Highly productive estuaries facilitate intense decomposition of dissolved organic matter (DOM) as a carbon source. However, the specific impacts of typhoons on DOM decomposition in eutrophic bays remain unclear. To address this issue, we investigated the spectral characteristics of DOM before and after Typhoon Ewiniar in Zhanjiang Bay, a eutrophic semi-enclosed bay in the northwestern South China Sea. The results revealed that intense microbial decomposition of DOM occurred during the pre-typhoon period because high nutrient inputs facilitated the mobilization of DOM in the bay. However, the intrusion of external seawater induced by the typhoon diluted the nutrient levels in Zhanjiang Bay, reducing the impact of microbial decomposition on DOM during the post-typhoon period. Nevertheless, the net addition of DOM occurred in Zhanjiang Bay during the post-typhoon period, possibly because of the decomposition of particulate organic matter (POM) and desorption of particulate matter. In addition, an increase in apparent oxygen utilization, a decrease in DO saturation and the reduced level of Chl a indicated that organic matter (OM) decomposition was enhanced and OM decomposition shifted to POM decomposition in Zhanjiang Bay after the typhoon. Overall, our study highlighted the shift in the intense OM decomposition from DOM to POM decomposition before and after typhoons in eutrophic bays, providing new insights into the response of typhoons to biogeochemistry.

Ocean fronts play important roles in nutrient transport and in the shaping ecological patterns. Frontal zones in small bays are typically small in scale, have a complex structure, and they are spatially and temporally variable, but there are limited data on how biological communities respond to this variation. Hangzhou Bay, a medium-sized estuary in China, is an ideal place in which to study the response of plankton to small-scale ocean fronts, because three water masses (Qiantang River Diluted Water, Changjiang River Diluted Water, and the East China Sea current) converge here and form dynamic salinity fronts throughout the year. We investigate zooplankton communities, and temperature, salinity and chlorophyll a (Chl a) in Hangzhou Bay in June (wet period) and December (dry period) of 2022 and examine the dominant environmental factors that affect zooplankton community spatial variability. We then match the spatial distributions of zooplankton communities with those of salinity fronts. Salinity is the most important explanatory variable to affect zooplankton community spatial variability during both wet and dry periods, in that it contributes >60% of the variability in community structure. Furthermore, the spatial distributions of zooplankton match well with salinity fronts. During December, with weaker Qiantang River Diluted Water and a stronger secondary Changjiang River Plume, zooplankton communities occur in moderate salinity (MS, salinity range 15.6 ± 2.2) and high salinity (HS, 22.4 ± 1.7) regions, and their ecological boundaries closely match the Qiantang River Diluted Water front. In June, different zooplankton communities occur in low salinity (LS, 3.9 ± 1.0), MS (11.7 ± 3.6) and HS (21.3 ± 1.9) regions. Although the LS region occurs abnormally in the central bay rather than its apex because of the anomalous influence of rising and falling tides during the sampling period, the ecological boundaries still match salinity interfaces. Low-salinity or brackish-water zooplankter taxa are relatively more abundant in LS or MS regions, and the biomass and abundance of zooplankton is higher in the MS region.

The sinking of phytoplankton is critical to organic matter transportation in the ocean and it is an essential process for the formation of coastal hypoxic zones. This study was based on a field investigation conducted during the summer of 2022 in the Changjiang River (Yangtze River) Estuary (CJE) and its adjacent waters. The settling column method was employed to measure the sinking velocity (SV) of different size fractions of phytoplankton at the surface of the sea and to analyze their environmental control mechanisms. The findings reveal significant spatial variation in phytoplankton SV (−0.55–2.41 m/d) within the CJE. High-speed sinking was predominantly observed in phosphate-depleted regions beyond the CJE front. At the same time, an upward trend was more commonly observed in the phosphate-rich regions near the CJE mouth. The SV ranges for different size-fractionated phytoplankton, including micro- (>20 μm), nano- (2–20 μm), and picophytoplankton (0.7–2 μm), were −0.50–4.74 m/d, −1.04–1.59 m/d, and −1.24–1.65 m/d, respectively. Correlation analysis revealed a significant negative correlation between SV and dissolved inorganic phosphorus (DIP), implying that the influence of DIP contributes to SV. The variations in phytoplankton alkaline phosphatase activity suggested a significant increase in SV across all size fractions in the event of phosphorus limitation. Phytoplankton communities with limited photosynthetic capacity (maximum photochemical efficience, Fv/Fm < 0.3) were found to have higher SV than that of communities with strong capacity, suggesting a link between sinking and alterations in physiological conditions due to phosphate depletion. The findings from the in situ phosphate enrichment experiments confirmed a marked decrease in SV following phosphate supplementation. These findings suggest that phosphorus limitation is the primary driver of elevated SV in the CJE. This study enhances the comprehension of the potential mechanisms underlying hypoxic zone formation in the CJE, providing novel insights into how nearshore eutrophication influences organic carbon migration.

