## 1 Introduction  
Saline water intrusion is a common phenomenon in estuaries, which is affected by different spatial-temporal hydrodynamic forces such as river discharge, sea level rise, tides, waves and cyclonic conditions [1]. Severe saline water intrusion, which may be caused by low river discharge and sea level rise (SLR), can impair fresh water supplies in local regions and threaten aquatic plants and animals [2]. Occasionally, estuaries are exposed to meteorological forces. The perturbation of winds and the fluvial flow interacting with tides increase the complexity of the dynamics and salinity distributions in the estuary system. River discharge is one of the dominant forces used to control saline water intrusion in estuaries [3; 4; 5; 6]. Generally, saline water intrusion is inversely correlated to river discharge, i.e., a low river discharge results in an increased saline water intrusion. For example, in the dry season, low river discharge can result in the salinity front shift about 20 km toward upstream in the Bengal delta [7]. The relationship between saline water intrusion, length and river discharge follows a power law with an exponent of n, which varies in different estuaries [8; 9; 10; 11]. The asymmetries of saline water intrusion responding to increasing and decreasing river discharge were observed in the Modaomen estuary by [PERSON] and [PERSON] [11]. River discharge also affects water vertical mixing. For example, the Hong Kong coastal water is stratified or has a large salinity gradient in the vertical direction in the wet season due to the large runoff from the Pearl River [12; 13]. In contrast, it is fully mixed in the vertical direction in the dry season due to low river discharge [14; 15]. Salinity distribution in an estuary depends not only on river discharge, but also wind and tidal mixing. The relationship between saline water intrusion and tidal mixing is complex [5; 6] and depends on different estuary types due to the different salinity transport mechanisms. For a well-mixed or salt wedge estuary, larger saline water intrusion occurs during the spring tides [16; 17; 18]. However, for a partially mixed estuary, larger saline water intrusion happens during near tides [3; 10; 17; 19]. Compared with the extensive literature on saline water intrusion variability induced by the spring-neap tidal cycle and seasonal variations in river discharge [17; 18; 19; 20; 21; 22], studies of climate events, such as wind-induced salt transport are surprisingly lacking, and the mechanism of salt transport is not sufficiently documented. Wind increases the complexity of dynamics and saline water intrusion in the estuary [23; 24], which has long been considered to cause a decrease in stratification. Several field surveys conducted during summer have shown that a change in wind direction to the E or NE can cause the vertical mixing of Hong Kong coastal water, and it may also change the spatial distribution of phytoplankton biomass [25]. [PERSON] et al. [26] studied in detail effect of wind speed and direction on summer tidal circulation and vertical mixing in Hong Kong waters using a 3D numerical model. They revealed that a strong NE wind pushed the river plume westward away from Hong Kong waters and more saline coastal waters entered Hong Kong waters; the water only became vertically well mixed after a 10 m/s NE wind blew for 5 days, but wind speeds of 5 and 7.5 m/s did not result in the same extent of mixing. [PERSON] and [PERSON] [27] summarized the wind's effect on stratification and suggested the two mechanisms of direct wind mixing and wind straining on stratification and salt transport. They used a horizontal Richardson number modified to include wind straining/mixing to reasonably represent the transition, and a regime diagram to classify the wind's effect on stratification. The variability of salt transport ultimately affects exchange flow because it governs the variability of salt intrusion, and thus the large-scale salt gradient that drives gravitational circulation.