Track variability of South China Sea-formed tropical cyclones modulated by seasonal and intraseasonal circulations

Abstract

Tracks of tropical cyclones (TCs) forming in the South China Sea (SCS) are examined in terms of their seasonal variability and associated modulations by seasonal and intraseasonal circulations. Three major TC tracks appear in the SCS: northeastward (NE) toward the Philippines and Taiwan, northward (N) toward southern China, and westward-northwestward (W-NW) toward the Indochina Peninsula and southwestern China. Seasonally, the dominant tracks are NE in May, from NE into W-NW in June, W-NW and N in July to September (JAS), and W-NW in October to November (ON). Formation of TCs with W-NW tracks is associated with a southeastward-extending monsoon trough (MT) and a northwestward-expanding western Pacific subtropical high (WPSH) in May to June (MJ), a southward-intensified MT and a westward-strengthened WPSH in JAS, and a northward and westward enhanced equatorial trough in ON. Formation of TCs with NE tracks in MJ is related to an eastward-intensified MT and an eastward-retreated WPSH. For TCs with N tracks, their formation is associated with an eastward and northward intensification of the MT and a northwestward extension of the WPSH. In terms of modulations of intraseasonal oscillations (ISOs), the 10-24-day and 30-60-day ISOs exert different effects on TC movement. For all track types, TCs tend to follow the propagation of the 10-24-day cyclonic anomaly and move along different tracks. The above movement comes under the influence of favorable environments in terms of moisture convergence provided by 30-60-day cyclonic anomalies distributing along TC tracks. The 10-24-day and 30-60-day ISOs exhibit comparable intensity to jointly modulate TC activity in the SCS.

Key points

• TCs forming inside the SCS exhibit clear seasonal variability in their tracks

• MT and WPSH show different variability features for different TC tracks

• Propagations and distributions of ISO anomalies effectively modulate TC tracks