What Caused the Record‐High Frequency of South China Sea Tropical Cyclones in July–September 2025?

Abstract The frequency of tropical cyclones (TCs) occurring over the South China Sea (SCS) during July–September was at its historical maximum in 2025. We found that this record‐high SCS TC activity was due to a simultaneous increase in both TCs forming in the SCS (local SCS TCs) and TCs entering into the SCS (non‐local SCS TCs). These increases were further linked to anomalous westerly 850‐hPa zonal wind anomalies over the Indian Ocean and a westward shift in the western Pacific subtropical high at 500 hPa. A statistical analysis for the period of 1961–2024 finds that when 850‐hPa westerly zonal wind anomalies occur over the equatorial western Indian Ocean, these anomalies can propagate eastward into the SCS and subsequently generate positive vorticity anomalies, enhancing the genesis of local SCS TCs. When the western Pacific subtropical high extends westward, it induces anomalous easterly flow spanning from the northern SCS to east of the Philippines, favoring TCs moving westward into the SCS. As a result, although there are only slight changes in the number of TCs forming outside the SCS, there is a significantly greater likelihood for these TCs to move into the SCS, leading to an increase in non‐local SCS TCs. By building a 1961–2024 diagnostic scheme, including only an index of 850‐hPa zonal wind anomalies over the western Indian Ocean and 500‐hPa geopotential height anomalies over the western Pacific, we can successfully anticipate the historical peak activity of SCS TCs in 2025.