Hanshi Wang

Ocean turbulence, as a critical physical process in marine hydrodynamics, holds significant guiding implications for the development of multiple disciplines and has emerged as a research hotspot in recent years. However, constrained by the limitations of traditional oceanographic instruments that rely solely on single-point measurements, our observations and studies of oceanic turbulence remain considerably inadequate. To extend the scope of oceanic turbulence observations from acquiring single-point information to capturing three-dimensional (3D) field distributions, this study builds upon previous work by introducing Holographic Astigmatic Particle Tracking Velocimetry (HAPTV) and successfully measuring a 3D micro-scale turbulence field generated under laboratory conditions. Analysis of the measured results indicates that this technique has yielded reliable turbulence field parameters, demonstrating its capability to measure 3D turbulence fields and its potential for conducting in-depth research in this area.

During tropical cyclones (TCs), intense exchanges of momentum, heat, and moisture occur across the air–sea interface. The present study was conducted to investigate the role of surface roughness parameterizations under such conditions. To this end, a series of sensitivity experiments was conducted with a focus on Tropical Cyclone Biparjoy, which originated from the Indian Ocean in 2023. The experiments evaluate the impact of different schemes for momentum, thermal, and moisture roughness length on TC track, intensity, significant wave height, and air–sea heat fluxes. The results indicate that the momentum roughness length scheme is critical for accurately forecasting TC track and intensity and for simulating significant wave height; furthermore, Drennan’s parameterization yielded slightly better results in this case, with the smallest track error (72.0 km MAE) among the momentum schemes. Under the premise that Drennan’s parameterization scheme has high accuracy in momentum roughness, sensitivity experiments on thermal and moisture roughness parameterization were conducted. The Drennan–Fairall2014 combination achieved the lowest errors in TC central pressure (4.25 hPa RMSE) and the maximum sustained wind speed (5.31 m/s RMSE). Thermal and moisture roughness mainly affects the efficiency of turbulent heat transfer between the ocean and the atmosphere and thus has a limited impact on the cooling of sea surface temperature, with SST RMSE differences among schemes within 0.3 °C. This effect is mainly confined to the uppermost ocean layer and does not significantly change the thermal structure of the upper layers.

Salinity extremes (SEs) play a crucial role in marine ecosystems, ocean circulation, and climate variability. Understanding their distribution and drivers is essential for predicting changes in ocean salinity under climate change, particularly in dynamic regions such as the Hawaiian Islands, where mesoscale eddies significantly modulate water mass properties. This study investigates the three-dimensional characteristics of SEs and their responses to mesoscale eddies using mooring observations and sea surface salinity data. We find that high salinity extremes (HSEs) generally occur more frequently than low salinity extremes (LSEs) in the study region, though LSEs exhibit greater duration and intensity. Mesoscale eddies modulate SEs significantly—anticyclonic eddies (AEs) enhance LSEs, whereas cyclonic eddies (CEs) promote HSEs in the upper layer. This relationship reverses in the deeper layer, with AEs favoring HSEs and CEs enhancing LSEs. These opposing effects are driven by a vertical displacement of the subsurface salinity maximum layer, where CEs lift high-salinity subsurface water to the upper ocean via upwelling, creating HSEs in the upper layer and LSEs in the deeper layer, while AEs subduct high-salinity water downward, reducing upper-layer salinity (LSEs) but increasing deeper-layer salinity (HSEs) via downwelling. Spatially, CEs exhibit a single-core high-salinity anomaly, displaced westward by 0.3 times of the eddy radius from the eddy center, with HSEs peaking in frequency and intensity near the core. In contrast, AEs display a dipole salinity anomaly (low northwest/high southeast), aligning with LSE frequency distribution, while HSEs show an inverse pattern. Mooring data further reveal that AE-LSE co-occurrence is highest within 1.2 times of the eddy radius, whereas CE-HSE probability declines with eddy intensity. Notably, AE-HSE and CE-LSE probabilities, though initially weaker, surpass AE-LSE and CE-HSE at certain depths, underlining the complexity of depth-dependent eddy modulation. These findings may advance understanding of ocean salinity dynamics and provide insights into how mesoscale processes modulate extreme events, with implications for marine biogeochemistry and climate modeling.

Wave-induced mixing can enhance vertical mixing in the upper ocean, facilitating the exchange of heat and momentum between the surface and deeper layers, thereby influencing ocean circulation and climate patterns. Building on previous research, this study proposes a wave-induced mixing parameterization scheme (referred to as EXP3) specifically designed for typhoon periods. This scheme was integrated into the fully coupled ocean–wave–atmosphere model COAWST and applied to analyze Typhoon In-Fa (2021) as a case study. The simulation results were validated against publicly available data, demonstrating a good overall match with observed phenomena. Subsequently, a comparative analysis was conducted between the EXP3 scheme, the previous scheme (EXP2) and the original model scheme (EXP1). Validation against Argo and Drifter buoy data revealed that both EXP2 and EXP3, which include wave-induced mixing effects, resulted in a decrease in the simulated mixed layer depth (MLD) and mixed layer temperature (MLT), with EXP3 showing closer alignment with the observed data. Compared to the other two experiments, EXP3 enhanced vertical motion in the ocean due to intensified wave-induced mixing, leading to increased upper-layer water divergence and upwelling, a decrease in sea surface temperature and accelerated rightward deflection of surface currents. This phenomenon not only altered the temperature structure of the ocean surface layer but also significantly impacted the regional ocean dynamics.

The drag coefficient of the sea surface is crucial for the exchange of momentum between the ocean and atmosphere. The wave state significantly influences the variability of the drag coefficient. In the past, researchers commonly employed single-parameterization methods to describe this influence. However, the influence of wave conditions on drag coefficient is complex and variable, and it is difficult to accurately describe it with a single parameter alone. Wave age represents the ability of wind-induced waves to input energy, while wave steepness reflects the stability of the waves. By simultaneously considering wave age and wave steepness, a more accurate characterization of the dynamic nature of waves can be achieved. Additionally, the presence of sea spray profoundly impacts the distribution of the momentum flux between the ocean and atmosphere, thereby influencing the drag coefficient of the sea surface. In this study, we established a novel sea spray generation function that bases on both the wind speed and wave states (wave steepness and wave age). Considering this function, the momentum flux of sea spray droplets was analyzed under different wave states. Moreover, with increasing wave age or wave steepness, the effective sea surface drag coefficient is attenuated at low to moderate wind speeds. Considering the challenge of simultaneously obtaining wave age and wave steepness data, this paper proposes a relationship equation between the two wave state parameters. When the wave age is greater than 0.4, the correlation between the wave age and the wave steepness is strong. As the wind speed increases from low to high, there is a noticeable decrease in the effective sea surface drag coefficient with the corresponding increase in wave age. When the wave age is less than 0.4, the wave steepness reaches a maximum value, and the effective sea surface drag coefficient increases with the increase of the wave age at medium and low wind speed. With further increases in wind speed, the momentum flux derived from the air also increases. Simultaneously, the effective sea surface drag coefficient exhibits a decrease as wave age increases.