Internal Solitons in the Northeastern South China Sea Part I: Sources and Deep Water Propagation

Abstract

Abstract-A moored array of current, temperature, conductivity, and pressure sensors was deployed across the Chinese continental shelf and slope in support of the Asian Seas International Acoustics Experiment. The goal of the observations was to quantify the water column variability in order to understand the along-and across-shore low-frequency acoustic propagation in shallow water. The moorings were deployed from April 21-May 19, 2001 and sampled at 1-5 min intervals to capture the full range of temporal variability without aliasing the internal wave field. The dominant oceanographic signal by far was in fact the highly nonlinear internal waves (or solitons) which were generated near the Batan Islands in the Luzon Strait and propagated 485 km across deep water to the observation region. Dubbed trans-basin waves, to distinguish them from other, smaller nonlinear waves generated locally near the shelf break, these waves had amplitudes ranging from 29 to greater than 140 m and were among the largest such waves ever observed in the world's oceans. The waves arrived at the most offshore mooring in two clusters lasting 7-8 days each separated by five days when no waves were observed. Within each cluster, two types of waves arrived which have been named type-a and type-b. The type-a waves had greater amplitude than the type-b waves and arrived with remarkable regularity at the same time each day, 24 h apart. The type-b waves were weaker than the type-a waves, arrived an hour later each day, and generally consisted of a single soliton growing out of the center of the wave packet. Comparison with modeled barotropic tides from the generation region revealed that: 1) The two clusters were generated around the time of the spring tides in the Luzon strait; and 2) The type-a waves were generated on the strong side of the diurnal inequality while the type-b waves were generated on the weaker beat. The position of the Kuroshio intrusion into the Luzon Strait may modulate the strength of the waves being produced. As the waves shoaled, the huge lead solitons first split into two solitons then merged together into a broad region of thermocline depression at depths less than 120 m. Elevation waves sprang up behind them as they continued to propagate onshore. The elevation waves also grew out of regions where the locally-generated internal tide forced the main thermocline down near the bottom. The "critical T. Y. Tang is with the Institute of Oceanography, National Taiwan University, Taipei, ROC.H. point" where the upper and lower layers were equal was a good indicator of when the depression or elevation waves would form, however this was not a static point, but rather varied in both space and time according to the presence or absence of the internal tides and the incoming trans-basin waves themselves.

The formation and fate of internal waves in the South China Sea

“…Over a decade of studies [8][9][10][11] have targeted the South China Sea, where the oceans' most powerful internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their generation mechanism, variability and energy budget, however, due to the lack of in-situ data from the Luzon Strait, where extreme flow conditions make measurements challenging.…”

“…When they shoal onto the continental slope to the west, the downward displacement of the ocean's layers associated with these solitary waves can exceed 250 m in 5 minutes 8 . On such a scale, these waves pose hazards for underwater navigation and offshore drilling 4 , and supply nutrients from the deep ocean that nourish coral reefs 1 and pilot whale populations that forage in their wakes 13 .…”

Interaction and generation of long-crested internal solitary waves in the South China Sea

“…Large ISWs are observed daily and their generation is due to the diurnal K 1 tide in the Luzon Strait that connects the SCS to the Western Pacific [Ramp et al, 2004;Klymak et al, 2006]. The induced flow dominates the upper ocean and hence an ISW can be observed by satellite as dark and light bands.…”

Current Observations of Internal Tides and Parametric Subharmonic Instability in Luzon Strait

“…It is well known that the barotropic tides in the SCS are dominated by diurnal constituents (Fang, Kwok, Yu, & Zhu, 1999;Yanagi & Takao, 1998). However, baroclinic tides are prominent in the semi-diurnal band of the NSCS (Du, Tseng, & Yan, 2008;Liao et al, 2010;Ramp et al, 2004;Zhao et al, 2004). Lien, Tang, Chang, and D'Asaro (2005) suggested that the strong internal tides generated in LS propagate into the SCS as a narrow tidal beam and produce nonlinear internal waves through interactions with islands and the continental slope.…”