GEFS tercile probabilities
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 stacked bar chart shows the percent of GEFS members in each tercile anomaly category, based on daily 1979-2021 ERA5 climatology, where: 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.
GEFS 35-day timeseries
Note that this is initialised the preceding day to the bar chart above, due to the time taken for the data to appear on the NCEP servers. No mean-state bias correction is applied, which may influence the interpretation with respect to ERA5 at longer lead-times.
GEFS 35-day ensemble mean forecast evolution