Abstract
Examining the deep-water exchange through the Ulleung Interplain Gap (UIG) between the Ulleung Basin (UB) and the Japan Basin in the East Sea (Sea of Japan) is critical for understanding the vigorous circulation and material cycles of the sea. The exchange features an asymmetric flow structure across the UIG: a broad and weak inflow (into the UB) in the western UIG, and a narrow and strong outflow (out of the UB) in the eastern UIG, with the latter closely associated with the Dokdo Abyssal Current (DAC), a long-term mean, strong abyssal current near Dokdo. In this study, the linear theory of bottom-trapped topographic Rossby waves (TRWs) is applied to explain the previously unexplored longer intraseasonal band (30–50 days) DAC variability by analyzing multi-year moored current-meter observations and HYCOM reanalysis data, as they are significantly correlated in the period band (though not at the shorter intraseasonal band of 5–25 day explored previously). Bottom-intensified DAC variability is characterized by TRW parameters with a vertical trapping scale of 1100–2100 m, a horizontal wavelength of 49–111 km, a propagating speed of 1.3–3.0 km day-1, and a propagating direction aligned with isobaths within a 2°–23° range (shallower water on the right). The departure angle between the energy-propagating direction of the waves and the isobath direction is estimated from the spectra of the along- and cross-slope abyssal currents and from the TRW theoretical dispersion relation for a given buoyancy frequency and bottom slope. These values are then compared to examine the significance of the bottom-trapped TRW dynamics, yielding a small (< 16°) difference. The results support the significance of bottom-trapped TRWs on the longer intraseasonal variability of abyssal currents near the steeply sloped eastern side of the UIG, and an asymmetric abyssal flow structure across the UIG in the southwestern East Sea.
Highlights
Bottom-trapped topographic Rossby waves (TRWs), generated by the stretching and squeezing of deep-water columns, propagate in the direction corresponding to the shallower water on the right-hand side in the northern hemisphere
This study aims to characterize and address the bottomtrapped TRWs underlying the longer-period intraseasonal Dokdo Abyssal Current (DAC) variability from multi-year moored current-meter observations and HYbrid Coordinate Ocean Model (HYCOM) reanalysis, and to discuss the possible cause
The difference between maximum current speeds at U5 and EC1 is due to the asymmetric flow structure across the Ulleung Interplain Gap (UIG), with a wider and weaker inflow into the Ulleung Basin (UB) located in the western UIG, and a narrower and stronger outflow into the Japan Basin (JB) located in the eastern UIG, which is consistent with the findings of previous studies (Chang et al, 2009) and confirmed from HYCOM (Figure 4A)
Summary
Bottom-trapped topographic Rossby waves (TRWs), generated by the stretching and squeezing of deep-water columns, propagate in the direction corresponding to the shallower water on the right-hand side in the northern hemisphere. TRWs proceed along isobaths over sloping topography and are modified by stratification (Rhines, 1970; Reid and Wang, 2004). Both observations and numerical models suggest that bottom-trapped TRWs dominate the abyssal current variability on a timescale ranging from several to hundreds of days. The ranges of frequencies and wavenumbers from previous studies are shown in Figure 1, along with the theoretical dispersion relations for ranges of different parameters accounting for intraseasonal oscillations
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