## 3 Results
### Salinity Structure Change by the Typhoon  
Figure 13 is surface salinity distribution with and without the typhoon at 5:00 on 19 September in the rising period, 7:00 on 23 September in the stable period and 9:00 on 25 September 2014 in the falling period of Typhoon Fongwong. Those three different times are at the same tidal phase. The six surface salinity distributions have similar patterns, i.e., high salinity in an offshore area and a salinity reduction towards the upstream area, except for shoal areas with high salinity. The main differences at three typical times of the typhoon focus on the area around the mouth. Compared with the scenario without the typhoon, the salinity distribution at the mouth area has a significant difference. Figure 3 shows the wind directions are NW, SW and NW on 19, 23 and 25 September, respectively, which indicate that the wind drives the saline water more to the open sea, making the salinity decrease at NB1. According to Figure 10a, salinity variations have significantly different styles that, at the surface salinity at NB2, have a semi-diurnal changing cycle, while this becomes irregular at NB1. Meanwhile, the surface salinity at NB1 is much higher than that at NB2, which means the effect of the typhoon on salinity at the mouth is much stronger than that in the upstream area of the NB. The surface salinity at NB1 also clearly presents a decreasing trend around 24 September 2014 due to the wind direction changing from S/SW to W/NW, which corresponds to the varying storm surge. The wind speed and air pressure also has a decreasing trend around 24 September, which indicates that the salinity variation is also related to the atmospheric pressure due to the wind speed having a direct relationship with the pressure gradient. Hence, the wind is a key influence factor on salinity change. A high surface salinity is maintained during the typhoon falling period (the wind speed is below 6 m/s), which shows that salinity change does not synchronize with the typhoon process and the impacts of the typhoon on salinity can be longer lasting. Figure 12: Contours of difference of tidal level (DTL) between typhoon and without typhoon and surface current velocity vector fields at 5:00 on 19 September 2014 (**a**) and at 7:00 on 23 September 2014 (**b**) with the strongest storm surge. Figure 10b is the time history of surface salinity, tidal level, current speed and air pressure without the typhoon. Compared with Figure 10a, the surface salinities under the scenarios with the typhoon and without the typhoon at NB1 have significant differences during the period from 22 September to 25 September due to different dominant factors. The former is dominated by wind with a low salinity, while the latter is dominated by tide with a high salinity. It also indicates that the salinity in the mouth area of NB is more sensitive to the impact of the typhoon. Water column stratification was determined by estimating the stratification parameters from each salinity profile. To better explain the salinity vertical distribution during the typhoon period, a stratification parameter \(n_{s}\), defined by [PERSON] et al. [54], was given by:  
\[n_{s}=\frac{S_{bott}-S_{surf}}{\frac{1}{2}(S_{surf}+S_{bott})} \tag{4}\]  
where \(S_{surf}\) and \(S_{bott}\) are the surface and bottom salinity, respectively. If \(n_{s}<0.1\), the water column is well mixed; if \(0.1<n_{s}<1.0\), the water column is partially mixed and if it is above 1.0, stratification with the presence of the salt wedge is evident. Table 2 is the \(n_{s}\) at points along the talweg on 19 and 25 September, respectively. Before the typhoon, except for point 1, point 3 and point 4, the \(n_{s}\) value is entirely above 0.1 and the maximum value reaches 0.528, which means that the salinity stratification exists. However, after the typhoon, the \(n_{s}\) value is entirely below 0.1, which indicates that the water column is well mixed. Figure 13: With typhoon and without typhoon.