Impacts of the multichannel river network on plume dynamics in the Pearl River estuary were examined using a high‐resolution 3‐D circulation model. The results showed that during the dry season the plume was a distinct feature along the western coast of the estuary. The plume was defined as three water masses: (a) riverine water (22 psu), respectively. A significant amount of low‐salinity water from Hengmen and Hongqimen was transported through a narrow channel between the QiAo Island and the mainland of the Pearl River delta during the ebb tide and formed a local salinity‐gradient feature (hereafter referred to as a discharge plume). This discharge plume was a typical small‐scale river plume with a Kelvin numberK = 0.24 and a strong frontal boundary on its offshore side. With evidence of a significant impact on the distribution and variability of the salinity and flow over the West Shoal, this plume was thought to be a major feature of the Pearl River plume during the dry season. The upstream multichannel river network not only were the freshwater discharge sources but also played a role in establishing an estuarine‐scale subtidal pressure gradient. This pressure gradient was one of the key dynamical processes controlling the water exchange between discharge and river plumes in the Pearl River estuary. This study clearly showed the role of the river network and estuary interaction on river plume dynamics.

The generation of nonlinear internal waves (NLIWs) by stratified tidal flow over an isolated topography has been a subject of research for many years. However, the enhanced complexity of the dynamics as a consequence of flow over two topographical features has been less studied and understood. Here, we report results of the generation of type‐A and type‐B NLIWs in the South China Sea that originate from a double‐ridge system. Based on a realistic three‐dimensional nonhydrostatic simulation, it is found that type‐A NLIWs are generated at the western ridge through the mechanism of internal tide release, while the generation of type‐B NLIWs occurs at the eastern ridge as a consequence of internal wave interaction. Associated with the ridge height, separation distance, and modulated tidal forcing, the generation of type‐B NLIWs is strongly modified by the resonant process between the two ridges. As a result, type‐A and type‐B NLIWs appear in the observations with alternating large and small amplitudes. In addition, with a detailed knowledge of the generation, it is also found that the different arrival time of these waves could be due to their different generation times. Finally, the reflection/transmission of type‐B NLIWs at the subcritical western ridge is suggested as a key feature of resonance. However, it is governed by hydraulically controlled flow instead of the ratio of the local topographic slope to the internal wave beam angle.

The 11 March 2011 tsunami triggered by the M9 and M7.9 earthquakes off the Tōhoku coast destroyed facilities at the Fukushima Dai-ichi Nuclear Power Plant (FNPP) leading to a significant long-term flow of the radionuclide ¹³⁷Cs into coastal waters. A high-resolution, global-coastal nested ocean model was first constructed to simulate the 11 March tsunami and coastal inundation. Based on the model's success in reproducing the observed tsunami and coastal inundation, model experiments were then conducted with differing grid resolution to assess the initial spread of ¹³⁷Cs over the eastern shelf of Japan. The ¹³⁷Cs was tracked as a conservative tracer (without radioactive decay) in the three-dimensional model flow field over the period of 26 March–31 August 2011. The results clearly show that for the same ¹³⁷Cs discharge, the model-predicted spreading of ¹³⁷Cs was sensitive not only to model resolution but also the FNPP seawall structure. A coarse-resolution (∼2 km) model simulation led to an overestimation of lateral diffusion and thus faster dispersion of ¹³⁷Cs from the coast to the deep ocean, while advective processes played a more significant role when the model resolution at and around the FNPP was refined to ∼5 m. By resolving the pathways from the leaking source to the southern and northern discharge canals, the high-resolution model better predicted the ¹³⁷Cs spreading in the inner shelf where in situ measurements were made at 30 km off the coast. The overestimation of ¹³⁷Cs concentration near the coast is thought to be due to the omission of sedimentation and biogeochemical processes as well as uncertainties in the amount of ¹³⁷Cs leaking from the source in the model. As a result, a biogeochemical module should be included in the model for more realistic simulations of the fate and spreading of ¹³⁷Cs in the ocean.

The impacts of climate change on ichthyoplankton have barely been examined in the Pearl River estuary (PRE). Using the fish larvae and eggs caught in the estuary in the summer from 2003 to 2018 (except for the years 2004 and 2008), the spatial and temporal variations in ichthyoplankton’s abundance, species composition, and community structure were examined and linked to the variability of the climate system. A positive/negative correlation was found between the abundance and taxonomic diversity of the fish larvae and the monthly Oceanic Niño Index. However, the variation in larval fish’s species diversity was in phase with climate change, while the variation in larval fish’s abundance had a 3-month lag behind the climate variability. The different phases of the correlations reflect a progressive change in the ichthyoplankton dynamics under the influence of climate. Furthermore, during the period of the study, the regional climate underwent an obvious transition from a “cold” to a “warm” state in the early 2010s. Associated with that, it was found that both the abundance and species composition of the ichthyoplankton exhibit a significant change around that time, particularly for the fish larvae. A further examination of the ichthyoplankton’s horizontal distributions suggests that the assemblage of ichthyoplankton in the PRE is sensitive to the La Niña conditions. This leads to an overall reduction in ichthyoplankton’s abundance and less seaward spreading of the assemblage, implying the possible impact of climate change on river discharge and then the estuarine environment.