There has always been a lot of debate about the effects of Arctic sea ice lack on the course of the winter season. Many studies have been carried out and all point in a similar direction: the sea ice deficit affects the jet stream (and vice versa, due to feedback mechanisms).
The jet stream, for those who do not remember what it is, it is a powerful flow of air that flows at about 8-11 km of altitude. It flows west to east around the entire hemisphere, affecting the distribution of high and low pressures and thereby also affecting ground-level weather conditions.
The movement from west to east is called “zonal flow”, if from north to south “northern flow”, vice versa “southern flow”. The jet stream is fueled by the temperature difference between the cold Arctic and warmer southern / tropical regions. As the Arctic warms, the north-south thermal gradient decreases and the jet stream loses strength.
The general idea behind melting sea ice is that freezing ice-free water can get warmer when exposed to the sun and temperatures that are consistently above normal.. Conditions that, for the uninitiated, we are observing quite often this year. It means Arctic temperatures are rising, potentially reducing the Arctic-Tropic temperature gradient thereby weakening the jet stream.
If the jet stream is weak it means that it can be easily interrupted and meridian exchanges can become more frequent. This can cause more dynamic and extreme weather conditions, this is because colder air can move more freely from north to temperate latitudes while warm air follows the reverse path.
But when winter comes, we also have other factors that mask or reverse the potential effects of sea ice melting. The first factor is that during the winter, of course, the Arctic sea ice reforms by limiting the exposure of the waters to the sun and higher temperatures. The second factor is the stratospheric polar vortex: a strong polar vortex can affect the jet stream and potentially make it stronger, bringing theoretically milder winters between North America and Europe. Then there would be a third fact, namely the influence of the ENSO.
So we can know for sure that the current lack of sea ice will have an effect to some degree, but it is difficult to say exactly how and how much. Even a small change or shift from current conditions can have a cascading effect throughout the winter, also known as the butterfly effect. Sometimes it’s the small changes like this that can cause a weather season to be different than expected, defying seasonal forecasts.
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