Andaman Sea

The Andaman Sea is a marginal sea of the northeastern Indian Ocean, bounded by the Andaman and Nicobar Islands (India) to the west, the coastlines of Myanmar (Burma) and Thailand to the north and east, the Malay Peninsula and the island of Sumatra (Indonesia) to the south, and communicating with the Bay of Bengal to the northwest through the Ten Degree Channeland the Preparis Channel. Covering approximately 797,700 km² — an area larger than Turkey — it is one of the most strategically consequential bodies of water in the world, serving as the indispensable northwestern approach to the Strait of Malacca, the world's second-busiest international shipping chokepoint.

Despite its moderate size, the Andaman Sea funnels an enormous volume of global commerce. Every vessel transiting from the Indian Ocean or Persian Gulf toward East Asia — carrying crude oil to Chinese and South Korean refineries, containerised goods to the factories and consumers of the Pacific Rim, and LNG to Japanese power stations — must pass through Andaman waters before entering the Malacca Strait. The sea's average depth of 1,096 m, with maxima exceeding 4,000 m in the southern basin, contrasts with the notoriously shallow northern reaches of the Malacca Strait, where controlling depth drops below 25 m in the Singapore Strait approaches.

The Andaman Sea is bounded by some of the region's most ecologically significant island groups. The Andaman Islands — 572 islands, islets, and rocks administered by India as a Union Territory — form a 650 km north-south chain dividing the sea from the Bay of Bengal. The Nicobar Islands, a further 22 islands extending south to within 150 km of Sumatra, complete the western boundary. To the east, Myanmar's Mergui Archipelago (Myeik Archipelago) — comprising over 800 islands between the Tenasserim coast and the open Andaman Sea — represents one of the last largely undisturbed maritime wilderness areas in Southeast Asia, its isolation maintained in part by decades of restricted access under Myanmar's military governments.

For maritime professionals, the Andaman Sea demands thorough knowledge of the Indian Ocean monsoon system, the maritime boundary complexities arising from overlapping claims, the navigational constraints of Myanmar's territorial waters, and the environmental sensitivity of reef and mangrove ecosystems that characterise its shores. The sea falls within NAVAREA VIII, coordinated by the National Hydrographic Office of India, and mariners operating in these waters must maintain a rigorous NAVTEX watch and be familiar with both Indian and Myanmar port state control requirements.

1. Geography & Physical Characteristics

The Andaman Sea is geographically defined by the International Hydrographic Organization (IHO) in Special Publication No. 23 as bounded to the north by the southern coast of Myanmar (Burma) and the Ten Degree Channel; to the west by the eastern coasts of the Andaman and Nicobar Islands; to the south by the northern coast of Sumatra between Ulu Man and Pedjantan Island, and by the northern entrance of the Strait of Malacca; and to the east by the coastlines of Thailand and the Malay Peninsula, extending to the Malay-Thai border on the Kra Isthmus. Its maximum north-south extent is approximately 1,200 km, and its maximum east-west breadth is roughly 650 km in the central portion.

The sea's most strategically important geographical feature is its role as the antechamber of the Strait of Malacca. The southern Andaman Sea narrows progressively as it approaches the northern entrance of the Malacca Strait between Phuket (Thailand) and Sabang/Weh Island (Indonesia), where the passage is approximately 250 km wide. As vessels proceed south through the Malacca Strait, this width contracts to just 2.8 km at the Phillips Channel in the Singapore Strait — the critical bottleneck constraining the draft of the largest vessels to approximately 20.5 m.

The Andaman Islands — the northern chain, collectively administered by India as part of the Andaman and Nicobar Islands Union Territory — consist of four principal island groups: North Andaman, Middle Andaman, South Andaman (the largest, on which Port Blair is located), and Little Andaman, plus numerous smaller islands. The islands are of mixed geological origin, incorporating both continental fragments and volcanic arc material associated with the eastern continuation of the Himalayan orogeny. The terrain is forested and hilly, reaching approximately 732 m at the summit of Saddle Peak on North Andaman. The surrounding seas are characterised by coral reefs, fringing reef platforms, and numerous shallow-water hazards requiring large-scale chart navigation.

The Ten Degree Channel — so named because it lies approximately at 10°N latitude — separates the Andaman Islands to the north from the Nicobar Islands to the south. It is approximately 150 km wide and constitutes the principal deep-water route between the Bay of Bengal and the northern Andaman Sea, with depths exceeding 1,000 m throughout. The Preparis Channel, to the north of North Andaman Island, provides a secondary passage between the Bay of Bengal and the Andaman Sea, used primarily by coastal and smaller vessels transiting between the Irrawaddy Delta region of Myanmar and the Bay of Bengal.

