Eddy Variability in the Kuroshio Extension as Revealed by Geosat Altimetry: Energy Propagation Away from the Jet, Reynolds Stress, and Seasonal Cycle

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Altimetric sea level data around the Kuroshio Extension, collected during the first year of the Geosat Exact Repeat Mission (ERM) (starting in November 1986), have been analyzed for energy propagation away from the jet, the Reynolds stress pattern, and the seasonal cycle of the eddy field. Space-time diagrams and wavenumber-frequency spectra are employed to discern the dominant direction of phase propagation, revealing westward and inward phase propagation towards the jet (i.e., energy propagation away from the jet), which is consistent with the notion that the widespread eddy variability from the jet is the result of transient Rossby wave response to forcing provided by growing, pulsating, and decaying meanders of the jet. It can be shown that any theory based on the linear instability cannot normally account for the energy propagation, because any linear instability that is wavelike away from the jet is by necessity not the fastest growing linear instability. Rather, a nonlinear instability mechanism is needed, whose nonstationary finite amplitude behaviors can excite Rossby waves away from the jet. The computed Reynolds stress pattern is consistent with previous observation indicating that, on the whole, the jet is reinforced, rather than weakened, thus favoring the baroclinic instability as the dominant source for meandering growth. A seasonal cycle in the intensity of both the eddy field and the surface transport of the Kuroshio Extension is found, which is weakest during winter and progressively strengthens from spring to fall.