An Overview of Solitons in the South China
Sea
by 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.
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Causes
capable of detecting solitons are not geostationary, this
technique cannot be used for real-time soliton detection.
The three main ingredients in soliton formation are:
1.
Density stratification: The area of the ocean where
can be used to determine areas that are affected by solitons.
solitons form and propagate must have at least two
Solitons visible from satellite images are typically 100's of km
layers of different density. The difference in density
long, and appear in groups 25 km wide, with separations of 5 to
may be caused by a difference in salinity (the near
0.5 km between crests [2] (See Fig 1)
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.
3.
However, because of their global coverage, satellite imagery
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
Obstruction: Solitons form to the lee of an
area once every 10 days. The narrow swath may reduce the
obstruction, usually an submarine sill or continental
probablilty of detection if the solitons travel in a perpendicular
shelf.
direction to the track; the long "revisit" times causes some
soliton events to be missed.
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 [1], and are thought to be caused
primarily by the diurnal tides that dominate the
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
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
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.
a good approximation governed by the Korteweg-deVries
Current Meters/Profilers: Current meters and especially
(KdV) equation, first proposed in 1895 to explain solitary
current profilers (eg ADCPs) offer detailed insight into the
waves on the sea surface.
currents associated with solitons. Figure 2 shows the current
profile a single soliton group passing a fixed observation point.
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
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Although taken in the Sulu Sea, Fig 2 is typical of the soliton
into the timing and effects of solitons. These data are especially
current profiles in general:
useful since they yield directly information on the effects of
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.
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
4. The soliton current reverses direction in the bottom
layer.
like FPSOs), the most practical detecting and measurement
methods are visual observations, current meters/profilers and
Conductivity-Temperature-Depth profiles (CTD): A CTD is
strain gauges.
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
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
discarded. XBT are useful because they do not hinder the
between Batan and Sabtang islands and the
movement of the vessel. They obviously have a limited role in
combination of tide and intrusion of the Kurishio
any operational soliton measurement system.
current into the SCS provide the necessary
Acoustic Echo Sounders: Solitons deflect sound waves in
ingredients for the generation of the largest solitons
ways that enable their physical properties to be deduced. This
in the SCS. From the Luzon straits, they propagate
technique has been used extensively in the few scientific
westwards towards Hainan. In this area, currents can
expeditions of solitons in the SCS.
reach 2m/s (4 knots) in the westward direction (in the
near-surface layer) and exceed 1m/s (2 knots) in the
Visual Observation: Detailed visual observations of solitons
eastward direction (in the bottom layer). [2] They
(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.).
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
Strain Gauges: For offshore drilling or production systems,
shelf break, whenever the slope is near 0.16 to 0.3
data from riser strain gauges may provide useful information
degrees [4]. These solitons are thought to be
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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.
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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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© Copyright 2013 Terra Weather Pte. Ltd. All rights reserved.
© Copyright 2013 Terra Weather Pte. Ltd. All rights reserved.