The ocean color elements refer to total suspended sediment (TSS) and chlorophyll-a (Chl-a), which are important parameters for the marine ecological environment. This study aims to examine the behavior of ocean color elements in response to a tropical cyclone in the case of typhoon Mangkhut (2018), which passed over the northern South China Sea (NSCS) on 16 September 2018, using satellite multi-sensor observations, Argo float profiles, and tidal gauge sea level data. The results indicate that typhoon Mangkhut (2018) resulted in TSS and Chl-a concentrations increasing, with the spatial and timing behavior different in the offshore, shelf, and basin areas. In the offshore area from the coast to isobath 50 m, the mean TSS concentration, i.e., C_TSS, reached 13.9 mg/L on 18 September 2018, two days after typhoon landfall, against about 3.5 mg/L before typhoon landfall. In the shelf area with depths from 50 m to 100 m, the mean C_TSS reached 2.5 mg/L, against about 0.8 mg/L before typhoon landfall. In the basin area with depths of 100 m and beyond, the mean C_TSS had only a little fluctuation. On the other hand, in the offshore area, the mean Chl-a concentration, i.e., C_Chl-a, was 7.3 mg/m³ on 21 September, five days after typhoon landfall, against 2.4 mg/m³ as the monthly mean value. Furthermore, TSS concentrations favorable for Chl-a bloom range from 6 to 7 mg/L in this area. In the shelf area, the mean C_Chl-a increased from 0.2 mg/m³ to 0.6 mg/m³ in two days. In the basin area, the C_Chl-a increased from 0.1 mg/m³ to 0.2 mg/m³ during typhoon passage. Concurrent dynamic condition analysis results indicate that, in the offshore area, typhoon-induced solitary continental waves may play a dominant role in determining the spatial distribution features of the TSS originating from the Pearl River runoff. The Chl-a bloom delayed rather than concurrently occurred with the terrigenous nutrient peak, which is attributed to the nonlinear relation between C_Chl-a and C_TSS. In the shelf and basin areas, typhoon-enhanced vertical mixing and upwelling may play dominant roles in determining the spatiotemporal behavior of the TSS and the Chl-a.

Strong typhoon winds enhance turbulent mixing, which induces sediment to resuspend and to promote chlorophyll-a (Chl-a) blooms in the continental shelf areas. In this study, we find limited Chl-a responses to three late autumn typhoons (typhoon Nesat, Mujigae and Khanun) in the northwestern South China Sea (NWSCS) using satellite observations. In climatology, the Chl-a and total suspended sediment (TSS) concentrations are high all year round with higher value in autumn in the offshore area of the NWSCS. After the typhoon passage, the Chl-a concentration increases slightly (23%), while even TSS enhances by 280% on the wide continental shelf of the NWSCS. However, in the southern area, located approximately 100 km from the typhoon tracks, both TSS and Chl-a concentrations increase 160% and 150% after typhoon passage, respectively. In the deeper area, the increased TSS concentration is responsible for the considerable increase of the Chl-a. An empirical analysis is applied to the data, which reveals the TSS and Chl-a processes during typhoon events. The results of this study suggest a different mechanism for Chl-a concentration increase and thus contribute toward further evaluation of typhoon-induced biological responses.

Using cruise observations before and after the typhoon Chebi in August 2013 and those without the typhoon in July 2012, this study investigates variations in current structure, nutrient distribution, and transports disturbed by a typhoon in a typical coastal upwelling zone east of Hainan Island in the northwestern South China Sea. The results show that along-shore northeastward flow dominates the coastal ocean with a volume transport of 0.64 × 10⁶ m³/s in the case without the typhoon. The flow reversed southwestward, with its volume transport halved before the typhoon passage. After the typhoon passage, the flow returned back northeastward except the upper layer in waters deeper than 50 m and the total volume transport decreased to 0.10 × 10⁶ m³/s. For the cross-shelf component, the flow kept shoreward, while transports crossing the 50 m isobath decreased from 0.25, 0.12 to 0.06 × 10⁶ m³/s in the case without the typhoon as well as before and after typhoon passage, respectively. For the along-shore/cross-shelf nutrient transports, SiO₃²⁻ has the largest value of 866.13/632.74 μmol/s per unit area, NO₃⁻ half of that, and PO₄³⁻ and NO₂⁻ one order smaller in the offshore water without the typhoon. The values dramatically decreased to about one-third for SiO₃²⁻, NO₃⁻, and PO₄³⁻ after the typhoon, but changed little for NO₂⁻. The disturbed wind field and associated Ekman flow and upwelling process may explain the variations in the current and nutrient transports after the typhoon.

An Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO) mesoscale eddy trajectory atlas product is used to analyze the path type and temporal variability of the eddies that entered the continental shelf area of the northern South China Sea (SCS) from 1993 to 2016. A total of 184 mesoscale eddies entered the continental shelf area of the northern SCS during a 24-year period. We classify the mesoscale eddies into four types according to the motion trajectories: along-the-isobath type, intrusion-of-continental-shelf type, local wandering type, and shelf-internal-generation type. The occurrence numbers of these four types were 87, 38, 23, and 36, respectively. The mean amplitude and radius of the along-the-isobath type are the largest, about 18 cm and 153 km, respectively; furthermore, their average lifetime is also the longest, about 93 days. The mean amplitude, radius, and lifetime are the smallest for the shelf-internal-generation type, about 16 cm, 146 km, and 74 days, respectively. The direction and velocity of the background flow field affects the intrusion path of the mesoscale eddies onto the continental shelf of the northern SCS. The seasonal distribution of the mesoscale eddies quantity is also related to the direction and velocity of the corresponding background flow field.

The evolution of mesoscale eddies is crucial for understanding the ocean energy cascade. In this study, using global reanalysis sea surface velocity data and a mesoscale eddy trajectory product tracked by satellite altimeters, we aimed to reveal the coherence of eddy kinetic energy (EKE) variation to low-frequency ageostrophic energy during the eddy life span. The variation in EKE throughout the eddy life span was highly coherent to that of the seven-day low-passed ageostrophic kinetic energy, with a correlation coefficient of −0.94. The low-frequency ageostrophic motions supplied 38% of the EKE variation in the growing stage of mesoscale eddies and absorbed 42% in the decaying stage. The evolution rate of the EKE during the eddy life span was consistent with the barotropic conversion rate of the low-frequency ageostrophic motions, further confirming the dominant role of low-frequency ageostrophic motions in eddy growth and decay.