LAUSR

The interannual variability of typhoon onset date in western North Pacific (WNP) during 1979–2015 is investigated. The average date in the earlier (later) onset group is early April (late July). A diagnosis of the genesis potential index shows that the key factor affecting the earlier and later onset lies in the background moisture field. According to their evolution feature, the earlier onset cases are further divided into two sub-groups. In the first sub-group, positive specific humidity anomalies were confined in the western WNP and caused primarily by circulation anomalies associated with a La Niña in the equatorial eastern Pacific. In the second sub-group, positive specific humidity anomalies were confined in the eastern WNP and caused by anomalous convection associated with the Pacific Meridional Mode (PMM) in the tropical and subtropical eastern Pacific. In the later onset composite, negative specific humidity anomalies appear in the WNP, and were accompanied by an anomalous anticyclone in situ and a V-shape negative SSTA pattern in the western Pacific. In addition, the change of background vertical wind shear (VWS) also influences the TC onset date. A decrease (increase) in the VWS magnitude and an easterly (westerly) shear anomaly occur in the earlier (later) onset composite. Physically it is argued that both the magnitude of the VWS and the sign of anomalous zonal wind shear may affect TC formation in the WNP.