Based on a two-level nested model from the global ocean to the western Pacific and then to the South China Sea (SCS), the high-resolution SCS deep circulation is numerically investigated. The SCS deep circulation shows a basin-scale cyclonic structure with a strong southward western boundary current in summer (July), a northeast-southwest through-flow pattern across the deep basin without a western boundary current in winter (January), and a transitional pattern in spring and autumn. The sensitivity model experiments illustrate that the Luzon Strait deep overflow is the main factor controlling the seasonal variation in the SCS deep circulation. The SCS surface wind can significantly influence the SCS deep circulation in winter. The Luzon Strait deep overflow transport from the Pacific into the SCS ranges from 0.68×10⁶ m³/s to 1.83×10⁶ m³/s, reaching its maximum in summer (July, up to 1.83×10⁶ m³/s), less in autumn and winter, and the minimum in spring (May, 0.68×10⁶ m³/s). In summer, the strong Luzon Strait deep overflow dominates the SCS deep circulation when the role of the SCS surface wind is small. In winter, the weaker Luzon Strait deep overflow and SCS surface wind jointly drive the SCS deep circulation into a northeast-southwest through-flow pattern. The potential vorticity (PV) dissipation in the SCS deep basin reaches its maximum (−0.122 m²/s²) in May and its minimum (−0.380 m²/s²) in July.

Using observations and numerical simulations, this study examines the intraseasonal variability of the surface zonal current (u ISV) over the equatorial Indian Ocean, highlighting the seasonal and spatial differences, and the causes of the differences. Large-amplitude u ISV occurs in the eastern basin at around 80°–90°E and near the western boundary at 45°–55°E. In the eastern basin, the u ISV is mainly caused by the atmospheric intraseasonal oscillations (ISOs), which explains 91% of the standard deviation of the total u ISV. Further analysis suggests that it takes less than ten days for the intraseasonal zonal wind stress to generate the u ISV through the directly forced Kelvin and Rossby waves. Driven by the stronger zonal wind stress associated with the Indian summer monsoon ISO (MISO), the eastern u ISV in boreal summer (May to October) is about 1.5 times larger than that in boreal winter (November to April). In the western basin, both the atmospheric ISOs and the oceanic internal instabilities contribute substantially to the u ISV, and induce stronger u ISV in boreal summer. Energy budget analysis suggests that the mean flow converts energy to the intraseasonal current mainly through barotropic instabilities.

The statistical characteristics and mechanisms of mesoscale eddies in the North Indian Ocean are investigated by adopting multi-sensor satellite data from 1993 to 2019. In the Arabian Sea (AS), seasonal variation of eddy characteristics is remarkable, while the intraseasonal variability caused by planetary waves is crucial in the Bay of Bengal (BOB). Seasonal variation of the eddy kinetic energy (EKE) is distinct along the west boundary of AS, especially in the Somali Current region. In the BOB, larger EKE occurs at the northwest basin from March to May, to the east of Sri Lanka from June to September, and along the east coast of India from November to December. The wind stress work (WW) is further studied to figure out the direct influence of wind forcing on EKE. The WW exerts positive effects on EKE along the west boundary of AS and in the south of India/Sri Lanka during the two monsoon seasons. Besides, the WW also has impact on EKE along the east coast of India in November and December. Eventually, we investigate the characteristics and the driving mechanisms of long lifespan eddies. In the AS, long lifespan anti-cyclonic eddies (AEs) mainly generate in the Socotra, the West Indian Coastal Current and the East Arabian Current regions, while cyclonic eddies (CEs) are concentrated in the northwest region. In the BOB, long lifespan AEs mostly form near the west of Myanmar, while CEs are accumulated at the north and northwest basin. The instabilities caused by Rossby waves, coastal Kelvin waves, seasonal currents, together with wind stress forcing exert enormous efforts on the generation and evolution of these eddies.

