Chongguang Pang

Based on the novel energetic analysis tools, namely multiscale window transform and multiscale energy and vorticity analysis, this study investigates the seasonal variability of eddy kinetic energy (EKE) in the greater Agulhas Current system (GACS), which is divided into three subdomains based on the horizontal structures of background flows: the Mozambique Channel (MZC) subdomain, the northern Agulhas Current (NAC) subdomain, and the Agulhas retroflection (ARF) subdomain. Results show that the seasonality of EKE is spatially inhomogeneous. In the MZC subdomain, the EKE is strongest in spring and weakest in autumn, whereas in the NAC and ARF subdomains, the EKE level is highest in summer and lowest in winter. In all the three subdomains, the seasonal cycle of the barotropic instability of the mean flow corresponds well with that of the EKE. In contrast, the nonlocal transportation that mainly works on the redistribution of EKE is out of phase with the seasonality of EKE. Regarding the local wind forcing and baroclinic instability, they both have weak impacts on the EKE evolution, contributing power only about 10% of the barotropic instability. Moreover, neither of their seasonal cycles is consistent with the seasonality of EKE. Therefore, it is the barotropic instability of the mean flow that controls the seasonal variability of the EKE in the GACS.

Based on mooring observations from October 10 to 5 November 2017 at four stations in the Northwestern Pacific, the characteristics of four strong near‐inertial wave (NIW) packets generated by the typhoon Lan were examined. The wave‐packet analysis revealed that for the NIWs with larger horizontal wavelengths, as their interactions with the background currents were weakened, the observed frequency was close to the intrinsic frequency. The near‐inertial kinetic energy (NIKE) between two cyclonic eddies penetrated deeper (∼620 m) than that in a negative vorticity region. A ray‐tracing model suggested that it was the strong positive vorticity to the north that caused the northward propagating NIWs to be reflected, and then the reflected NIWs were accelerated to propagate downward at ∼70 m depth where the stratification was strongest. In these two cases, furthermore, the efficiency of the downward propagation of NIKE was at a comparable level of 21%–25%. Energy budget analysis indicated that about 5%–25% of the near‐inertial wind work was injected into the upper 50–200 m, approximately 6%–25% of which could be further radiated to the deeper ocean. On average, after the passage of the typhoon Lan, the dissipation rate increased by 3–8 times, and for the enhanced diapycnal mixing, ∼42% of the energy was provided by the typhoon‐induced strong NIWs.

The seasonal variability of the eddy kinetic energy (EKE) along the Kuroshio Current (KC) is examined using outputs from an eddy-resolving (1/10°) ocean model. Using a theoretical framework for climatological monthly mean EKE, the mechanisms governing the seasonal cycle of upper-ocean EKE are investigated. East of Taiwan, the EKE shows two comparable peaks in spring and summer in the surface layer; only the spring one is evident in the subsurface layer. The seasonality is determined by mixed barotropic (BTI) and baroclinic (BCI) instabilities. Northeast of Taiwan, the EKE is also elevated during spring–summer but with a sole peak in summer, which is dominated by the meridional EKE advection by the KC. In the middle part of the KC in the East China Sea, the mesoscale (>150 km) EKE (EKE_MS) is relatively strong during spring–summer, whereas the submesoscale (50–150 km) EKE (EKE_SM) is significantly enhanced during winter–spring. The seasonal cycles of EKE_MS and EKE_SM are primarily controlled by the external forcing and BCI, respectively. In particular, the higher EKE_MS level in summer is mainly due to the increased wind work. West of the Tokara Strait, the EKE exhibits a prominent peak in winter and has its minimum in summer, which is regulated by the BCI. As the submesoscale signals are partially resolved by the model, further studies with higher-resolution simulations and observations are needed for a better understanding of the EKE_SM seasonality and its contribution to the seasonally modulating EKE_MS along the KC.

Sedimentary processes in marginal seas play an important role in the biology, physics, and geochemistry as well as ecology of coastal environments and contain abundant information about the material transfer from land to ocean and the regional circulation. Due to the huge sediment discharge of the Yellow River, the Bohai Sea, China is one of the areas with the highest suspended sediment concentration (SSC) in the world. Interestingly, the SSC at the west of the Bohai Sea is low all year round. Thus, it is of great significance to examine the sedimentary dynamic process in this area for better understanding the circulation structure, material exchange and regional environment of the Bohai Sea. Using seabed base observation platform measurements obtained in February 2017 and August 2019, this study examines the winter and summer hydrography and suspended sediment concentration in the sea area off Qinhuangdao located to the west of the Bohai Sea. In summer, the relatively weak residual currents flowed northeastward and showed little correlation with the wind field, especially in the middle layer of the water column. In winter, the residual currents were strengthened, flowing to the northeast during strong wind periods, and predominantly to the southwest during intermittent periods. Moreover, driven by the pressure gradient force associated with the wind-induced sea surface height variations, the winter current was closely related to the wind speed, with correlation coefficients ranging from 0.4 to 0.6 and a time lag of 10 h. The summer SSC was lower and mainly controlled by the tidal current, whereas in winter, owing to the enhanced Reynolds stress and turbulent kinetic energy, strong wind bursts triggered significant sediment resuspension and led to a higher SSC. For the suspended sediment flux (SSF), the advection terms contributed more than 80% in the winter and summer, while the vertical circulation terms contributed 13% in winter, and approximately half that much in summer. Generally, the suspended sediment is transported back and forth, with a little net northward and northeastward motions in winter and summer, respectively. This may explain the low SSC sustaining in coastal Qinhuangdao all year round. These results provide a reference for sedimentary studies conducted in other coastal waters, especially in monsoon-dominating shelf seas.

The influence of sea ice on the hydrodynamics, sediment resuspension, and suspended sediment concentration (SSC) in the coastal area of Qinhuangdao was systematically investigated using 45-day in situ measurements at two stations (with ice at station M1 and without ice at station M2) in the Bohai Sea in the winter of 2018. It was found that the daily fluctuations of temperature and salinity at M1 are more significant than those at M2. During a typical seawater icing event on January 28, the temperature and salinity of the bottom water at M1 were decreased by 1.77 °C and increased by 0.4 psu, respectively. Moreover, due to the shielding effect of the sea ice, the residual current was much less affected by the wind at M1 than at M2. For the vertical distribution of current velocity, it changed from a traditional logarithmic type under ice-free conditions to parabolic type under ice-covered conditions due to the larger drag coefficient of the water body on the solid ice surface. For the SSC and turbidity at the bottom layer, the average values were 4.9 μL/L and 8.6 NTU at M1, respectively, approximately half of those at M2. The smaller SSC and turbidity at M1 can be attributed to the lower near-bottom turbulent kinetic energy (TKE). At M2, however, the larger SSC is closely related to the strong wind forcing, which could induce higher TKE without sea ice cover, and hence stronger turbulent resuspension. The seabed sediment analysis results showed that in the study area, fine sand is most likely to resuspend, while cohesive particles would resuspend only under strong hydrodynamic conditions.