Abstract In this study, we analyzed decreasing trends in the amplitude (− 0.185 m century − 1 , or − 12.1% century − 1 relative to the mean) and phase (− 7.69° century − 1 ) of M 2 tidal elevation over the period 1970–2024 in the semi-enclosed Ariake Sea located on the east coast of the East China Sea (ECS). We used a one-dimensional quarter-wavelength bay resonance model to identify the contributions of the incoming M 2 tide from the ECS, tidal amplification due to the quarter-wavelength bay resonance, and damping caused by bottom friction. In addition to the previously reported secular variation in the incoming M 2 tide, this study quantified the secular variations in the intrinsic resonance frequency and linear bottom friction coefficient, as well as their impact on tidal amplification in the Ariake Sea. A less resonant condition for the M 2 tide was detected after 1997, mainly due to the partial enclosure of Isahaya Bay, a sub-embayment of the Ariake Sea, by a dike. Furthermore, we found that bottom friction and M 2 energy dissipation decreased, especially after the 2000s, resulting in an increase in the amplitude and a decrease in the phase lag of the M 2 elevation. We discuss the impacts of the less resonant condition after 1997 and the decreasing incoming M 2 tide on the recent decrease in energy dissipation. These results have implications for mixing and material transport in the Ariake Sea and should be useful for understanding past environmental change and for predicting future changes in tides in semi-enclosed coastal seas.

Abstract In this study, the scattering and dissipation processes of diurnal coastal trapped waves (CTWs) are investigated using mooring observations and direct microstructure measurements in the Suruga Trough, Japan. First, we perform spectral and harmonic analyses of the observed data, confirming the presence of diurnal CTWs characterized by velocity and temperature perturbations that are most pronounced near the seafloor. The interaction of bottom-intensified CTW currents with a small-scale bathymetric bump generates high-frequency waves, exhibiting spectral peaks at the harmonics of the diurnal tidal constituents. These high-frequency waves are identified as internal lee waves based on the dispersion relationship and are responsible for strong mixing on the lee side of the bathymetric bump, as evidenced by microstructure measurements. Using moored velocity records, we further examine the parameter dependence of internal lee wave generation by CTWs propagating over the bathymetric bump. The results indicate that internal lee wave activity intensifies when the tidal excursion parameter $${T}_{e}$$ falls within an intermediate range between 1 and the stratification parameter $${N}^{*}$$ . In contrast, when $${T}_{e}$$ exceeds $${N}^{*}$$ , the energy of internal lee waves decreases markedly due to the formation of “evanescent waves” resulting from strong interactions between CTWs and the bathymetric bump.

Abstract One of the major sources of iron supply to the Okhotsk Sea and the Oyashio region, which both have high biological productivity, is thought to be regions of the East Sakhalin Current and its upstream, originating from the Amur River. This study suggests that the Soya Warm Current (SWC) region could also be a possible source of iron via resuspension of bottom sediments. We derived the volume backscatter strength from acoustic Doppler current profilers (ADCP) data that had previously been collected in the SWC region. The backscatter strength data indicate that prominent sediment resuspension occurs in both the upstream and downstream regions of the SWC when the bottom current exceeding 0.5 m/s continues to some extent. In the upstream region, strong current events that induced the sediment resuspension occurred through the SWC intensification caused by the increase in sea-level difference across the Soya Strait excited by the southerly winds. Such events frequently occur from September to December, because of frequent passages of cyclone. By contrast, in the downstream region, sediment resuspension occurred due to the strong northerly winds via the current increase caused by the coastal trapped waves over the shelf in winter. The sediment resuspension in the downstream region was also supported by direct observations of high turbidity near the bottom. High iron concentrations in the coastal Oyashio water possibly originate from the SWC water. The occurrence of the resuspension during the sea ice season suggests the potential local incorporation of sediment into sea ice.

Abstract The temporal drift of the ARO-FTs, the new optical dissolved oxygen (DO) sensors mounted on 13 Argo floats, was analyzed for two periods: the storage period before float deployment and the period after float deployment. ARO-FTs exhibited a common issue of optical DO sensors, resulting in greater deviations from the reference data at higher concentrations. The storage drift of the ARO-FTs mounted on floats in this study was on the order of 5 μmol kg –1 . Although this storage drift was larger than that of the Aanderaa Optode 4330, the first ARO-FT DO profiles could be corrected to within ± 2% of the ship-based bottle sampling data, even at layers with large vertical DO gradients, using a linear first-order equation, owing to the fast response time of ARO-FTs. The average temporal drift of ARO-FTs after float deployment was estimated at – 0.31 ± 0.17% year –1 in oxygen concentration using the method of a previous study. Although the drift of ARO-FTs was greater than that of the Aanderaa Optode 4330 for both periods, the DO profiles corrected for drift remained within ± 2% of the nearby reference data at 210 days after float deployment. The seasonal variations in oxygen saturation rate near the surface and their amplitudes during the study period were consistent with those observed in monthly climatology. These results indicate that the corrected DO measurement by ARO-FTs was comparable to that of the Aanderaa Optode 4330.