Physical Processes Controlling Early And Late Onset Of Typhoon Season In The Western North Pacific

The formation of first tropical cyclone (TC) in early summer signifies the onset of an active TC season. In this study the interannual variability of the TC onset date in western North Pacific (WNP) during 1979-2012 is investigated. The average onset date happens on May 6. A composite analysis was conducted to separate early and late onset groups according to one 0.7 standard deviation. A diagnosis of TC genesis potential index (GPI) indicates that the major difference between the two groups lies in the background moisture field. There is a positive (negative) water vapor anomaly over the tropical WNP in the early (late) onset composite.According to the position of water vapor positive anomalies, the early onset years can be further divided into two parts. In the first kind of the early onset composite, positive specific humidity anomalies are accompanied by La Nina-developing like SSTA in the eastern equatorial Pacific, which mainly exists over the west of the WNP. A cyclonic circulation over the west of the WNP is motivated by the anomalous warm and wet southeasterly of the anticyclonic circulation motivated by the cold SSTA in the equatorial eastern Pacific. It is the Ekman-pumping induced ascending motion associated with the anomalous cyclone that primarily moistens the air column through anomalous vertical advection. For the second type, there is clearly richer water vapor over the east of the WNP, contributed by the anomalous active convection accompanied by the persistent warm SSTA in the tropical central Pacific. In the late onset composite, negative specific humidity anomalies are accompanied by the positive SSTA in the tropical eastern Pacific and an anomalous anticyclone in the WNP.In addition, the vertical wind shear also plays an important role in controlling the onset timing of the TC season. There is a decreasing (increasing) vertical wind shear and anomalous easterly (westerly) shear of zonal wind in the early (late) onset composite. Both the magnitude of the vertical shear and the sign of zonal wind vertical shear are in favor of (not in favor of) the development of cyclonic disturbances in the early (late) onset years.