A Frequency-Dependent Description of Propagating Sea Level Signals in the Kuroshio Extension Region

Hilbert empirical orthogonal function analysis is used to provide a frequency-dependent description of observed sea level variability in the Kuroshio Extension region, 1993-2012 inclusive. The dominant high-frequency mode (periods between 140 and 350 days) describes signals that propagate westward with the largest amplitudes in the vicinity of the Shatsky Rise and Emperor Seamounts. Based on the close correspondence between the variance of the high-frequency variability and the underlying bathymetry, it is speculated that this mode is driven by jet bathymetry interactions. The dominant low-frequency mode (periods longer than 350 days) is explained in terms of wind-forced, jet-trapped Rossby waves that propagate along the mean Kuroshio Extension jet. One of the most surprising findings of this study is that sea level changes north of the jet in the meander region anticipate changes south of the jet by about 3 yr. Based on correlations of observed sea level with the Pacific decadal oscillation, and western boundary transport variability estimated from the Global Ocean Reanalysis and Simulations (GLORYS), it is speculated that this anticipation is due to the differences in time taken for (i) Rossby waves to travel from the eastern North Pacific to the meander region and (ii) the much faster barotropic response of western boundary transport, and sea level north of the jet, to large-scale forcing by the wind stress curl.