Saline water intrusion in relation to strong winds during winter cold outbreaks: North Branch of the Yangtze Estuary

The strong saline water intrusion in the North Branch of the Yangtze Estuary threatens the freshwater supply of the region in winter half year. Strong northerly winds have been identified as a factor increasing saline water intrusion. However, there are few studies on this subject, and the mechanisms of winds influencing saline water intrusion are still unclear. In the present contribution, we investigate the variation trend of strong wind events during cold outbreaks in winter half year and their correlation with saline water intrusion in the North Branch, together with the processes and mechanism of strong winds increasing saline water intrusion, based on observations and an analytical salt water intrusion model. The results indicate that the strong northerly and northeasterly wind events and saline water intrusion in the North Branch have similar variation trends in 1994–2008, both being relatively weak in the 1990s but being intensified dramatically after 1999. The significant correlation between these two trends suggests that the increase in strong wind events may be one of the factors inducing the enhanced saline water intrusion. Observations and model output show that the strong northerly and northeasterly winds can induce dramatic water level setup, increase of flood-tide current velocities, decrease of ebb-tide velocities, and decrease of freshwater inflow into the North Branch. These changes in combination cause the enhanced intensity of saline water intrusion. The Ekman transport from remote winds results in water level setup at the estuary mouth pumping more seawater into the North Branch, which should be a dominant mechanism inducing the change in hydrodynamics and increase of saline water intrusion.