This is MetO blog on the 2016 one

Professor Adam Scaife, Head of Monthly to Decadal Prediction explains:
“Sudden stratospheric warming events occur high up in the atmosphere and
involve a complete reversal of the high altitude polar jet stream – they
can even affect weather at the surface, and for the UK a sudden
stratospheric warming increases the risk of wintry weather.”

The phenomenon begins with a wave-like disturbance which travels up into
the high-altitude jet stream. Scaife said: “This disturbance can grow to
a point where it turns over and breaks, just like a wave on a beach.”

Normally the jet stream flows from west to east with some north and
south oscillation, but the force from this high altitude disturbance
pushes against the jet stream until the winds actually reverse and flow
from east to west instead. Air then falls into the Arctic and is
compressed so that it starts to warm: the temperature can rise by as
much as 50C in just a few days.

Professor Scaife added: “This reversal of high altitude winds can also
burrow down into the lower stratosphere. Once it is within reach of
weather systems in the lower atmosphere the Atlantic jet stream often
weakens and moves south. This allows cold air from the east into
northern Europe and the UK.”

https://blog.metoffice.gov.uk/2016/0...art-to-spring/