An Overview of Solitons in the South China Sea

Arnold Doray

Most of the world's oceans are characterised by two layers: The water in the near surface layer has a lower density while the deeper layer has a higher density. Because the top layer is 'lighter', it rides above the denser layer below, much like oil over water. In certain circumstances, waves can develop in the interface between these two layers. Such waves are variously known as solitons, internal solitons, internal waves or non-linear internal waves (NLIW).

Solitons manifest themselves on the sea surface as long narrow lines of propagating whitecaps or breaking waves [1]. More significantly, solitons are associated with very strong currents (up to 4 knots in the South China Sea) of short duration (minutes), and typically in groups (about 10 solitons in a group).

Fortunately, solitons do not usually pose a hazard to most maritime activities. However, they can damage fixed offshore structures like offshore platforms or riser systems. The strong currents may also give rise to downtime during operations requiring the coordination of multiple vessels.

Causes

The three main ingredients in soliton formation are:

  1. Density stratification: The area of the ocean where solitons form and propagate must have at least two layers of different density. The difference in density may be caused by a difference in salinity (the near surface layer receives more freshwater from rain or river runoff, while the lower layer does not), or a difference in temperature (the top layer is more readily heated by the sun). Most of the world's oceans have this density stratification, except in the very high or low latitudes.
  2. Obstruction: Solitons form to the lee of an obstruction, usually an submarine sill or continental shelf.
  3. Currents: Solitons are caused by the disruption to the current flow (usually tidal) over a submarine obstruction. Solitons over the South China Sea show a 14-day cycle [3], and are thought to be caused primarily by the diurnal tides that dominate the region.

The exact mechanism of soliton formation is still unknown (although limited to a few possibilities) [5], but once they are generated, their propagation and subsequent development is to a good approximation governed by the Korteweg-deVries (KdV) equation, first proposed in 1895 to explain solitary waves on the sea surface.

Detection

There are a number of ways to detect solitons:

Satellites: Satellite images give snapshots of the surface characteristics of solitons over large regions.

They do not offer insight into the speed, intensity and other characteristics of these phenomena. Also, since satellites capable of detecting solitons are not geostationary, this technique cannot be used for real-time soliton detection.

However, because of their global coverage, satellite imagery can be used to determine areas that are affected by solitons. Solitons visible from satellite images are typically 100's of km long, and appear in groups 25 km wide, with separations of 5 to 0.5 km between crests [2] (See Fig 1)

Satellite Altimetry: Satellite altimetry measures sea surface height to an accuracy of about 3cm. They may also be used to detect solitons, since solitons depress the local sea surface topography [4]. Unfortunately, such satellites sample sea surface height along a very narrow track (typically 2 - 3km wide, depending on the local wave height), and only revisit an area once every 10 days. The narrow swath may reduce the probablilty of detection if the solitons travel in a perpendicular direction to the track; the long "revisit" times causes some soliton events to be missed.

Certainly, satellite altimetry cannot be used for real-time detection. But their insensitivity to cloud cover means that it is easier for this data to be processed by automated means for past solitons to be detected.

Radar: On the sea surface, solitons have increased, highly localized sea surface roughness, so shipboard radar may be used to detect their approach. Radar allows the speed and wavelength of solitons to be accurately measured.

Current Meters/Profilers: Current meters and especially current profilers (eg ADCPs) offer detailed insight into the currents associated with solitons. Figure 2 shows the current profile a single soliton group passing a fixed observation point.

Although taken in the Sulu Sea, Fig 2 is typical of the soliton current profiles in general:

  1. The soliton group consists of about 10 individual solitons.
  2. The strength of each soliton decreases after the 'leader' on the left of the figure.
  3. The soliton current is strongest in the near surface layer.
  4. The soliton current reverses direction in the bottom layer.

Conductivity-Temperature-Depth profiles (CTD): A CTD is a device that measures conductivity (and hence, salinity) and temperature. The device is attached to a streamlined weight and tethered to the ship by a thin cable. As it drops to the sea floor, it records the temperature and conductivity profile. The instrument is typically 'cycled' (dropped, retrieved and dropped again) to provide in situ measurements of the soliton's temperature and salinity profile. Because the CTD has to be cycled, it cannot be used for continuous monitoring.

Expendable Bathythermography (XBT): This instrument is similar to the CTD, but measures temperature only, and is used just just once. Once the weight reaches a given depth, the tether (which relays data to the ship) breaks, and the device is discarded. XBT are useful because they do not hinder the movement of the vessel. They obviously have a limited role in any operational soliton measurement system.

