The preconditioning of winter mixed layer in the formation of the eastern subtropical mode water (ESTMW) in the North Pacific Ocean is investigated using an eddy-permitting ocean general circulation model. The result shows that convergence in the northward Ekman transport of saltier water and weak summertime heat fluxes due to the presence of stratus cloud in the ESTMW formation region are both central to the initiation of significant mixed layer deepening that subsequently evolves into the ESTMW pycnostad in the main thermocline. These two factors originate from air-sea interactions inherent in the region of Northeast Pacific Basin. The approach described here has significant potential for determining the water-mass distribution and may lead to an explanation of the Sverdrup's Eastern Gyral in the subtropical North Pacific.