A comparison of tropical cyclone formation over the western North Pacific in August between 1996 and 2014

The tropical cyclogenesis over the western North Pacific (WNP) displays distinct difference in August of 1996 and 2014. Total 9 tropical cyclones (TCs) formed in August of 1996 over the WNP (5 and 4 TCs in the northeastern and southwestern part, respectively), whereas no TC formed in August of 2014. The present study investigates the reasons for the above difference. In August 1996, an obvious wave train with a southwest-northeast orientation is detected at the lower-level over the WNP on the monthly time scale. Anomalous cyclone and enhanced convection are induced over the northeastern WNP by the wave train, providing a favorable condition for TC formation. Analysis shows that the intraseasonal positive vorticity anomalies contributed to the TC genesis over the southwestern WNP though monthly mean state was unfavorable. In August 2014, both monthly mean and daily environment conditions were hostile to the TC formation.