Acoustic Echo Sounders: Solitons deflect sound waves in ways that enable their physical properties to be deduced. This technique has been used extensively in the few scientific expeditions of solitons in the SCS.

Visual Observation: Detailed visual observations of solitons (based on the surface rips), along with time of their occurrence provide useful data to help tie their occurrence with environmental conditions (model currents, tides, etc.).

Strain Gauges: For offshore drilling or production systems, data from riser strain gauges may provide useful information into the timing and effects of solitons. These data are especially useful since they yield directly information on the effects of solitons on riser systems. However, these data should ideally be accompanied by visual observations or other supporting measurements, since the soliton signal may not be easily distinguishable from other environmental effects on the riser system.

For offshore platforms (or semi-permanently moored vessels like FPSOs), the most practical detecting and measurement methods are visual observations, current meters/profilers and strain gauges.

Solitons in the South China Sea

Over the SCS as a whole, solitons occur primarily during summer, especially during the months of June and July [3]. They move at a speed of 1.9 m/s to 2.9 m/s (4 - 6 knots)[3][6]. They have mostly been observed in the western portion of the basin. Observations in the deep sea in the eastern portion of the SCS are rare [6].

The areas where solitons have been observed:

  1. Luzon Strait to Hainan Island: The submarine sill between Batan and Sabtang islands and the combination of tide and intrusion of the Kurishio current into the SCS provide the necessary ingredients for the generation of the largest solitons in the SCS. From the Luzon straits, they propagate westwards towards Hainan. In this area, currents can reach 2m/s (4 knots) in the westward direction (in the near-surface layer) and exceed 1m/s (2 knots) in the eastward direction (in the bottom layer). [2] They propagate at about 1.9 m/s (4 knots) [3].
  2. Hainan and Taiwan Islands: Solitons may also be generated locally near Hainan and Taiwan due to the shelf break, whenever the slope is near 0.16 to 0.3 degrees [4]. These solitons are thought to be generated locally by the diurnal tide.
  3. The Vietnamese Coast: The internal waves propagate to the northwest, roughly perpendicular to the continental shelf break.
  4. South Vietnam to Borneo: Solitons also occur in the continental shelf between Vietnam and Borneo.
  5. Sulu Sea: Solitons also occur in the Sulu Sea. They are generated near Pearl Bank and propagate to the NW towards Palawan Is. [1]

Prediction

To the best of our knowledge, all present efforts on soliton prediction centre on the KdV equation. While this equation does not predict their formation, it does provide minimum criteria for solitons to propagate and develop. Thus, estimates or predictions of KdV parameters combined with current/tidal model data allow forecasts of solitons to be made. Such a system is the basis of an early warning system being developed by the US Navy [7] for the northern SCS.

However, the lack of continuous current measurements make it difficult for the accuracy of such forecasts to be readily assessed.

Another approach is to analyse observational or measurement data in order to yield statistical correlations to other environmental variables (tidal current, surface current). These correlations might then be used as the basis of an early warning system. This of course assumes that a sizable database of historical soliton events is available.

References

[1] Apel, J.R, et. Al (1985), The Sulu Sea Internal Soliton Experiment, Journal of Physical Oceanography, Vol. 15, Issue 12, pp.1625 - 1651

[2] Liu, A.K., Hsu, M.K, (2004), Internal wave study in the South China Sea using Synthetic Aperture Radar (SAR) Int. J. Remote Sensing, 10 - 20 Apr, 2004, Vol. 25, No. 7-8, 1261-1264

[3] Global Ocean Associates (2004), An Atlas of Oceanic Internal Solitary Waves.

[4] Duda, T. F., et. al. (2004), Internal tide and nonlinear internal wave behavior at the continental slope in the Northern South China Sea, IEEE J. Oceanic Engineering, ASIAEX Special Issue.

[5] Apel, J.R, (2003), A New Analytical Model for Internal Solitons in the Ocean, Journal of Physical Oceanography, Vol. 33, Issue 11 pp. 2247-2269.

[6] Klymak, J.M., et. al. (2006), Prototypical Solitons in the South China Sea, Geophysical Res. Lett. Vol. 33.

[7] Ramp, S. (2006), The Windy Islands Soliton Experiment, Naval Postgraduate School brochure, Dept. of Oceanography.

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