Offshore marine engineering, offshore aquaculture, and offshore environmental protection require periodic offshore surveys. At present, the main means of offshore marine surveys are mooring buoys and marine survey ships. Anchored buoys are fixed in place for a long time, which affects the navigation of ships. Therefore, mooring buoys cannot be deployed over a large area with high density. The cost of marine survey ships is high, especially when multipoint synchronous marine surveys are needed, and marine survey ships cannot perform offshore surveys under bad sea conditions. A profile autonomous underwater vehicle system is developed to meet the requirements of multipoint short-term synchronous offshore surveys. It is a small, reusable, low-cost equipment designed to move up and down at a mooring position while measuring temperature, salinity, depth, and other quantities along a vertical water section. Profile autonomous underwater vehicles can be commanded remotely and report their measurements in near real-time via wireless telemetry. The time it takes for a profile AUV to move up and down can indicate the current velocity. Tests were carried out on a wharf and in offshore areas, and the results were satisfactory.

Surface and Subsurface currents in the Philippine Sea have been reported previously, and the debate persists regarding whether they are related to the strength of ENSO (El Niño/Southern Oscillation). We investigated the mean vertical structure and variability of the currents along the western boundary of the Philippine Sea using mooring observations at about 8°N, 127°E from August 2014 to January 2016. The southward Mindanao Current (MC) exists in the upper 600 m countered by the northward Mindanao Undercurrent (MUC) observed below 600 m to the depth of 2,000 m, with a core at approximately 1,000 m. During the development of El Niño from July to December 2015, the core velocity and depth of the MC were noticeably increased, consistent with the Niño 3.4 index, and the current at 2,000 m was observed to be southward, which indicated that the MC may extend to 2,000 m under the influence of El Niño. The MUC was stronger around November 2015, corresponding to the peak phase of the Niño 3.4 index. The intraseasonal variability with a period of 70-110 days from the surface to 2,000 m is also discussed based on power spectral analysis. Subsurface eddies are likely the source of the intraseasonal variability of current in the Philippine Sea from the model outputs.

The deep scattering layer (DSL), a stratum of the marine diel vertical migration (DVM) organisms inhabiting the mesopelagic ocean, plays a crucial role in transporting carbon and nutrients from the surface to depth through the migration of its organisms. Using 18 months of in-situ observations and altimeter sea level data, we reveal for the first time the intraseasonal variations and underlying mechanisms of the DSL and the DVM to the east of the Taiwan Island. Substantial vertical speeds acquired from the Acoustic Doppler Current Profiler were used to examine the distribution and variation of the DVM. Innovatively, the results for the power spectrum analysis of the scattering intensity demonstrated a significant intraseasonal variability (ISV) with an 80-day period in the DSL. Furthermore, the variation in the DVM was closely linked to the DSL and showed an 80-day ISV during the observation. A dynamic relationship between the ISV of the DSL east of Taiwan Island and the westward-propagating mesoscale eddies was established. Anticyclonic (cyclonic) eddy movement toward Taiwan Island triggers downward (upward) bending of the local isotherms, resulting in a layer of DSL warming (cooling) and subsequent upper boundary layer deepening (rising). These findings underscore the substantial influence of mesoscale eddies on biological activity in the mesopelagic ocean, establishing a novel understanding of ISV dynamics in the DSL and their links to eddy-induced processes.

The Deep Scattering Layer (DSL), a stratum of marine Diel Vertical Migration (DVM) organisms inhabiting the mesopelagic ocean, plays a crucial role in transporting carbon and nutrients from the surface to depth through the migration of its organisms. Here, using 18 months in-situ observations and altimeter sea level data, we observed for the first time the intraseasonal variation of the DSL and the DVM of zooplankton and micronekton to the east of the Taiwan Island, and revealed the underlying dynamics. The vertical speeds returned from the Acoustic Doppler Current Profiler are used to investigate the distribution and variation of the DVM innovatively. Results of power spectrum analysis of the scattering intensity indicate that the DSL exhibits a significant intraseasonal variability (ISV) with an 80-day period. Simultaneously, the variation of the DVM is linked to the DSL, and also shows an 80-day ISV during the observation. Dynamically, the ISV of the DSL to the east of the Taiwan Island is correlated with the westward-propagating mesoscale eddies. When anticyclonic (cyclonic) eddies move towards the Taiwan Island, the local isotherms bend downward (upward), resulting in warming (cooling) in the DSL, and thus the upper boundary layer of the DSL gradually deepens (rises). These findings highlight the significant influence of mesoscale eddies on biological activity in the mesopelagic ocean.