Birner and Albers (2017) define a sudden stratospheric deceleration (SSD) to occur when the (de-seasonalised) 10 hPa 60°N zonal-mean zonal wind* undergoes a deceleration exceeding 20 m/s over 10 days (i.e. 2 m/s per day for 10 days). The analysis here is quite noisy, especially in the mesosphere, because it is simply the day-on-day change with no smoothing. As it is not de-seasonalised, a component of the tendency (particularly in autumn and spring) will arise from the seasonal cycle rather than dynamic wave driving.
*the study actually uses 45-75°N average winds but notes the results are “virtually the same” for 60°N.