This paper presents an efficient algorithm for generating a spherical multiple-cell (SMC) grid. The algorithm adopts a recursive loop structure and provides two refinement methods: (1) an arbitrary area refinement method and (2) a nearshore refinement method. Numerical experiments are carried out, and the results show that compared with the existing grid generation algorithm, this algorithm is more flexible and operable.

Knowledge of sediment variation processes is essential to understand the evolution mechanism of beach morphology changes. Thus, a field measurement was conducted at the Heisha Beach, located on the west coast of the Zhujiang River (Pearl River) Estuary, to investigate the short-term variation in suspended sediment concentrations (SSCs) and the relationship between the SSC and turbulent kinetic energy, bottom shear stress (BSS), and relative wave height. Based on extreme event analysis results, extreme events have a greater influence on turbulent kinetic energy than SSC. Although a portion of the turbulent kinetic energy dissipates directly into the water column, it plays an important role in suspended sediment motion. Most of the time, the wave-current interaction is strong enough to drive sediment incipience and resuspension. When combined, the wave-current interaction and wave-induced BSSs have a greater influence on suspended sediment transport and SSC variation than current-induced BSS alone. The relative wave height also has a strong correlation with SSC, indicating that the combined effect of water depth and wave height significantly impacts SSC variation. Water depth is mainly controlled by the tide on the beaches; thus, the effects of tides and waves should be conjunctively considered when analyzing the factors influencing SSC.

Threatening millions of people and causing billions of dollars in losses, tropical cyclones (TCs) are among the most severe natural hazards in the world, especially over the western North Pacific. However, the response of TCs to a warming or changing climate has been the subject of considerable research, often with conflicting results. In this study, the abilities of Coupled Model Intercomparison Project (CMIP) Phase 6 (CMIP6) models to simulate TC genesis are assessed through historical simulations. The results indicate that a systematic humidity bias persists in most CMIP6 models from corresponding CMIP Phase 5 models, which leads to an overestimation of climatological TC genesis. However, the annual cycle of TC genesis is well captured by CMIP6 models. The abilities of 25 models to simulate the geographical patterns of TC genesis vary significantly. In addition, seven models are identified as well simulated models, but seven models are identified as poorly simulated ones. A comparison of the environmental variables for TC genesis in the well-simulated group and the poorly simulated group identifies moisture in the mid-troposphere as a key factor in the realistic simulation of El Niño-Southern Oscillation (ENSO) impacts on TC genesis. In contrast with the observations, the poorly simulated group does not reproduce the suppressing effect of negative moisture anomalies on TC genesis in the northwestern region (20°–30°N, 120°–145°E) during El Niño years. Given the interaction between TC and ENSO, these results provide a guidance for future TC projections under climate change by CMIP6 models.

Seasonal and interannual variability of ocean bottom pressure (OBP) in the Southern Ocean was investigated using Gravity Recovery and Climate Experiment (GRACE) data and a Pressure Coordinate Ocean Model (PCOM) based on mass conservation. By comparing OBP, steric sea level, and sea level, it is found that at high latitudes the OBP variability dominates the sea level variability at seasonal-to-decadal time scales. The diagnostic OBP based on barotropic vorticity equation has a good correlation with the observations, indicating that wind forcing plays an important role in the variability of the OBP in the Southern Ocean. The unique interannual patterns of OBP in the Southern Ocean are closely associated with El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM). Regression analysis indicates that ENSO and SAM influence the OBP through altering the Ekman transport driven by surface wind. The leading pattern of OBP from PCOM are very similar to observations. Sensitive experiments of PCOM show that surface wind forcing explains the observed OBP variability quite well, confirming the importance of wind forcing and related oceanic processes. In the eastern South Pacific, the averaged OBP shows a decrease (increase) trend before (after) 2011, reflecting the reverse trend in westerly wind. In the South Indo-Atlantic Ocean, the averaged OBP has a weak increase trend during 2003–2016.