Minimize Oceanic Internal Waves - Andaman Sea

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Andaman Sea Andaman Sea

Latitude: 08° 48' N - Longitude: 96° 22' E

The front of a packet of internal solitary waves (left-hand section of the image) interacting with the tail of another internal solitary wave packet (right-hand section of the image). The leading soliton of the western packet seems to sweep clean the trailing part of the previous packet.
Andaman Sea Andaman Sea

Latitude: 07° 01' N - Longitude: 97° 25' E

Sea surface manifestations of two internal wave packets generated at successive semi-diurnal tidal cycles. The variation of the NRCS along the profile AB is shown in the figure below.
Andaman Sea Variations of the NRCS in dB along the scan line A-B shown in the ERS image above.
Andaman Sea Andaman Sea

Latitude: 07° 17' N - Longitude: 98° 15' E

The irregular shape of the sea surface manifestations of this packet of internal solitary waves result from the interaction with the shallow bottom topography in this region of the Malayan shelf. The position where the sea surface manifestation of the leading solitary wave suddenly disappears coincides with the 90m depth line. It is suggested that at this position the leading solitary wave breaks. According to theory, wave breaking occurs where the water depth is equal to twice the depth of the upper layer.
# Orbit Frame(s) Satellite Date Time Location
1 9477 3429 ERS-2 11-Feb-1997 03:58
2 05426 3447-3465-3483 ERS-2 04-May-1996 03:52
3 07514 0135 ERS-1 22-Dec-1993 16:03

If you have any comments on these images please write an e-mail to alpers@ifm.uni-hamburg.de.


Internal Solitary Waves in the Andaman Sea


The Andaman Sea of the Indian Ocean is known to be one of the sites in the world's ocean where extraordinarily large internal solitons are encountered.

For centuries seafarers passing through the Strait of Malacca on their journeys between India and the Far East have noticed that in the Andaman Sea bands of strongly increased surface roughness often occur. These have also been referred to as bands of choppy water or ripplings and have been found mainly between the Nicobar Islands and the north east coast of Sumatra. A description of such bands of choppy water observed from ships in the western approaches of the Malacca Strait can, e.g., be found in the book of Mauray which was published in 1861 and which is quoted in Osborne and Burch (1980): "The ripplings are seen in calm weather approaching from a distance, and in the night their noise is heard a considerable time before they come near. They beat against the sides of a ship with great violence, and pass on, the spray sometimes coming on deck; and by carrying out oceanographic measurements from a ship, a small boat could not always resist the turbulence of these remarkable ripplings".

Perry and Schimke (1965) were the first to show by oceanographic measurements carried out from a ship that these bands of choppy water in the Andaman Sea are associated with large amplitude oceanic internal waves. Later Osborne and Burch (1980) analysed oceanographic data collected by the Exxon Production Research Company in the southern Andaman Sea with the aim to assess the impact of underwater current fluctuations associated with oceanic internal waves on drilling operations carried out from a drill ship. They concluded that the visually observed roughness bands are caused by internal solitons which can be described by the Korteweg - de Vries equation (1885).

The oceanographic measurements of Osborne and Burch (1980) showed that the time interval between the first solitons in the packets was typically 40 minutes and then decreased towards the end. In one event, the amplitude (peak-to-trough distance) of the foremost soliton was estimated to be 60 m, i.e., warm water from above was pushed down by the internal soliton by 60 m. The roughness bands associated with one of the internal soliton packets extended from horizon to horizon and were 600 to 1200 m wide. The first band of choppy water consisted of breaking waves about 1.8 m high. The background wave field preceding this band had only a waveheight of 0.6m. Behind this band of strongly increased surface roughness, the waveheight gradually decreased and a band of reduced surface roughness followed, which had a waveheight of less than 0.1 m and looked "as smooth as a millpond".

Occasionally, long narrow stripes of rough water have also been identified in the Andaman Sea on satellite images acquired in the visible band, e.g., from the Russian-US space station Apollo-Soyus (Apel, 1979), the Landsat satellites (Osborne and Burch, 1980), and the SPOT satellites. However, no systematic investigations of internal solitons could be carried out with optical satellite images because they can be acquired only sporadically when there are no clouds present and when the sun elevation angle is favourable.

A large number of spaceborne SAR images of the Andaman Sea have become available after the ERS receiving station in Singapore became operational in Sept. 1995. These ERS SAR images allow for the first time to study systematically the spatial distribution of internal solitons in the Andaman Sea and thus to obtain information on their generation and propagation characteristics.

Figure 1: A SAR strip inserted into a map of the Andaman Sea which was acquired on 11 Feb. 1997. The sea surface manifestations of several internal solitary wave packets having their origin in three locations in the shelf region of the Andaman and Nicobar islands are visible. Inserted is also the location of the ERS-2 SAR strip shown in Figure 2.

References

  • Alpers, W., Heng,W.-C. & Hock, L., Observation of internal waves in the Andaman Sea by ERS SAR, Proc. 3rd ERS Symp., Florence, Italy, 17-21 March 1997, ESA publication SP-414, 1287-1291 (1997).
  • Apel, J.R., Thomson, D.R., Tilley, D.G. & van Dyke, P., Hydrodynamics and radar signatures of internal solitons in the Andaman Sea, John Hopkins APL Technical Digest, Vol.6, 4, 3330-3337 (1985).
  • Osborne, A.R. & Burch, T.L., Internal solitons in the Andaman Sea, Science, 1980, 208, 451-460.
  • Perry, B.R. & Schimke, G.R., Large-amplitude internal waves observed off the northwest coast of Sumatra, J. Geophys. Res., 70, 2319-2324 (1965).