Skill of the MJO and Northern Hemisphere Blocking in GEFS Medium-Range Reforecasts

Forecast characteristics of Northern Hemisphere atmospheric blocking and the MJO were diagnosed using an extensive time series (Dec-Jan-Feb 1985-2012) of daily medium-range ensemble reforecasts based on a version of the NCEP Global Ensemble Forecast System (GEFS).

For blocking : (a) inter-annual variability of analyzed blocking frequency was quite large; (b) the GEFS slightly under-forecasted blocking frequency at longer leads in the Euro-Atlantic sector; (c) predictive skill of actual blocking was substantially smaller than its perfect-model skill; (d) block onset and cessation were forecast less well than overall blocking frequency; (e) there was substantial variability of blocking skill between half-decadal periods; and (f) the reliability of probabilistic blocking forecasts degraded with increasing lead time.

For the MJO: (a) forecasts of strong Indian Ocean MJOs propagated too slowly, especially the component associated with outgoing longwave radiation (OLR), i.e., convection; (b) tropical precipitation was greatly over-forecast at early lead times; (c) the ensemble predictions were biased and/or under-dispersive, manifested in U-shaped rank histograms of MJO indices. Magnitude forecasts were especially U-shaped. (d) MJO correlation skill was larger for its wind than for its OLR component, and was larger for the higher-amplitude MJO events; (e) there was some half-decadal variability in skill; (f) probabilistic skill of the MJO forecast was modest, and skill was larger when measured relative to climatology than when measured relative to a lagged persistence forecast.

For longer-lead forecasts, the GEFS demonstrated little ability to replicate the changes in blocking frequency due to a strong MJO that were noted in analyzed data.