Intra-tidal upwelling variability off Zhoushan Islands, East China Sea

The coastal upwelling that takes place off the Zhoushan Islands is of great importance in the transport of nutrients, nourishing local fisheries and exerting a substantial environmental influence. While it is known that tides play a central role in triggering and controlling the Zhoushan upwelling, its short-term variability within the tidal cycle remains uncertain, thereby constituting the primary focus of this study, which relies on the analysis of high-resolution Sea Surface Temperature (SST) satellite images and in-situ data. In a tidal cycle, cold upwelling patches exhibit a westward shift during the flood tide and an eastward shift during the ebb tide. The extent of cold upwelling patches is greatest during maximum flood and ebb periods. At maximum ebb, the core temperature within the cold patches notably decreases, which indicates intensified upwelling. At the same time, a similar pattern of sudden increases is also observed throughout the entire water column in the upwelling region. The strong ebb current swiftly transports the upper water mass during maximum ebb, facilitating the compensation of lower water and augmenting the intensity of the upwelling. The calculated vertical velocity within the study area ranged from −9.04 × 10−4 m/s to 12.66 × 10−4 m/s during the observational period, when upwelling showed a significant congruence with low-temperature phenomena. Alternating upwelling and downwelling phases were discernible within a tidal cycle, with upwelling being prevalent. The influence of surface heat flux on seawater below the thermocline and night-time SST was negligible. Furthermore, compared to the vertical velocity, the calculated wind-induced upwelling velocity was much lower, reaching just 3.38 × 10−6 m/s during the observed tidal cycle, which suggests that wind does not exert a primary influence on upwelling within the tidal cycle. This study contributes to an enhanced understanding of upwelling variability under the influence of tides, a topic of paramount importance in the marine environment.