Seasonal to interannual variations of the western boundary current of the subarctic North Pacific by a combination of the altimeter and tide gauge sea levels

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 largescale atmospheric forcing and fluctuates on a decadal timescale almost in phase with atmospheric changes and influences the oceanic condition east of Japan.