The macroalgal blooms of floating brown algae Sargassum horneri are increasing in the Yellow Sea and East China Sea during the past few years. However, the annual pattern of Sargassum bloom is not well characterized. To study the developing pattern and explore the impacts from hydro-meteorologic environment, high resolution satellite imageries were used to monitor the distribution, coverage and drifting of the pelagic Sargassum rafts in the Yellow Sea and East China Sea from September 2019 to August 2020. Sargassum blooms were detected from October 2019 to June 2020 and presented two successive drifting paths that both initiated from around 37°N. The first path spanned smaller spatial scale and shorter period, starting with a bloom of 3 km² distribution area near the eastern tip of Shandong Peninsula in late October 2019 and drifted southwards, hit the Pyropia aquaculture area in early January 2020, then vanished in the northwest of East China Sea (ca. 32°N) around end of January. The second path began with a large distribution area of 23 000 km² east of 123°E in late January 2020, firstly moved southwards in the central Yellow Sea and northern East China Sea (north of 29°N) till late April, then turned northwards with monsoon wind and vanished from late June to August. The mean sea surface temperature of 8°C to 20°C in the Sargassum bloom areas corresponded to in situ observed temperature range for vegetative growth and floating of S. horneri. There was no observed floating Sargassum blooms during July through September in the Yellow Sea and East China Sea. The results indicate that floating S. horneri is unable to complete life cycle in the Yellow Sea and East China Sea, and provide insights to the future management of Sargassum blooms. Further studies are needed to validate the pattern and source of annual Sargassum bloom in the Yellow Sea and East China Sea.

The Zhujiang River Estuary is becoming eutrophic due to the impact of anthropogenic activities in the past decades. To understand nutrient dynamics and fluxes to the Lingdingyang water via four outlets (Humen, Jiaomen, Hongqimen and Hengmen), we investigated the spatial distribution and seasonal variation of dissolved nutrients in the Zhujiang River Estuary, based on fourteen cruises conducted from March 2015 to October 2017, covering both wet (April to September) and dry (October to March next year) seasons. Our results showed that riverine fluxes of dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi) into the Lingdingyang water through four outlets varied seasonally due to the influence of river discharge, with the highest in spring and the lowest in winter. However, riverine flux of phosphate exhibited little significant seasonal variability. Riverine nutrients into the Lingdingyang water most resulted through Humen Outlet. The estuarine export fluxes of DIN out of the Zhujiang River Estuary derived from a box model were higher than fluxes of riverine nutrients in May, likely due to the influence of local sewage, while lower than riverine flux in August. The export fluxes of phosphate were higher than the fluxes of riverine phosphate in May and August. In contrast, large amounts of DSi were buried in the estuary in May and August. Although excess DIN was delivered into the Zhujiang River Estuary, eutrophication effect was not as severe as expected in the Zhujiang River Estuary, since the light limitation restricted the utilization of nutrients by phytoplankton.

The coastal upwelling has profound influence on the surrounding ecosystem by supplying the nutrient-replete water to the euphotic zone. Nutrient biogeochemistry was investigated in coastal waters of the eastern Hainan Island in summer 2015 and autumn 2016. From perspectives of nutrient dynamics and physical transport, the nutrient fluxes entered the upper 50 m water depth (between the mixed layer and the euphotic zone) arisen from the upwelling were estimated to be 2.5−5.4 mmol/(m²·d), 0.15−0.28 mmol/(m²·d), and 2.2−7.2 mmol/(m²·d) for dissolved inorganic nitrogen (DIN), phosphate (DIP), and dissolved silicate (DSi), respectively, which were around 6- to 12-fold those in the background area. The upwelled nutrients supported an additional plankton growth of (14.70±8.95) mg/m² for chlorophyll a (Chl a). The distributions of nitrate δ¹⁵N and δ¹⁸O above the 300 m water depth (top of the North Pacific Intermediate Water) were different among the upwelling area, background area in summer, and the stations in autumn, and the difference of environmental and biogeochemical conditions between seasons should be the reason. The higher DIN/DIP concentration ratio, nitrate concentration anomaly, and lower nitrate isotope anomaly (Δ(15, 18)) in the upper ocean in summer than in autumn indicated the stronger nitrogen fixation and atmospheric deposition, and the following fixed nitrogen regeneration in summer. The higher values of Chl a and nitrate δ¹⁵N and δ¹⁸O within the euphotic zone in autumn than the background area in summer suggested the stronger nitrate assimilation in autumn. The differences in relatively strength of the assimilation, nitrogen fixation and atmospheric deposition, and the following remineralization and nitrification between the two seasons made the higher δ¹⁸O:δ¹⁵N and larger difference of enzymatic isotope fractionation factors ¹⁵ε and ¹⁸ε for nitrate assimilation in summer than in autumn above the North Pacific Tropical Water.

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.

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.

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.

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.

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.

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.

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.