The tercile-category anomaly of the lower-stratospheric polar vortex, defined by the 100 hPa 60°N zonal-mean zonal wind, has been used as a diagnostic for the behaviour in the ‘coupling layer’ between the stratosphere and troposphere, i.e. the level in the stratosphere where circulation anomalies are important for influencing tropospheric weather regimes (e.g. Charlton-Perez et al. 2018, Lee et al. 2019). The chart below shows the percent of GEFS members in each tercile anomaly category, based on daily 1979-2019 ERA5 climatology.
Weak = lower tercile; neutral = middle tercile; strong = upper tercile. The letter and number at the top of each bar indicates the category with the greatest ensemble support and the percent of members in that category.
Update 29 Sep: fixed a bug which was causing incorrect tercile assignment following the new GEFSv12 implementation – apologies. U100 was in fact much weaker than this chart had previously been suggesting!