Prepare other classification

[kuchaale]

Prepare classification according to polar-night jet oscillation events (Hitchcock and Shephard, 2013). These PJO events often are related to strong major SSWs, but sometimes they can be related also to minor warmings (Ern et al, 2016). Related to #4.

[kuchaale]

Or prepare classification defined as sudden stratospheric deceleration events (SSDs; Birner&Albers, 2017). SSDs are used instead of the conventional SSWs to better capture the mechanically forced explosive dynamics of these events.