<p>This study analyzes the effects of interannual variation in surface heat flux on the variability of the upper layer circulation in the East Sea (Sea of Japan) via a series of numerical experiments. Comparing the intrinsic variability of the upper layer circulation, the interannual variation in the surface heat flux amplifies the variability in the Yamato Basin, but not in the Ulleung Basin. The variability in the water temperature in the northern region is highly correlated with the variability in the surface heat flux with a 1-month time lag. Under strong cooling conditions, the cyclonic gyre and convection in the northern region are intensified, and the expansion of the cold water area facilitates a more pronounced southward intrusion compared to weak cooling conditions. However, the interannual variation in surface heat flux does not significantly impact the northernmost latitude of the western boundary current (i.e., the East Korea Warm Current). Instead, the northernmost latitude of the East Korea Warm Current is influenced primarily by the magnitude of the strong positive winter wind stress curl in the northern region, which greatly affects the southward flow of cold water along the Korean coast. An increased (decreased) volume transport or lateral heat flux through Korea/Tsushima Strait promotes the eastward (westward) propagation of eddies distributed in the Yamato Basin, thereby affecting the meandering pattern of the Tsushima Warm Current. This study is expected to enhance understanding of the respective contributions of interannual variation in individual external forcing factors to the variability of the upper layer circulation in the East Sea.</p>