Now there is no longer any doubt: the 2020/2021 winter season will be influenced by La Nina, for better or for worse. We are talking about a phenomenon that will modify the jet stream over North America and the Pacific Ocean, extending the climatic influence on a planetary level. All major seasonal models, at this time, see a heavily altered jet stream.
That said, we must remember that when it comes to seasonal projections there is no “magic wand”. The global weather system is extremely complex, a perfect live simulation of chaos theory, with many large-scale and small-scale climate factors. Therefore, limiting oneself to the analysis of a single event, even though it has an impact like the Nina, can be misleading.
ENSO, for those who are not aware of it, has an important impact on tropical convection models and on the complex interaction of the ocean-atmosphere system. We usually observe a shift, on a global scale, in the distribution of High and Low Pressures. Historically, the most typical effect of La Nina is a strong anticyclonic blockade system in the North Pacific.
After passing Canada and the United States, the jet stream moves into the Atlantic. There are several paths it can take: a lot depends on the Arctic oscillation pattern and the distribution of high and low pressures in the Atlantic. This is where La Nina likely loses its direct influence, which is when the Atlantic takes over. However, some influence remains, often with a change in the positioning of the jet stream. The problem is that the end result is much more unpredictable in the Euro-Atlantic area than in North America (which has a more direct influence).
We have carefully analyzed two of the most important seasonal models, namely the European ECMWF and the American CFSv2. We can give you a summary, or rather, an average of the two projections. It could be argued that Europe could experience higher than normal temperatures. This does not mean that there will be no cold raids and harsh winter days, but that certain episodes will be less frequent.
The huge modeling discrepancy to date lies in establishing the influence of oceanic high pressure. Not a small discrepancy, given that if it were to frequently go north it could give rise to massive Arctic irruptions. On the contrary, were to remain stretched on parallels or stretch over Europe, the perturbations would be deflected northwards and at that point conditions of atmospheric stability would dominate with all the consequences of the case.
To conclude, we must not forget, but we had already talked about it, that this year there could be the La Nina-QBO + pairing, or a pairing that in the past gave birth to winter seasons that were anything but mild …
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