Impact of the Boreal Summer Intraseasonal Oscillation on Typhoon Tracks in the Western North Pacific and the Prediction Skill of the ECMWF Model

The tropical intraseasonal oscillation (ISO) strongly influences tropical cyclone (TC) genesis. However, little is known concerning its impact on TC tracks. Here, we examine how TC tracks in the western North Pacific are modulated by the boreal summer ISO (BSISO) during each month of the TC season (June-October) using the best track data and how well the modulation are reproduced in the European Centre for Medium-Range Weather Forecasts (ECMWF) model forecasts. The results reveal that the impact of the BSISO on the typhoon tracks varies month to month. The ECMWF model successfully reproduces this impact up to a month in advance. A simple advection model shows that the reproduction of the modulations in the TC genesis location leads to accurate TC track forecasts. These results suggest that the BSISO is one of the major sources of TC track predictability at the subseasonal time scale (2 weeks to 2 months). Plain Language Summary A 30-90-day oscillation of convective activity in the tropical atmosphere affects the formation of tropical cyclones (TCs) in the western North Pacific (WNP) in summer. However, how this oscillation modulates the TC tracks is not yet well documented. In this study, we analyze the observational data and how well the modulation is simulated by a state-of-the-art operational numerical weather prediction model to discuss further improvements in TC predictions beyond 2 weeks that are necessary to mitigate the risk of TCs. The results show that the modulation of the TC track by the oscillation varies month to month. For example, TCs formed in the enhanced convection phases in the WNP have a lower chance of hitting Japan than those formed in the suppressed phases in September. In October, however, TCs formed in the enhanced phases have a higher chance of hitting Japan. The model can simulate the TC track modulation well in June, July, and September but not in August and October. Further detailed analyses suggest that model error in the modulation of the TC formation distribution caused by the oscillation causes errors in the TC tracks. These findings will help further improvement of the TC predictions beyond 2 weeks. Key Points The impact of the boreal summer intraseasonal oscillation on the typhoon tracks varies month to month The European Centre for Medium-Range Weather Forecasts model successfully reproduces the impact up to a month in advance Reproduction of modulations in the typhoon genesis location leads to accurate tropical cyclone track forecasts

Automated summary

The study reveals varying impacts of the boreal summer intraseasonal oscillation on typhoon tracks each month, with the ECMWF model successfully predicting these impacts up to a month in advance. Reproduction of modulations in the typhoon genesis location leads to accurate tropical cyclone track forecasts.