W EUROPE: Atlantic Still Rules / Explanation About The Stratosphere & Role In Cold!

Polar vortex, the stratosphere and wave breaking (very briefly):Something you will see me mention many times over the course of the next few months is wave 1 and wave 2 activity. The following should help explain what is meant by these terms:…

Before we get into this, I will briefly explain the role of the stratosphere on the troposphere below it. Each winter the stratosphere gets very cold. The difference in temperatures between the arctic and warmer latitudes further south results in the formation of a strong area of low pressure, called the stratospheric polar vortex. Below this is the tropospheric polar vortex (the area of low pressure located around the north pole that we see on weather charts). The colder the stratosphere is, the tighter/stronger the stratospheric vortex becomes and consequently, the tropospheric vortex too. The strength/position of the tropospheric vortex influences the AO (Arctic Oscillation), which is a measure of pressure between the north pole and the Azores, as shown in attached graphic, courtesy of NASA.
A stronger vortex can result in a positive Arctic Oscillation which, for Europe, equates to a less cold winter. The opposite is the case with a weaker vortex.

Sometimes, during winter, a strong vortex is put under pressure, warming it and displacing cold air from the pole down to the mid latitudes. Rossby/planetary waves circumnavigating the globe, and during winter, when a large wave encounters a mountain range, if the wave is large enough, some energy is deflected poleward (mountain torque event). It needs to be a sizeable deflection to achieve this, but these waves can penetrate into the stratosphere, creating a warming disruption to the otherwise usually cold and stable wintertime stratospheric environment. There are two main types of disruption to the vortex via this process; a displaced vortex, where wave breaking and consequent warming moves the core of the vortex away from the pole; or a split vortex where the vortex is put under even more pressure and is split in two. These events are often referred to as sudden stratospheric warmings (SSW) where the zonal winds at 60N/10HPA are reversed from westerly to easterly. In both cases, the warming and movement of the vortex, pushes cold air into the middle latitudes as higher pressure builds over more northern latitudes. Generally speaking the number of the wave (1,2,3) refers to the number of waves at that time. Wave 1 usually displaces, and strong wave 2 can cause splits. Following a displacement, if wave activity subsides, it is common for the vortex to fairly rapidly regroup and cool. Following a split, if wave activity wanes, it can take much longer for the vortex to recover and regroup. So we want strong wave 2.
So far this Autumn, we have seen wave 1 activity creating some warming and consequent weakening of the vortex. This coming at a time when the vortex is usually cooling dramatically is great news and in stark contrast to what we saw last year. (the QBO another important factor here but i wont go into that now).
Going into December, i hope to see strong wave 2 activity (currently weak) which should continue to disrupt the vortex, ultimately resulting in a SSW roughly mid winter.