This study documents a weakening of the relationship between the spring Arctic Oscillation(AO) and the following summer tropical cyclone(TC) formation frequency over the eastern part(150?–180?E) of the western North Pacific(WNP). The relationship is strong and statistically significant during 1968–1986,but becomes weak during 1989–2007. The spring AOrelated SST,atmospheric dynamic,and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. Results indicate that the spring AO leads to an El Ni ?no-like SST anomaly,lower-level anomalous cyclonic circulation,upper-level anomalous anticyclonic circulation,enhanced ascending motion,and a positive midlevel relative humidity anomaly in the tropical western–central Pacific during 1968–1986,whereas the AOrelated anomalies in the above quantities are weak during 1989–2007. Hence,the large-scale dynamic and thermodynamic anomalies are more favorable for TC formation over the eastern WNP during 1968–1986 than during 1989–2007. This study documents a weakening of the relationship between the spring Arctic Oscillation (AO) and the following summer tropical cyclone (TC) formation frequency over the eastern part (150? -180? E) of the western North Pacific (WNP). The relationship is strong and statistically significant 1968-1986, but becomes weak during 1989-2007. The spring AOrelated SST, atmospheric dynamic, and thermodynamic conditions are compared between the two epochs to understand the possible reasons for the change in the relationship. the spring AO leads to an El Ni? no-like SST anomaly, lower-level anomalous anticyclonic circulation, enhanced ascending motion an and positive midlevel relative humidity anomaly in the tropical western-central Pacific during 1968- 1986, and the AOrelated anomalies in the quantities were weak during 1989-2007. Therefore, the large-scale dynamic and thermodynamic anomalies are more favorable for TC for mation over the eastern WNP during 1968-1986 than during 1989-2007.