**5. Conclusions**

In this paper, an ISW with extreme current velocity northeast of Dong-Sha Atoll is reported and discussed, whose velocity is the largest among the ISWs ever observed in the global ocean. The peak westward velocity (u) was 2.94 m/s, and the peak downward velocity (w) was 0.63 m/s, respectively. The amplitude of ISW was about 97 m, propagating as a mode 1 depression wave. A strong trailing wave was captured 2.2 h later and suggested the ISW was an A-type wave. The barotropic tide had little influence on the current field of the ISW, which indicated that the velocity induced by the ISW was extremely large indeed. The KdV theoretical phase speed and the estimated phase speed by the satellite image were 1.76 m/s and 1.59 m/s, respectively. By comparing the peak current velocity *umax* and phase speed *c*, it is inferred that the ISW reported in our paper was close to or already in the process of breaking and may have developed a trapped core.

In the deep basin of northern SCS, a strong ISW was reported with an amplitude of 240 m and a peak westward current velocity of 2.55 m/s [33], with a larger amplitude but lower peak velocity than the one discussed in this paper. However, the ISW reported in [33] was captured in the deep basin of northern SCS where the depth was 3847 m, rather than in the vicinity of Dong-sha Atoll.

ISWs with such a large velocity are rare, with previous observations near Dong-Sha Atoll reporting amplitudes ranging from 70 to 106 m and peak westward velocities from 0.73 to 2.4 m/s. In the vicinity of Dong-sha Atoll, 41 ISWs were identified with moorings, among which only one ISW with a large peak westward current exceeding 2.4 m/s was captured, whose characteristics were similar to the ISW discussed in this paper [5]. Both ISWs in the two studies were near breaking. Thus, the ISW discussed in this paper has the largest velocity so far reported in the global ocean.

According to [11], ISWs in a marginal convectively unstable state (which leads to a trapped core) can contribute to turbulence mixing with long distances, along with the propagation. We supposed that the turbulence mixing would appear if the breaking state continued, but further observations are needed to prove it.

**Author Contributions:** Conceptualization, A.X. and X.C.; data curation, A.X.; formal analysis, A.X.; investigation, A.X.; methodology, A.X.; software, A.X.; validation, A.X.; visualization, A.X.; funding acquisition, X.C.; resources, X.C.; supervision, X.C.; writing—original draft, A.X.; writing review and editing, A.X. and X.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key Research and Development Plan, Grant 2016YFC1401300, "Oceanic Instruments Standardization Sea Trials (OISST)", and the Taishan Scholar Program.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The in situ data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors appreciate the National Ocean Partnership Program (NOPP) for providing the HYCOM reanalysis products and the Open spatial data sharing system of the Aerospace Information Research Institute (AIR) for providing the satellite image data. We are grateful to the Earth & Space Research (ESR) and Oregon State University (OSU) for providing the Tidal Model Driver (TMD) Matlab toolbox. We also thank the helpful suggestions from two anonymous reviewers for improving the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
