A Comparison of the Downstream Predictability associated with ET and

The impact of the extratropical transition (ET) of tropical cyclones and baroclinic cyclogenesis in the Western North Pacific (WNP), Atlantic, and Southern Indian Ocean (SIO) basins on the predictability of the downstream midlatitude flow is assessed using 30 y of cases from the Global Ensemble Forecast System (GEFS) Reforecast Version 2. In all three basins, ET is associated with statistically larger 500 hPa geopotential height forecast standard deviation (SD) compared to the forecast climatology. The higher SD values originate from where the TC enters the midlatitudes and spreads downstream at the group velocity of the associated wave packet. Of the three basins, WNP ET is associated with the largest amplitude and longest-lasting SD anomalies. Forecasts initialized 2-4 d prior to the onset of ET have larger SD anomalies compared to forecasts initialized during or after the onset of ET. By contrast, the region of positive SD anomaly associated with winter baroclinic cyclones is confined to the upstream trough, with fall cyclones exhibiting some downstream propagation characteristics similar to ET. ET cases with the larger downstream SD anomaly are characterized by a more amplified ridge downstream of the TC as it enters the midlatitudes. By contrast, ET cases with large TC position variability at the onset of ET or upper-tropospheric PV advection by the irrotational wind are not characterized by significantly larger downstream SD. 9