The vital role of current-topography interaction and tidal mixing in the upwelling around Zhoushan Archipelago

Coastal upwelling plays a critical role in the marine biogeochemical environment by bringing nutrient-rich deep water to the surface, thereby fostering the development of high-biomass fishing grounds, such as the Zhoushan upwelling system in China. High-resolution Level-2 satellite-derived Sea Surface Temperature (SST) data captured notable upwelling events in the Zhoushan Archipelago, even under persistent northerly upwelling-unfavorable winds lasting over one week. Further analysis revealed that these upwelling-unfavorable winds not only generated onshore Ekman transport but also induced a negative wind stress curl, both of which acted to suppress upwelling. However, the weighted cumulative topographic position index demonstrated a quasi-steady upwelling intensity despite the unfavorable wind forcing. Under upwelling-favorable winds, a northward coastal current dominates the Zhoushan Archipelago offshore region, whereas a southward coastal current prevails under upwelling-unfavorable winds. The Zhoushan Archipelago features two prominent meridional topographic slopes formed by its two island chains. When northward or southward coastal current interacts with these slopes, upslope motion occurs on the stream-facing side, facilitating the upwelling of deeper cold water. Additionally, strong tidal current within the shallow waters surrounding the islands generates intense tidal mixing, further enhancing the upward transport of colder water to the surface. The locally enhanced current-topography interaction, tidal mixing, and leeward upwelling nearby the islands collectively trigger multiple cold upwelling cores around the islands, leading to a heterogeneous spatial distribution of upwelling around Zhoushan Archipelago.