Extreme Heatwave Affects the Saltwater Intrusion and River Plume Extension in the Changjiang River Estuary

In 2022, China experienced an extreme hot summer accompanied by record-breaking droughts. Although extensive attentions have been paid on its influences on the urban and watershed systems, how the heatwave affects the hydrodynamics in marginal seas, which is essential for the environment and security in the coastal area, remains unclear. Here in this study, we examined effects of the heatwave on the Changjiang River Estuary and adjacent shelf seas with a numerical model. The results indicated that direct heating over the sea has dual effects on both saltwater intrusion into the estuary and river plume extension. First, strong heating causes intensified stratification, increasing the potential energy anomaly (PEA) value by approximately 7%. This weakens the vertical mixing, thus maintains a low-salinity in the surface. Second, on the contrary, extreme heating induces strong evaporation, which increases the sea surface salinity. In the estuary area, the heating-induced stratification and evaporation-induced salinization are both in favor of a stronger estuarine circulation, and therefore increase the salinity in the estuary. This worsened the already severe situation of freshwater supplement in that year. In the river plume area, the heating-induced stratification resulted a stronger plume extension, but was overwhelmed by the evaporation-induced salinization. Consequently, the plume area decreased by 8.52%. In contrast, in normal years, precipitation dominates in summer due to the heavy rainfall, and thus the co-work of heating-induced stratification and precipitation-induced freshening further enhances the plume extension. This study contributes to understanding of the interlinked effects of extreme climate events on shelf and estuarine systems.