Three typhoon events were selected to examine the impact of energy transformation on near-inertial oscillations (NIOs) using observations from a subsurface mooring, which was deployed at 125° E and 18° N on 26 September 2014 and recovered on 11 January 2016. Almost 16 months of continuous observations were undertaken, and three energetic NIO events were recorded, all generated by passing typhoons. The peak frequencies of these NIOs, 0.91 times of the local inertial frequency f, were all lower than the local inertial frequency f. The estimated vertical group velocities (C_gz) of the three NIO events were 11.9, 7.4, and 23.0 m d⁻¹, and were relatively small compared with observations from other oceans (i.e., 100 m d⁻¹). The directions of the horizontal near-inertial currents changed four or five times between the depths of 40 and 800 m in all three NIO events, implying that typhoons in the northwest Pacific usually generate high-mode NIOs. The NIO currents were further decomposed by performing an empirical orthogonal function (EOF) analysis. The first and second EOF modes dominated the NIOs during each typhoon, accounting for more than 50% of the total variance. The peak frequencies of the first two EOF modes were less than f, but those of the third and fourth modes were higher than f. The frequencies of all the modes during non-typhoon periods were more than f. Our analysis indicates that the relatively small downward group velocity was caused by the frequent direction changes of the near-inertial currents with depth.

Using a ray tracing model, acoustic propagation through a subsurface eddy is investigated. The eddy with a 460 km diameter lies at approximately 23° N, 126° E. The presence of the subsurface eddy is found to result in significant alterations to the received acoustic field. Affected by the subsurface eddy, the location of a given convergence zone can be different from usual with optimum propagation conditions. For a source deployed outside of the eddy, the subsurface eddy creates a surface duct by modifying the sonic layer depth. A maximum of 15 km difference in the range of a given convergence zone is found between the propagations with the presence and absence of the eddy when the source is placed around 100 m depth. However, for a source within the eddy, the intense shift in convergence zone patterns occurs only when the source is deployed around 300 m depth. These ocean-acoustic results can provide essential information for operating technologies such as communication devices, underwater sonar, and navigation systems.

Editorial: Eddy-current interactions in the ocean and their impacts on climate, ecology, and biology

The Gravity Recovery and Climate Experiment (GRACE), and its follow-on mission (GRACE-FO), provides a novel measurement of the variations in ocean bottom pressure (OBP) at global and basin scales, including those in marginal seas. However, these measurements have not yet been validated rigorously for the South China Sea (SCS). In this study, the accuracy in the monthly GRACE-FO mascon solutions in the SCS from the Jet Propulsion Laboratory (JPL), Center for Space Research (CSR), and Goddard Space Flight Center (GSFC) was validated with the results of the comparison with the in situ OBP records from an array of 25 pressure-recording inverted echo sounders (PIESs) that are located west of the Luzon Strait (LS). The correlation coefficient (Cor) and root mean square difference (RMSD) between the 10-month period of GSFC and PIES, spanning from July 2018 to June 2019 (with missing satellite data for August and September 2018), were 0.77 (p-value = 0.005) and 0.41 mbar (1 mbar = 100 Pa), respectively. These values suggest that the accuracy of GSFC in the SCS in this period was substantially better than that of JPL (Cor = 0.35, p-value = 0.16; RMSD = 0.74 mbar) and CSR (Cor = 0.25, p-value = 0.24; RMSD = 0.89 mbar). Moreover, the volume transport anomaly of the SCS abyssal circulation was estimated and compared based on the OBP records from GSFC and PIES observations, indicating that the GRACE-FO OBP (GSFC) can be used to monitor seasonal or longer-period variations in the SCS abyssal volume transport. Additionally, the variations in OBP from GRACE-FO were significantly overestimated on the continental shelf of the SCS, which may be attributed to signal leakage. Our findings provide reliable evidence for the application of long-term, fully covered OBP records from GRACE-FO in the SCS, and also offer a valuable reference for the application of GRACE-FO in other regions.

North Pacific subtropical mode waters (STMWs), which are subducted with air-sea interaction signals and characterized by low potential vorticity (PV), play an important role in climate change issues. STMW (PV<2×10−10 m−1· s−1) can be advected westward by large-scale ocean circulation and mesoscale eddies. However, whether and how the STMW intrudes into the South China Sea (SCS) remains unclear. Low PV signals that originate from the STMW and intruding into the SCS are investigated by comparing observations and ocean general circulation models during 2003-2017. These signals mostly move across the Luzon Strait (LS) into the SCS during the 2008-2009 and 2014-2016 periods. The results of case studies selected from these two periods indicate that the low PV signals can move to the LS and are trapped by anticyclonic eddies (AEs). When the velocity vorticity is negative southwest of Taiwan Island, vertical stratification becomes weaker west of the coming AE (AE1), and low PV signals can consequently be transported along the STMW layer (25.0 σθ- 25.5 σθ isopycnal layer). Thus, an AE (AE2) forms east of AE1 and isolates the low PV signal from AE1, and then AE2 transports the low PV water southwestward in the SCS. In contrast, the low PV signal has difficulty migrating eastward when the velocity vorticity is positive west of AE1, as vertical stratification strengthens and then weakens that signal. The results suggest that only AEs can relay low PV signals from east of the LS and carry them over long distances in the SCS.