The Andaman Sea (historically also known as the Burma Sea) is a marginal sea of the northeastern Indian Ocean bounded by the coastlines of Myanmar and Thailand along the Gulf of Martaban and the west side of the Malay Peninsula, and separated from the Bay of Bengal to its west by the Andaman Islands and the Nicobar Islands. Its southern end is at Breueh Island just north of Sumatra, with the Strait of Malacca further southeast.
Traditionally, the sea has been used for fishery and transportation of goods between the coastal countries and its coral reefs and islands, which are popular tourist destinations. The fishery and tourist infrastructure was severely damaged by the December 2004 Indian Ocean earthquake and tsunami.
Geography
Location The Andaman Sea, which extends over 92°E to 100°E and 4°N to 20°N, occupies a very significant position in the Indian Ocean, yet remained unexplored for a long period. To the south of Myanmar, west of Thailand, and north of Indonesia, this sea is separated from the Bay of Bengal by the Andaman and Nicobar Islands and an associated chain of sea mounts along the Indo-Burmese plate boundary. The Strait of Malacca (between the Malay Peninsula and Sumatra) forms the southern exitway of the basin, which is wide and deep.
Extent
The International Hydrographic Organization defines the limits of the "Andaman or Burma Sea" as follows:
On the Southwest. A line running from "Oedjong Raja" ["Ujung Raja" or "Point Raja"] () in Sumatra to Poeloe Bras (Breuëh) and on through the Western Islands of the Nicobar Group to Sandy Point in Little Andaman Island, in such a way that all the narrow waters appertain to the Burma Sea.
On the Northwest. The Eastern limit of the Bay of Bengal [A line running from Cape Negrais (16°03'N) in Burma [Myanmar] through the larger islands of the Andaman group, in such a way that all the narrow waters between the islands lie to the Eastward of the line and are excluded from the Bay of Bengal, as far as a point in Little Andaman Island in latitude 10°48'N, longitude 92°24'E].
On the Southeast. A line joining Lem Voalan (7°47'N) in Siam [Thailand], and Pedropunt (5°40'N) in Sumatra.
Oedjong means "cape" and Lem means "point in the Dutch language on maps of the Netherlands East Indies (Indonesia).Glossary of Terms Appearing on Maps of the Netherlands East Indies, United States Army Map Service, page 115, 93. Lem Voalan [Phromthep Cape] is the southern extremity of Goh Puket (Phuket Island).
Exclusive economic zone Exclusive economic zones in Andaman Sea:
NumberCountryArea (Km2)1 - Andaman and Nicobar Islands 659,5902 - Mainland 511,3893 - Andaman Sea 118,7144 - Northeast Aceh 76,500TotalAndaman Sea -
Channels
See Channels in Bay of Bengal, Laccadive Sea, and Arabian Sea.
Geology
The northern and eastern side of the basin is considered shallow, as the continental shelf off the coast of Myanmar and Thailand extends over (marked by isobath). About 45 percent of the basin area is shallower (less than depth), which is the direct consequence of the presence of the wider shelf. The continental slope which follows the eastern shelf is quite steep between 9°N and 14°N. Here, the perspective view of the submarine topography sectioned along 95°E exposes the abrupt rise in depth of sea by about within a short horizontal distance of a degree. Isobaths corresponding to are also shown in the figure to emphasize the steepness of the slope. Further, it may be noted that the deep ocean is also not free from sea mounts; hence only around 15 percent of the total area is deeper than .S. R. Kiran (2017) General Circulation and Principal Wave Modes in Andaman Sea from Observations, Indian Journal of Science and Technology
The northern and eastern parts are shallower than due to the silt deposited by the Irrawaddy River. This major river flows into the sea from the north through Myanmar. The western and central areas are deep. Less than 5% of the sea is deeper than , and in a system of submarine valleys east of the Andaman-Nicobar Ridge, the depth exceeds . The sea floor is covered with pebbles, gravel, and sand.
The western boundary of the Andaman Sea is marked by islands and sea mounts, with straits or passages of variable depths that control the entry and exit of water to the Bay of Bengal. There is a drastic change in water depth over a short distance of , as one moves from the Bay of Bengal (around deep) to the vicinity of islands (up to depth) and further into the Andaman Sea. Water is exchanged between the Andaman Sea and the Bay of Bengal through the straits between the Andaman and Nicobar Islands. Out of these, the most important straits (in terms of width and depth) are Preparis Channel (PC), Ten Degree Channel (TDC), and Great Channel (GC). PC is the widest but shallowest () of the three and separates south Myanmar from north Andaman. TDC is deep and lies between Little Andaman and Car Nicobar. GC is deep and separates Great Nicobar from Banda Aceh.
Ocean floor tectonics
Running in a rough north–south line on the seabed of the Andaman Sea is the boundary between two tectonic plates, the Burma Plate and the Sunda Plate. These plates (or microplates) are believed to have formerly been part of the larger Eurasian Plate, but were formed when transform fault activity intensified as the Indian Plate began its substantive collision with the Eurasian continent. As a result, a back-arc basin center was created, which began to form the marginal basin which would become the Andaman Sea, the current stages of which commenced approximately 3–4 million years ago (Ma).
