Seasonal to interannual variations of the East Kamchatka Current (EKC) and the Oyashio are examined by focusing on their barotropic response to wind forcing by a combined use of altimeter-derived and tide gauge sea levels. An empirical orthogonal function (EOF) analysis is performed on the 9-year altimeter sea level maps with thermosteric signals removed. A second EOF (EOF2) shows a spin-up and spin-down of the subarctic gyres, and its temporal variation is almost accounted for by the time-dependent Sverdrup balance. Tide gauge sea levels at Petropavlovsk-Kamchatsky (PK) agree with EOF2 and the Sverdrup transports in terms of not only the seasonal variation but also its year-to-year variability in winter when the subarctic gyre is spun up most. We also detect two types of EKC/Oyashio variations from the altimeter data: drifting velocities of sea level disturbances and geostrophic velocity anomalies. These two EKC/Oyashio temporal variations are also accounted for by the Sverdrup balance and agree with the PK sea levels and EOF2. The results imply that the PK sea levels can be a good representative of the subarctic gyre and EKC/Oyashio variations. On the basis of this relation, interannual variations during winter are discussed. The 44-year wintertime sea levels at PK correlate with the wintertime Sverdrup transport and springtime sea surface temperature off the northeastern coast of Japan with decadal-scale variability. This demonstrates that EKC/Oyashio is primarily explained by a barotropic response to large-scale atmospheric forcing and fluctuates on a decadal timescale almost in phase with atmospheric changes and influences the oceanic condition east of Japan.