspondingly strong. We won't feel the "westerly weather" in the mild, damp mountain winter version because the frontal zone is too far to the north. Meanwhile, the stormy mountain winter with warm slushy weather at lower altitudes is taking place in Scandinavia.

The Alpine region lies to the south of the main event in a high-pressure bridge that stretches between the northern frontal zone and a cut-off low over Spain (which causes a lot of fresh snow and strong storms there). The low pressure is shoveling in a little more humid air in the southwestern Alps and the sunshine there is a little clouded, everywhere else it is brightly sunny and mild at altitude, while the now familiar inversion hangs in the valleys. The city of Innsbruck is already asking its residents to leave their cars at home, as the fine dust pollution in the inversion is now very high.

The low pressure system over Spain is expected to move a little to the north on Friday and then become part of the frontal zone. At the weekend, it will move across the Alps in this form and provide clouds and a few raindrops or snowflakes here and there. Heavy precipitation is not expected.

Far away in the crystal ball, there are currently one or two ideas of turning the anticyclonic situation into a similar, but in this country more cyclonic version (mildly wet instead of mildly dry), but this is so far away that there is no real need to wait for it at the moment.

Finally, a recommended read:Interview with Marc Olefs from ZAMG (who is also primarily responsible for the beautiful SNOWGRID maps) on the future of winters in the Alps.