Abstract:The East Asian Winter Monsoon (EAWM) was weaker than normal during the 2014/2015 winter with a phase transition from strong to weak at the end of December 2014. A warmer winter was experienced in most areas of China due to the effect of weaker EAWM. In association with the phase transition of EAWM, a reverse of temperature anomalies over China occurred simultaneously with negative temperature anomalies in December 2014 and positive temperature anomalies in January and February 2015. Further study indicates that the weakening of EAWM is related to the extension of Arctic sea ice cover (without linear trend) in September 2014. The expanded sea ice coverage before winter restrains the development of Siberian high, leading to a weaker EAWM and a warmer winter over China. Besides, the combination of central Pacific El Ni〖AKn~D〗o and positive phase of PDO is also important in reducing the intensity of EAWM. The phase of PDO has an influence on the effect of El Ni〖AKn~D〗o on EAWM. Positive anomalies of SST over central Pacific is favorable for the negative anomalies of meridional wind over the east and eastern coast of China and a weak EAWM. El Ni〖AKn~D〗o event tends to exert stronger influence on the circulation over East Asia when the event is accompanied with a positive phase of PDO. The intraseasonal variation of EAWM is related to the response of atmosphere to El Ni〖AKn~D〗o event.