An extreme internal solitary wave event observed in the northern South China Sea
With characteristics of large amplitude and strong current, internal solitary wave (ISW) is a major hazard to marine engineering and submarine navigation; it also has significant impacts on marine ecosystems and fishery activity. Among the world oceans, ISWs are particular active in the northern South China Sea (SCS). In this spirit, the SCS Internal Wave Experiment has been conducted since March 2010 using subsurface mooring array. Here, we report an extreme ISW captured on 4 December 2013 with a maximum amplitude of 240 m and a peak westward current velocity of 2.55 m/s. To the authors’ best knowledge, this is the strongest ISW of the world oceans on record. Full-depth measurements also revealed notable impacts of the extreme ISW on deep-ocean currents and thermal structures. Concurrent mooring measurements near Batan Island showed that the powerful semidiurnal internal tide generation in the Luzon Strait was likely responsible for the occurrence of the extreme ISW event. Based on the HYCOM data-assimilation product, we speculate that the strong stratification around Batan Island related to the strengthening Kuroshio may have contributed to the formation of the extreme ISW.
Spatial structure and temporal variability of the zonal flow in the Luzon Strait
One year long, full-depth velocity measurements were used to examine the spatial structure and temporal variability of the zonal flow in the Luzon Strait (LSZF). The observation revealed a renewed mean flow structure: in the upper ( 2000 m) layers, the LSZF was mostly westward; in the intermediate layer (500–2000 m), it was dominated by an eastward flow in the south but a westward flow in the north. The volume transport across the observed section between 19.8°N and 21.2°N exhibited strong seasonal and intraseasonal variability. On the seasonal time scale, the upper-layer transport showed a clear annual cycle, strongest in January and weakest in June; the intermediate-layer transport also showed a semiannual cycle, attaining its peaks (troughs) in January and June (April and October). On the intraseasonal time scale, both the upper-layer and intermediate-layer transports showed significant energy peaks at about 60 and 10–30 days. Further analysis indicated that the ∼60 day variability might be attributed to the impinging mesoscale eddies from the Pacific, while the 10–30 day variability appeared to obtain its energy from local intraseasonal wind forcing and baroclinic instability of the background current. The 10–100 day upper-layer and intermediate-layer transport variabilities were highly anticorrelated, suggesting a baroclinic nature of the intraseasonal variabilities of the LSZF.
Advances in research on the deep South China Sea circulation
Deep sea circulation is important for world climate and has been a substantial research area in ocean science, leading to various breakthroughs and discoveries. With the rapid advance in research on ocean science, these matters have received increasing attention from the oceanography community. In this article, we attempt to convey the progress made in recent years. We first provide an overview of existing observations, theories, and simulations of deep South China Sea circulation. Finally, we discuss remaining issues.