Strong tropical cyclone (TC) Ockhi occurred in the southeastern Arabian Sea (AS) in 2017. Ockhi greatly changed the oceanic conditions and induced large variation in chlorophyll-a (Chl-a). The dynamic mechanisms of the long-term phytoplankton bloom after the passage of the TC were investigated in this study. Prominent surface ocean responses, e.g., decreasing temperature and salinity, were identified from Argo data by comparing the pre-and post-conditions of the TC. A phytoplankton bloom was observed in southeastern AS after the passage of TC Ockhi within the area of (11˚N-14˚N, 67˚E-70˚E) and lasted for seven days. Interestingly, there were two weaker cyclonic eddies, with an average vorticity of less than 0.14 s-1 , on the TC trajectory from November 28 to December 2. As Ockhi approached, strong vertical mixing occurred on December 3, increasing the eddy vorticity to 0.26 s-1. After the passage of Ockhi, both eddies, with a two-day oscillation period, were substantially enhanced. Especially from December 11 to 16, the vorticity above 70 m was as high as 0.2 s-1 in the thermocline. Because of the high photosynthetically available radiation (PAR) and low precipitation, the enhanced cyclonic eddies induced upwelling for the entire thermocline for over ten days and uplifted nitrates into the mixed layer. This study offers new insights on the influence of eddies in regulating the impacts of typhoons on Chl-a, and the results can help evaluate typhoon-induced biological responses in the future.
The western South China Sea (WSCS) exhibits intense mesoscale eddy activity, especially in summer. However, the influence of the eddies on regional ecosystems is not well understood. By analyzing historical in situ observations, satellite-derived near-surface chlorophyll (CHL) data and eddy data, we investigated the composite response of CHL to eddies in the WSCS. The results show that eddies decrease CHL in the WSCS and that nearly half of this decrease is attributed to the activity of anticyclonic eddies during summer when they are much more energetic than cyclonic eddies. A comparison between local and nonlocal eddies suggests that the CHL response to the eddies is dominated by local dynamics rather than being trapped and transported from neighboring regions. In situ observations provide evidence that intense convergence and downwelling in anticyclonic eddies deepen the mixed layer depth and subsurface CHL maximum depth. The decreased nutrients in the euphotic layer inhibits the growth of phytoplankton, resulting in a weakened subsurface CHL maximum (by 54%) and decreased CHL in the entire euphotic layer (by 41% for the upper 100 m). Considering the greater number and area of anticyclonic eddies compared with cyclonic eddies, eddy activity might cause an upper ocean CHL decrease of~7% in the WSCS in summer. This study demonstrated that the response of CHL to eddies in the WSCS differs in both spatial distribution and seasonal variation from those in other subregions of the SCS, emphasizing the importance of considering regional variability in eddies and large-scale environments when evaluating the biogeochemical influences of eddies.Plain Language Summary Mesoscale eddies, both local and nonlocal, enrich the circulation system in the western South China Sea (WSCS), giving rise to complex regional biogeochemical environments. To gain insights into the role of eddies in regional ecosystems, we investigated the composite near-surface chlorophyll (CHL) anomalies within thousands of eddies in the WSCS. The results show a noticeable near-surface CHL decrease induced by the eddies, and nearly half of this decrease is attributed to the activity of anticyclonic eddies in summer. During this period, intense downwelling in anticyclonic eddies results in much deeper and weaker subsurface CHL maximums than those in cyclonic eddies, as well as the backgrounds. This study demonstrates that the CHL response to eddies in the WSCS differs in both spatial distribution and seasonal variation from those in the northern and eastern SCS, emphasizing the importance of considering regional variability in eddies and large-scale environments when evaluating the biogeochemical influences of eddies.
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