Typhoon Chan-Hom (2015) experienced a sudden northward track change around 0000 UTC 3 July 2015, with a rapid weakening of its intensity during the 12 hours of the track change. Analysis shows that its activity was associated with a large-scale monsoon gyre on the intraseasonal (15-30-day) time scale during sudden track and intensity changes. The monsoon gyre was located to the east of the Philippines and nearly stationary before the typhoon made the sudden northward turn. Then the monsoon gyre moved northward together with Chan-Hom after the sudden turn. In agreement with the previous result of the idealized numerical experiments, the cyclonic rotation of the beta-gyres was identified prior to its coalescence process with the monsoon gyre, suggesting the importance of the change of the orientation of the beta gyres. The rapid weakening of the Chan-Hom was related to the strengthening of outer convection associated with the monsoon gyre during the interaction of two systems, while the large-scale environmental factors including sea surface temperature, humidity in low and middle levels and vertical wind shear had little influences. It is found that the development of eastern outer convection associated with the monsoon gyre made the weakening of the inner-core convection and enlargement of the eye of Chan-Hom, thus leading to the rapid weakening of Chan-Hom within an relative favorable environment for intensification.