Subseasonal Tropical Cyclone Prediction and Modulations by MJO and ENSO in CESM2
Skillful subseasonal forecasts are valuable to decision-makers of various industries and societal sectors. Here we aim to evaluate the subseasonal tropical cyclone (TC) reforecasts from the Community Earth System Model version 2 (CAM6) subseasonal prediction system. We evaluate the modeled TC climatology and the probabilistic forecast skill of basin-wide TC genesis, utilizing the Brier skill score. We investigate the sources of skills and errors, including the number of TC “seeds”, the mean-state climate condition, and the modulations by MJO and ENSO.
Reliable subseasonal TC prediction is important for guiding storm preparedness and protecting public safety. We use the CESM2 subseasonal hindcast dataset to better understand the sources of skills and errors for subseasonal TC predictability. The model shows good skill when compared to the annual mean climatology but has limited skill in predicting TC genesis beyond the TC annual cycle. CESM2's prediction skill is generally comparable with the NCEP model but does not match the ECMWF model. We find that the influence of the large-scale environment varies across different basins, and the TC seeds are the dominant factor for the decline of TC genesis at week 2 and longer. The MJO-TC relationship is well captured at up to 3 weeks lead time, indicating that the MJO provides a robust source of predictability for TC genesis in the model.
We examine the subseasonal prediction of TCs in the CESM2(CAM6) subseasonal prediction system. We evaluate the modeled TC climatological characteristics and the model's skill in forecasting weekly TC genesis in each TC basin. We find that the model skillfully captures the observed basin-wide climatological TC seasonality and spatial distributions, but TC genesis is largely underestimated from week 2 onward. For some basins and lead times, the predicted TC genesis is primarily controlled by the number of TC “seeds” and the mean-state climate condition. The model has good prediction skill compared with the constant climatology across all the basins and lead times but has limited skill in predicting deviations from the TC seasonal cycle. We find that the Madden-Julian Oscillation has a strong modulation on TC activity and provides the main source of predictability for subseasonal TC genesis. The interannual variability of predicted TC activity is skillfully predicted in the North Atlantic and the Northwestern Pacific, with a strong modulation by the El Nino-Southern Oscillation.