The interdecadal variations in the North Pacific sea level field were investigated on the basis of two historical tide-gauge data sets: long-term sea level records spanning 100 years but spatially limited in the midlatitudes (the CL data set) and records from a wider area but with a data length of about 50 years (the HCL data set). A seesaw-like sea level oscillation between the eastern and the western Pacific was extracted as the first mode of empirical orthogonal functions (EOF) from the CL data set. This mode shows a 22.5-year period of variation that has continued throughout the 20th century. A similar variation was found in the HCL data set as the first mode of complex EOF (CEOF), though its dominant period was 14.5 years. This interdecadal sea level oscillation is the steric sea level change associated with the Pacific Inter-Decadal Oscillation (PDO) in the upper ocean temperature field, because the temporal evolution and spatial structure in the interdecadal oscillation resemble those of the dominant modal pattern in the temperature field. However, this mode shows a standing wave characteristic in the midlatitudes rather than propagating, though the clockwise phase propagation around the subtropical gyre has been reported in the temperature field. Another interdecadal oscillation is found in the CEOF second mode, which was dominant in the 1960s when the first mode was relatively weak. The second mode shows a 15- to 16-years period of variation and phase lags of 3-4 years at the subtropical stations relative to those in the midlatitudes.
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