The boundary between two major tectonic plates results in high seismic activity in the region (see List of earthquakes in Indonesia). Numerous earthquakes have been recorded, and at least six, in 1797, 1833, 1861, 2004, 2005, and 2007, had a magnitude of 8.4 or higher. On 26 December 2004, a large portion of the boundary between the Burma plate and the Indo-Australian plate slipped, causing the 2004 Indian Ocean earthquake. This megathrust earthquake had a magnitude of 9.3. Between of the boundary underwent thrust faulting and shifted by about , with the sea floor being uplifted several meters. This rise in the sea floor generated a massive tsunami with an estimated height of Tom Paulson (7 February 2005) New findings super-size our tsunami threat. 80-foot waves blasted Indonesia, scientists now say, Seattle Post-Intelligencer that killed approximately 280,000 people along the coast of the Indian Ocean.Indonesia quake toll jumps again, BBC, 25 January 2005 The initial quake was followed by a series of aftershocks along the arc of the Andaman and Nicobar Islands. The entire event severely damaged the fishing infrastructure.
Volcanic activity
Within the sea, to the east of the main Great Andaman island group, lies Barren Island, the only currently active volcano associated with the Indian subcontinent. This island volcano is in diameter and rises above sea level. Its recent activity resumed in 1991 after a quiet period of almost 200 years.D. Chandrasekharam, Jochen Bundschuh (2002) Geothermal energy resources for developing countries, Taylor & Francis p. 408 It is caused by the ongoing subduction of the India plate beneath the Andaman island arc, which forces magma to rise in this location of the Burma plate. The last eruption started on 13 May 2008 and continues.Barren Island , Smithsonian National Museum of Natural History The volcanic island of Narcondam, which lies further north, was also formed by this process. No records exist of its activity.Narcondum , Smithsonian National Museum of Natural History
Sediments to the Sea Collectively, the modern Ayeyarwady (Irrawaddy) and Thanlwin (Salween) rivers deliver >600 Mt/yr of sediment to the sea. Most recent studies show: 1) There is little modern sediment accumulating on the shelf immediately off the Ayeyarwady River mouths. In contrast, a major mud wedge with a distal depocenter, up to in thickness, has been deposited seaward in the Gulf of Martaban, extending to approximately water depth into the Martaban Depression. Further, 2) There is no evidence showing that modern sediment has accumulated or is transported into the Martaban Canyon; 3) a mud drape/blanket is wrapping around the narrow western Myanmar Shelf in the eastern Bay of Bengal. The thickness of the mud deposit is up to nearshore and gradually thins to the slope at water depth, and likely escapes into the deep Andaman Trench; 4) The estimated total amount of Holocene sediments deposited offshore is approximately . If we assume this has mainly accumulated since the middle Holocene highstand (~6000 yr BP) like other major deltas, the historical annual mean depositional flux on the shelf would be per year, which is equivalent to ~35% of the modern Ayeyarwady-Thanlwin rivers derived sediments; 5) Unlike other large river systems in Asia, such as the Yangtze and Mekong, this study indicates a bi-directional transport and depositional pattern controlled by the local currents that are influenced by tides, and seasonally varying monsoons winds and waves.
Climate
The climate of the Andaman Sea is determined by the monsoons of southeast Asia, as the prevailing winds reverse with the start of either season. The region experiences north-easterlies with an average windspeed of from November to February. During these months, the western part of the domain experiences maximum wind intensity. It weakens by March–April and reverses to strong south-westerlies from May to September, with mean wind speeds touching in June, July, and August, distributed near-uniformly over the entire basin. Wind speeds plummet by October and switch back to north-easterlies from November.
Air temperature is stable over the year at in February and in August. Precipitation is as high as /year and mostly occurs in summer. Sea currents are south-easterly and easterly in winter and south-westerly and westerly in summer. The average surface water temperature is in February and in May. The water temperature is constant at at the depths of and below. Salinity is 31.5–32.5‰ (parts per thousand) in summer and 30.0–33.0‰ in winter in the southern part. In the northern part, it decreases to 20–25‰ due to the inflow of freshwater from the Irrawaddy River. Tides are semidiurnal with an amplitude of up to .
The effect of wind stress on the ocean surface can be explained by wind stress curl. The net divergence of water in the ocean mixed layer results in Ekman pumping. The contrast between the two seasons elicits a very strong negative pumping velocity of more than per day along the north coast of Indonesia from May to September (shown here, June). This may signify coastal downwelling in the summer. It is also observed that the region develops a weak but positive pumping velocity of less than per day at the mouth of GC in winter (here, December).
Current and wave fluid dynamics
Generally, currents are found to be stronger in the south than any other part of the basin. An intense surface outflux through GC, of the order of , occurs during summers and winters. While this flow is directed westwards in winter, it is southwards along the west coast of Indonesia in summer. On the other hand, the TDC has strong surface influx in summer, which weakens by October. This is followed by a sturdy outflux in winter, which wanes by the month of April. Although the surface flow through PC is generally inward during summer monsoon, the preceding and succeeding months experience outflow (strong outflow in October, but weak outflow in April). During April and October, when the effects of local winds are minimal, Andaman Sea experiences the intensification of meridional surface currents in the poleward direction along the continental slope on the eastern side of the basin. This is characteristic of the propagation of Kelvin Waves.