Spatiotemporal Distributions of Ocean Color Elements in Response to Tropical Cyclone: A Case Study of Typhoon Mangkhut (2018) Past over the Northern South China Sea

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., CTSS, 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 CTSS 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 CTSS had only a little fluctuation. On the other hand, in the offshore area, the mean Chl-a concentration, i.e., CChl-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 CChl-a increased from 0.2 mg/m3 to 0.6 mg/m3 in two days. In the basin area, the CChl-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 CChl-a and CTSS. 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.