We examined the convective activity and dynamical field over the subtropical North Pacific by focusing on the relationship between upper cold lows (UCLs) and the formation of the Marcus Convergence Zone (MCZ). In mid-July, we detected a UCL near Marcus Island that was associated with the formation of the MCZ. The convective activity associated with the MCZ followed the UCL, implying that the UCL contributed to the MCZ formation. Analysis of the temporal evolution of the dynamic field revealed that the UCL appeared in a large-scale, high-potential vorticity (PV) air mass migrating from the east. We had already identified the high-PV region near the mid-Pacific trough in late July. It stretched westward and reached Marcus Island in mid-July, suggesting that the UCL in the high-PV air mass over the North Pacific contributed to the formation of the MCZ.