The PDS-IESs were deployed in October 2020 to monitor the active mesoscale eddies in the southwestern South China Sea. PDSs for C1 (serial number (SN) 85) and C2 (SN 81) were released and sent data at the scheduled time February 10 and 1 of 2021, respectively. All the measured data are successfully retrieved, which proved the robustness of the PDS technique. From the retrieved data (
Fig. 2), it is found that
τ and
$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over u } $,
$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over v} $ of the two stations (C1 and C2) show quite different variability, although their distance is not quite large (around 180 km). For C1,
τ dropped from October to December in 2020, then increased slowly till late February. It is interpreted that temperature in the upper ocean increased first but decreased after December. It seemed to be the opposite for C2. Strong mesoscale eddies are likely to account for above phenomenon, which are identified from merged sea surface height product released by Copernicus. For
$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over u } $ and
$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over v } $, time-mean eastward components are evident for both stations, with mean speeds of 2.0 cm/s (C1) and 1.2 cm/s (C2), which seemed to be caused by the Vietnam Offshore Current; however, meridional velocities of the two stations showed quite different variability. From the power spectrum analysis (figures not shown), near 20 d period variability are strong for both
τ and
$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over u } $,
$\mathord{\buildrel{\lower3pt\hbox{$\scriptscriptstyle\rightharpoonup$}} \over v } $. Tidal signals with maximum amplitude of near 1×10
4 Pa are quite significant in bottom pressure records. In subtidal frequency band, near 5 d period bottom pressure variability are quite significant for both stations. The PDSs continued to report its location once a day after its surface. Till February 20, they both headed southwestward with an average speed of 0.15 m/s (SN 85) and 0.13 m/s (SN 81), respectively.