## 4 Sea Experiment  
In order to verify the performance of the real-time communication subsurface mooring system during long-term operation, from 15 December 2021 to 18 March 2022, a sea experiment was carried out in an area near Xisha, China. The deployment point was located at the coordinates 113\({}^{\circ}\) E, 19\({}^{\circ}\) N, with a water depth of up to 1165 m. The total length of the real-time communication subsurface mooring system was 1220 m. As shown in Figure 11, the top of the system was the SCB, which was integrated with the Beidou second-generation satellite module, and the communication rate reached up to 400 Byte/min. The data return frequency was 1 h/time in this test. The SCB was connected with the inductive coupling cable, and the length of the cable was 650 m. The cable was equipped with 15 sensors, include 12 TD-IM and 3 CTD-IM. Due to the dramatic changes in the temperature and salinity data of the sea surface thermocline, five TD-IMs and three CTD-IMs were installed on the inductive coupling cable with a water depth from 40 m to 110 m. Four TD-IMs were installed on the inductive coupling cable with a water depth of 200 m to 500 m. Table 1 shows the estimated operating depth of each sensor. The lower end of the inductive coupling cable was connected to the mooring device with a total length of about 570 m. The mooring device consisted of a glass float, a Kevlar rope, a parallel acoustic release, and an anchor. The real-time communication subsurface mooring system was deployed for 93 days, and 2034 sets of CTD data have been successfully returned, of which 1489 sets of data were sent back immediately after sampling, and 545 sets of data were successfully re-issued through the efficient decision data recovery strategy; the data return success rate was 91.1%. Figure 10: Flowchart of efficient decision data recovery strategy. The temperature and salinity profiles from the in situ observations match excellently with those from the HYbrid Coordinate Ocean Model (HYCOM) output ([[https://www.hycom.org/datasetver/gofs-3](https://www.hycom.org/datasetver/gofs-3) pt1/reanalysis]([https://www.hycom.org/datasetver/gofs-3](https://www.hycom.org/datasetver/gofs-3) pt1/reanalysis), accessed on 5 September 2022) and more detailed information because of the high sampling frequency of the CTD. The temperature profiles from the CTD observations and the HYCOM output are illustrated in Figure 12a,b, and both 15\({}^{\circ}\)C isotherms show an upward trend with time. The salinity profiles from the CTD observations and the HYCOM output are illustrated in Figure 12c,d, and both profiles revealed that the maximum salinity is in the subsurface layer. The salinity also shows an upward trend between February 29 and March 15. In addition to the entire uplift of the isotherm, the temperature and salinity profiles have an obvious fluctuating signal. The power spectrum of the density variety has the obvious distribution of internal tide energy (Figure 13). As shown in Figure 13, the diurnal (mainly K1) and semi-diurnal (mainly M2) peaks are remarkable, and this is consistent with the results of the velocity spectrum of previous studies [28, 29]. \begin{table}
\begin{tabular}{c c} \hline
**Sensor Type** & **Depth/m** \\ \hline TD & 40 \\ CTD & 50 \\ TD & 60 \\ TD & 70 \\ CTD & 80 \\ TD & 90 \\ CTD & 100 \\ TD & 110 \\ TD & 200 \\ TD & 300 \\ TD & 400 \\ TD & 500 \\ \hline \end{tabular}
\end{table} 
Table 1: The operating depth of each sensor. Figure 11: Real-time communication subsurface mooring system experiment. According to the sea level anomaly maps obtained from the Copernicus Marine and Environment Monitoring Service ([[https://doi.org/10.48670/moi-00148](https://doi.org/10.48670/moi-00148)]([https://doi.org/10.48670/moi-00148](https://doi.org/10.48670/moi-00148))), the entire uplift of the isotherm revealed the movement of a cyclonic mesoscale eddy passing by the mooring site (Figure 14), and our moored CTD observed precisely the thermohaline structure of this process beneath the surface.