Over the Northwestern (NW) Pacific, a subtropical jet stream develops at ~30¿N in midwinter, as the East Asian winter monsoon intensifies. Eddies with subweekly periods propagating through the midlatitude tropopause into the NW Pacific storm track tend to be trapped into the jet core ~1000 km away from a surface baroclinic zone at ~40¿N, while lifted up by ~3 km. The trapping appears to weaken the interaction of the upper-level eddies with the baroclinic zone, thus acting against eddy amplification and thereby contributing to the midwinter minimum in the eddy activity as observed before the late 1980s. Since then, under the reduced trapping effect by the weakened jet, upper-level eddies tended to propagate right above the surface baroclinic zone, while strongly interacting it, even in midwinter as in fall and spring. This midwinter tendency acted to enhance the eddy amplification that led to the recent disappearance of the activity minimum.