Impacts of the Madden-Julian Oscillation and Genesis Pathways on Tropical Cyclone Predictability in the GEFS Reforecasts Version-2

The impacts of the Madden-Julian Oscillation (MJO) and synoptic-scale environments on tropical cyclone (TC) predictability were examined in the NCEP Global Ensemble Forecasting System (GEFS) Reforecast Version-2 (GEFS-R). A non-degenerate skill score for rare binary events, “Symmetric Extremal Dependence Index” (SEDI), was used to examine the TC prediction skill. It was found that the GEFS-R has overall higher skill in predicting TC genesis over the eastern North Pacific and the North Atlantic than the other basins for all forecast lead times. Among the five genesis pathways identified by McTaggart-Cowan et al. (2013), the strong tropical transition pathway, in which baroclinicity significantly impacts TC genesis, has the lowest prediction skill, while the low-level baroclinic pathway exhibits the highest prediction skill. The GEFS-R captures the MJO signal with realistic variance, frequency range and spatial scales out to 5-day lead times, but the MJO signal attenuates quickly and has slower propagation speed beyond one-week lead time. The Week-1 reforecasts show higher TC prediction skill over the Atlantic when the model is initialized at the MJO phase 2 or 3 (based on the Wheeler-Hendon index). Various environmental variables are examined to better understand the sensitivity of the TC prediction skill to the MJO and genesis pathways.