Characteristics and mechanism of deep mesoscale variability south of the Kuroshio Extension

To clarify the characteristics and mechanism of mesoscale variability in the deep ocean, we analyzed historical mooring observations conducted at Site R (30°N, 147°E) south of the Kuroshio Extension during 1978-85 and the 10-year output of an eddy-resolving ocean general circulation model. The power spectral density (PSD) of current velocity at a depth of 5000 m had a peak at periods of 45-75 days at two mooring stations that were zonally 98 km apart and at 54 days at Site R in the model. In the model, the PSD at 54 days at 5000 m had a horizontal maximum 100 km north of Site R as well as in the Kuroshio Extension region 500 km north of Site R. Within the high-PSD region near Site R, variability in the velocity was coherent, and its phase propagation was characterized by zonal and meridional wavenumbers of −2.9×10−5 and 0.1×10−5 rad m−1, respectively, the former of which was comparable to the value of −2.6×10−5 rad m−1 obtained from the mooring observations. Such wavenumbers matched well with the dispersion relation of barotropic topographic Rossby waves (TRWs). Backward ray tracing and PSD distribution suggested that the barotropic TRWs at Site R were generated in the Kuroshio Extension region, and their energy propagated southward.