Oxygen facilitates the breakdown of the organic material to provide energy for life. The concentration of dissolved oxygen (DO) in the water must exceed a certain threshold to support the normal metabolism of marine organisms. Located in the northern Beibu Gulf, Qinzhou Bay receives abundant freshwater and nutrients from several rivers which significantly influence the level of the dissolved oxygen. However, the spatial-temporal variations of DO as well as the associated driving mechanisms have been rarely studied through field observations. In this study, a three-dimensional coupled physical-biogeochemical model is used to investigate the spatial and seasonal variations of the DO and the associated driving mechanisms in Qinzhou Bay. The validation against observations indicates that the model can capture the seasonal and inter-annual variability of the DO concentration with the range of 5−10 mg/L. Sensitivity experiments show that the river discharges, winds and tides play crucial roles in the seasonal variability of the DO by changing the vertical mixing and stratification of the water column and the circulation pattern. In winter, the tide and wind forces have strong effects on the DO distribution by enhancing the vertical mixing, especially near the bay mouth. In summer, the river discharges play a dominant role in the DO distribution by inhibiting the vertical water exchange and delivering more nutrients to the Bay, which increases the DO depletion and results in lower DO on the bottom of the estuary salt wedge. These findings can contribute to the preservation and management of the coastal environment in the northern Beibu Gulf.

Massive bodies of low-oxygen bottom waters are found in coastal areas worldwide, which are detrimental to coastal ecosystems. In summer 2020, the response of coastal hypoxia to extreme weather events, including a catastrophic flooding, an extreme marine heatwave, and Typhoon Bavi, is investigated based on multiple satellite, four cruises, and mooring observations. The extensive fan-shaped hypoxia zone presents significant northward extension during July−September 2020, and is estimated as large as 13 000 km² with rather low oxygen minimum (0.42 mg/L) during its peak in 28−30 August. This severe hypoxia is attributed to the persistent strong stratification, which is indicated by flood-induced larger amount of riverine freshwater input and subsequent marine heatwave off the Changjiang River Estuary. Moreover, the Typhoon Bavi has limited effect on the marine heatwave and coastal hypoxia in summer 2020.

Except for conventional mesoscale eddies, there are also abundant warm cyclonic eddies (WCEs) and cold anticyclonic eddies (CAEs) in the global ocean. Based on the global mesoscale eddy trajectory atlas product, satellite altimetric and remote sensing datasets, and three-dimensional temperature/salinity dataset, spatiotemporal features of WCEs and CAEs are compared with traditional cold cyclonic eddies and warm anticyclonic eddies in the Kuroshio Extension (KE; 28°−43°N, 140°−170°E) region. Characteristics of abnormal eddies like radius, amplitude, eddy kinetic energy, and proportion in all eddies behave in significant asymmetry on the north and south sides of the KE jet. Unlike eddies in the general sense, temporal feature analysis reveals that it is more favorable to the formation and maintenance of WCEs and CAEs in summer and autumn, while winter is the opposite. The spatiotemporal variation of abnormal eddies is likely because the marine environment varying with time and space. Statistically, proportion of abnormal eddies increases rapidly in decaying stage during the whole eddy lifespan, resulting in smaller average radius, amplitude, sea surface temperature anomaly and sea surface height anomaly compared to normal ones. The three-dimensional composite structures for four types of eddies expose that the difference between abnormal and conventional eddies is not just limited to the sea surface, but also exists within the water below the sea surface. Vertical structures also indicate that the anomalous temperature signal is confined in the water from the sea surface to layers at about 30 m in the KE region.

