DECADAL CHANGES IN WESTERN NORTH PACIFIC TROPICAL CYCLONES ASSOCIATED WITH MADDEN-JULIAN OSCILLATION

This study focuses on the decadal variability of tropical cyclones（TC） over the Western North Pacific（WNP）and how these changes are related to the Madden-Julian Oscillation（MJO）.It was done with the help of the Real-time Multivariate MJO index from the Australian Government Bureau of Meteorology of the Centre for Australian Weather and Climate Research,TC data from the Joint Typhoon Warming Center best track datasets,and daily and monthly datasets from the NCEP/NCAR reanalysis center.The results show that the TC frequency in the WNP exhibited a statistically significant decrease during 1998-2010 compared to during 1979-1997.The decrease in TC frequency in the WNP mainly occurred during MJO active phases（i.e.,phases 4,5,6,and 7）.Further investigation of the climate background and the propagation differences of the MJO between 1979-1997 and 1998-2010 was performed.The La Nina-like tropical sea surface temperature cooling caused stronger Walker circulation and thus induced unfavorable atmosphere conditions for WNP TC genesis including a low-level easterly anomaly,a negative relative vorticity anomaly,an increase in sea-level pressure,and stronger vertical wind shear.Moreover,shortening of the MJO cycle,decline in the duration of the active phases in the WNP,and easterly anomaly and shrinkage of the convection area during MJO active phases may also partly explain the decadal variation of TC.