Relationship between the Tropical Cyclone Forecast Skill and the Western North Pacific Summer Monsoon in the ECMWF Monthly Ensemble

In this study, week-1 to week-4 forecasts of tropical cyclones (TCs) in the western North Pacific are evaluated. The CWB TC Tracking System 2.0 (Lo et al. 2021) is used to objectively detect TCs in the 46-day ECMWF ensemble (ENS) forecasts in the 2021 season and also the reforecasts during 2001-2020. Preliminary evaluations of the probabilistic TC activity forecasts in the 20-year reforecasts show promising forecast skills. The reliability diagrams indicate slight over-forecasting bias in the weeks 1-4 forecasts, and the AUCs (Area Under Curves) are ranging from 0.91 (week-1) to 0.80 (week-4). The relationship between the TC activity forecast skill and the western North Pacific summer monsoon (WNPSM) is also investigated. The WNP monsoon index (WNPMI) proposed by Wang et al. (2001) is computed to provide a measure for the summer monsoon, and the TC forecast skills are evaluated under different levels of the WNPMI. To identify the potential false alarms, a spatial-temporal track clustering technique (Tsai et al. 2019) is implemented to objectively group similar vortex tracks in the 51-member forecasts. The corresponding ensemble mean track for each cluster is then used for performing the event-based verifications after the end of season. More details about the TC forecast verifications in weeks 1-4 using the ECMWF monthly ensemble will be presented in the meeting.