Local topographic Rossby modes observed in the abyssal Japan Sea

Abstract

The short-period current fluctuations (topographic wave fluctuations, TWFs) on the southern rim slope of the abyssal Japan Sea were investigated using current meter datasets from closely spaced mooring arrays. The TWFs occurred almost continuously throughout the year with short periods in a narrow band (1.5–5-d), showing a seasonal modulation in their amplitude. The TWFs were attributable to alternate passage of cyclonic and anti-cyclonic eddies on the rim slope, which propagated eastward at a speed of 0.15–0.23 m s−1. In addition, the TWFs showed a bottom-intensified characteristic, along with the two-layer structure consisting of an almost barotropic lower layer and a marginally baroclinic upper layer. The lowest topographic Rossby mode, which is a normal mode of the topographic Rossby waves prescribed by the two ridges on the rim slope, was considered as a cause of the TWFs because of its eastward-propagating eddy train structure along the rim slope and the eigenperiod (3–5-d) near the TWF-band. In addition, the local time-dependent Sverdrup balance was considered as a mechanism of the TWF generation, since the TWFs significantly correlated with the wind stress curl variations over the observation area with time lags. That is, the current fluctuations near the eigenperiod were selectively amplified via the resonance between the lowest topographic Rossby mode and the Ekman pumping variations induced by the TWF-band wind stress curl. We concluded that the observed TWFs were a manifestation of the wind-induced lowest topographic Rossby mode prescribed by the bottom topography.