## 3 Results and Discussion
### Reconstruction Results of Ocean Front Sound Speed Field Model  
We used FCM to cluster the surface sound speed in the study area, and the results are shown in the figure below (Figure 5). We find that the position, width, and range of the KEF are different at different times. In order to verify the validity of the results of FCM, we calculated the horizontal temperature gradient of the sea surface in the study area to obtain the detection results of the frontal zone under the traditional gradient method (the critical value is 0.05 \({}^{\circ}\)C/km) (Figure 6). We found that the detection results of the frontal zone were similar to those of the FCM, especially in winter (Figure 6a,b). Compared with those of the gradient method, the detection results of FCM led to a more continuous frontal zone in the range, which is helpful for us to obtain the starting and ending positions of the frontal zone on the relevant sections and realize effective modeling. However, the continuity of the judgment results of the temperature gradient method is very poor, and this discontinuity may become worse due to different judgment standards. Figure 4: The technical route of the research process. We used FCM to cluster the surface sound speed and detect the frontal zone of KEF; At the same time, sound speed profiles were used to establish the model of the sound speed field through the melt function. Next, the model was compared with the actual ocean front calculated by reanalysis data, which proved its validity. Finally, we proposed the melt function in the model to forecast the depth change of the acoustic convergence area. The 144 \({}^{\circ}\)E was selected as the characteristic section (the direction is 38\({}^{\circ}\) N-36\({}^{\circ}\) N). According to the results of FCM (Figure 5) and the function curve of melt function \(m\) with melt parameter \(a\) (Figure 3), we identified the position and range of the frontal zone of KEF at different times as well as the selection of the corresponding melt parameters (Table 1). The melt parameters \(a\) of the model of the sound speed field of the ocean front in four periods were determined as 0.2, 0.3, 0, and \(-\)0.1, respectively. Taking the experiment on 1 January as an example, we obtained the sound speed profiles of water masses on both sides of the frontal zone ((38\({}^{\circ}\) N, 144\({}^{\circ}\) E) and (36\({}^{\circ}\) N, 144\({}^{\circ}\) E)) and the sound speed profiles on the edge of the frontal zone (Figure 7). We found obvious differences between the sound speed profiles of water masses on both sides of the frontal zone. The depth of the SOFAR axis (the depth of the minimum sound speed) of the SSP on the north side was deeper, and the surface sound speed difference between the water masses on the south side and the north side was more than 18 m/s. Between the front edge, 1 and 2 are the sound speed profiles in the frontal zone. Figure 5: The results of clustering the surface sound speed of the study sea area via FCM (the blue points represent the north water mass of the frontal zone, the red points represent the south water mass of the frontal zone, and the green points represent the frontal transition water mass): (**a**) 1 January 2019; (**b**) 1 February 2019; (**c**) 1 July 2019; (**d**) 1 August 2019. The position, width, and range of the KEF are different at different times. Taking the experiment on the 144 \({}^{\circ}\)E characteristic section as an example, the widths of the frontal zone at different times are 30 km, 60 km, 50 km, 70 km, respectively. Figure 6: The ocean frontal zone detected by the method of the horizontal temperature gradient (the critical value is 0.05 \({}^{\circ}\)C/km): **(a)** 1 January 2019; **(b)** 1 February 2019; **(c)** 1 July 2019; **(d)** 1 August 2019. The detection results of the frontal zone were similar to those of the FCM, especially in winter.