Synoptic numerical modeling was conducted to evaluate the open ocean influences on the thermal environment of five aligned embayments (Osaka Bay, Ise Bay, Suruga Bay, Sagami Bay, and Tokyo Bay) along the Pacific coast of Japan. We exclusively focused on the open ocean effects by analyzing the heat flux exchange through each bay's mouth, thereby disregarding contributions from tidal and fluvial influences. The mean heat flux (MHF), predominantly comprising seasonal variations, emerged as a primary driver of seasonal thermal fluctuations, exerting a more significant impact on heat exchange compared to the eddy component. Warm water diverging from the Kuroshio reached the bays east of Sagami Bay through the Oshima West Channel, where the MHF was intensified during the Kuroshio berthing period, correlating with its axis fluctuations. In the Osaka Bay-Suruga Bay region, topographically constrained horizontal circulation mitigated direct intrusion. The direction and magnitude of MHF into and out of the bays varied depending on the degree of closure exhibited by each bay.