The ocean conditions beneath the ice cover play a key role in understanding the sea ice mass balance in the polar regions. An integrated high-frequency ice-ocean observation system, including Acoustic Doppler Velocimeter, Conductivity-Temperature-Depth Sensor, and Sea Ice Mass Balance Array (SIMBA), was deployed in the landfast ice region close to the Chinese Zhongshan Station in Antarctica. A sudden ocean warming of 0.14℃ (p < 0.01) was observed beneath early-frozen landfast ice, from (−1.60 ± 0.03)℃ during April 16–19 to (−1.46 ± 0.07)℃ during April 20–23, 2021, which is the only significant warming event in the nearly 8-month records. The sudden ocean warming brought a double rise in oceanic heat flux, from (21.7 ± 11.1) W/m² during April 16–19 to (44.8 ± 21.3) W/m² during April 20–23, 2021, which shifted the original growth phase at the ice bottom, leading to a 2 cm melting, as shown from SIMBA and borehole observations. Simultaneously, the slowdown of ice bottom freezing decreased salt rejection, and the daily trend of observed ocean salinity changed from +0.02 d⁻¹ during April 16–19, 2021 to +0.003 d⁻¹ during April 20–23, 2021. The potential reasons are increased air temperature due to the transit cyclones and the weakened vertical ocean mixing due to the tide phase transformation from semi-diurnal to diurnal. The high-frequency observations within the ice-ocean boundary layer enhance the comprehensive investigation of the ocean’s influence on ice evolution at a daily scale.

Accurate simulation of the evolution of freak waves by the wave phase focusing method requires accurate linear and nonlinear properties, especially in deep-water conditions. In this paper, we analyze the ability to simulate deep-water focused waves of a two-layer Boussinesq-type (BT) model, which has been shown to have excellent linear and nonlinear performance. To further improve the numerical accuracy and stability, the internal wave-generated method is introduced into the two-layer Boussinesq-type model. Firstly, the sensitivity of the numerical results to the grid resolution is analyzed to verify the convergence of the model; secondly, the focused wave propagating in two opposite directions is simulated to prove the symmetry of the numerical results and the feasibility of the internal wave-generated method; thirdly, the limiting focused wave condition is simulated to compare and analyze the wave surface and the horizontal velocity of the profile at the focusing position, which is in good agreement with the measured values. Meanwhile the simulation of focused waves in very deep waters agrees well with the measured values, which further demonstrates the capability of the two-layer BT model in simulating focused waves in deep waters.

Synthetic aperture radar (SAR) and wave spectrometers, crucial in microwave remote sensing, play an essential role in monitoring sea surface wind and wave conditions. However, they face inherent limitations in observing sea surface phenomena. SAR systems, for instance, are hindered by an azimuth cut-off phenomenon in sea surface wind field observation. Wave spectrometers, while unaffected by the azimuth cutoff phenomenon, struggle with low azimuth resolution, impacting the capture of detailed wave and wind field data. This study utilizes SAR and surface wave investigation and monitoring (SWIM) data to initially extract key feature parameters, which are then prioritized using the extreme gradient boosting (XGBoost) algorithm. The research further addresses feature collinearity through a combined analysis of feature importance and correlation, leading to the development of an inversion model for wave and wind parameters based on XGBoost. A comparative analysis of this model with ERA5 reanalysis and buoy data for of significant wave height, mean wave period, wind direction, and wind speed reveals root mean square errors of 0.212 m, 0.525 s, 27.446°, and 1.092 m/s, compared to 0.314 m, 0.888 s, 27.698°, and 1.315 m/s from buoy data, respectively. These results demonstrate the model’s effective retrieval of wave and wind parameters. Finally, the model, incorporating altimeter and scatterometer data, is evaluated against SAR/SWIM single and dual payload inversion methods across different wind speeds. This comparison highlights the model’s superior inversion accuracy over other methods.

The Agulhas system is the strongest western boundary current system in the Southern Hemisphere and plays an important role in modulating the Indian-to-Atlantic Ocean water exchange by the Agulhas leakage. It is difficult to measure in situ transport of the Agulhas leakage as well as the Agulhas retroflection position due to their intermittent nature. In this study, an innovative kinematic algorithm was designed and applied to the gridded altimeter observational data, to ascertain the longitudinal position of Agulhas retroflection, the stability of Agulhas jet stream, as well as its strength. The results show that the east-west shift of retroflection is related neither to the strength of Agulhas current nor to its stability. Further analysis uncovers the connection between the westward extension of Agulhas jet stream and an anomalous cyclonic circulation at its northern side, which is likely attributed to the local wind stress curl anomaly. To confirm the effect of local wind forcing on the east-west shift of retroflection, numerical sensitivity experiments were conducted. The results show that the local wind stress can induce a similar longitudinal shift of the retroflection as altimetry observations. Further statistical and case study indicates that whether an Agulhas ring can continuously migrate westward to the Atlantic Ocean or re-merge into the main flow depends on the retroflection position. Therefore, the westward retroflection may contribute to a stronger Agulhas leakage than the eastward retroflection.