Skill of the MJO and Northern Hemisphere blocking in GEFS medium-range reforecasts

Abstract

Forecast characteristics of Northern Hemisphere atmospheric blocking and the Madden-Julian oscillation (MJO) were diagnosed using an extensive time series (December-February 1985-2012) of daily mediumrange ensemble reforecasts based on a version of the NCEP Global Ensemble Forecast System (GEFS) For blocking, (i) interannual variability of analyzed blocking frequency was quite large, (ii) theGEFS slightly underforecasted blocking frequency at longer leads in the Euro-Atlantic sector, (iii) predictive skill of actual blocking was substantially smaller than its perfect-model skill, (iv) block onset and cessation were forecast less well than overall blocking frequency, (v) there was substantial variability of blocking skill between half-decadal periods, and (vi) the reliability of probabilistic blocking forecasts degraded with increasing lead time For the MJO, (i) forecasts of strong Indian Ocean MJOs propagated too slowly, especially the component associated with outgoing longwave radiation (OLR), that is, convection; (ii) tropical precipitation was greatly overforecast at early lead times; (iii) the ensemble predictions were biased and/or underdispersive, manifested in U-shaped rank histograms of MJO indices (magnitude forecasts were especially U shaped); (iv) MJO correlation skill was larger for its wind than for its OLR component, and was larger for the higheramplitude MJO events; (v) there was some half-decadal variability in skill; and (vi) 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, theGEFS demonstrated little ability to replicate the changes in blocking frequency due to a strong MJO that were noted in analyzed data.