**1. Introduction**

As one of the important dynamic processes in continental shelf seas, coastal upwelling is capable of bringing the cold and nutrient-rich subsurface water to the surface [1–3]. Thus, it plays a crucial role in the local fishery [4, 5], sedimentation process [6, 7], and weather system [8, 9]. Meanwhile, the coastal upwelling is of significance for the air-sea CO2 flux and the global carbon cycle [10, 11]. Hence, oceanographers have paid much attention to the coastal upwelling in various ocean areas [12–18].

The wind-forced offshore Ekman transport is the general mechanism for the coastal upwelling [16, 19–21]. In recent decades, with global climate change, the long-term and interannual variability of upwelling, particularly its response to extreme events, such as the El Niño-Southern Oscillation (ENSO) events, has been paid more and more attentions [22–27]. On the other hand, the coastal upwelling in low latitudes is susceptible to frequently passing tropical cyclones. The sudden impacts by typhoon passages on the upwelling process are worth pursuing [28–31].

As shown in **Figure 1**, upwelling east of Hainan Island (UEH) is seasonal coastal upwelling in the northwestern South China Sea (SCS). It occurs mostly within a

#### **Figure 1.**

*Climatological monthly mean SST (a–c), wind stress vector (d–f), and wind stress curl (g–i) in summer months of June, July, and August in northern SCS. Black boxes A and B represent the coastal upwelling zone of the UEH and the background zone without upwelling in deep water, respectively.*

range of 40 km off the coastline in summer months as the southwesterly monsoon prevails [16, 21, 32]. The wind-driven offshore Ekman transport and Ekman pumping, as well as the topography effect, are main mechanisms of the UEH [33–35]. Since pioneer studies in the 1960s, researchers have investigated the hydrographic features and the variability of the UEH [16, 31, 33, 36–39]. In recent years, the 3D vertical circulation and diapycnal turbulent mixing in the UEH zone have been investigated from cruise observations [21, 40].