The Mergui Archipelago (Myeik Archipelago), extending along the Tenasserim coast of southern Myanmar from approximately 9°N to 14°N, is the defining geographical feature of the eastern Andaman Sea. The archipelago comprises over 800 islands with a combined coastline of extraordinary complexity — intricate channels, submerged reefs, and mangrove-lined inlets alternating with deep-water passages between the major islands. The largest islands include King Island (Kenght), Domel Island, and Lampi Island. Access to the Mergui Archipelago by foreign vessels has been restricted by the Myanmar government, and mariners must obtain permits through Myanmar Ports Authority before entering territorial waters in this region. Charting quality is variable and some older charts covering Mergui waters are of limited accuracy; mariners should use the latest British Admiralty or Thai Hydrographic charts and exercise heightened caution.

The bathymetry of the Andaman Sea reflects its complex tectonic setting on the eastern margin of the Indian Plate, adjacent to the Sunda subduction zone. The northern and central basin reaches average depths of 800–1,200 m, while the southern basin deepens progressively toward the Andaman Trench — a submarine trench associated with the subduction of the Indian Plate beneath the Burma microplate — reaching maximum depths of approximately 4,198 m. This trench was the locus of the December 2004 Sumatra-Andaman earthquake, the third-largest earthquake ever recorded instrumentally, with a moment magnitude of 9.1–9.3.

2. Oceanography & Climate

The Andaman Sea lies entirely within the tropical monsoon belt and is dominated oceanographically and meteorologically by the Indian Ocean monsoon system — one of the world's most powerful and consistent atmospheric circulation patterns. The monsoon creates two distinct maritime seasons that govern navigation, fishing, tourism, and trade throughout the region with a regularity that has underpinned maritime activity in this part of the world for thousands of years.

The Southwest (SW) Monsoon — locally called the “wet season” or “summer monsoon” — prevails from approximately May to October, driven by the thermal low pressure system that develops over the heated Asian landmass. The resulting pressure gradient draws moisture-laden southwesterly airflow across the Indian Ocean and into the Andaman Sea with considerable force. Sustained wind speeds of Beaufort Force 4–6 (11–27 knots) are typical throughout the SW Monsoon, with episodic strengthening to Force 7–8 (28–40 knots) during June and July. Significant wave heights of 2–4 m are standard in the open Andaman Sea during this period; in the northern basin, fetch effects and interaction with the continental coast can generate seas of 4–5 m. Visibility is frequently reduced to 2–5 nautical miles or less in heavy monsoon rain squalls, which can be of short duration but intense. Small craft and passenger ferries — including the inter-island services of the Andaman and Nicobar Administration — are frequently disrupted during the SW Monsoon.

The Northeast (NE) Monsoon — the “dry season” running from approximately November to April — reverses the circulation, bringing drier, cooler northeasterly airflow from the Asian continental interior across the Bay of Bengal and into the Andaman Sea. Wind speeds during the NE Monsoon are generally Force 3–4 (7–16 knots), with calmer periods of Force 2 or less common in January and February. Sea states are typically 1–2 m, with occasional short-period swells generated by distant Bay of Bengal weather. The NE Monsoon season is the operational preference for all forms of maritime activity in the region: dive tourism at the Similan and Surin Islands, small-craft navigation through the Mergui Archipelago, and coastal shipping.

Tropical cyclones originating in the Bay of Bengal represent the most severe meteorological hazard to the Andaman Sea. The Bay of Bengal is the world's most cyclone-active semi-enclosed sea, with peak seasons in April–May (pre-monsoon) and October–November (post-monsoon). Cyclones that track west-northwestward from their formation areas in the central and eastern Bay may enter the northern Andaman Sea before making landfall on the Myanmar or India (Andaman Islands) coast. Cyclone Nargis (May 2008) — which killed approximately 138,000 people in the Irrawaddy Delta of Myanmar — followed a track across the Andaman Sea before making landfall near Haing Gyi Island. Mariners in the Andaman Sea during cyclone season must monitor Joint Typhoon Warning Center (JTWC) advisories and India Meteorological Department (IMD) cyclone warnings via NAVTEX, SafetyNET, and HF radio.

The thermocline structure of the Andaman Sea reflects its tropical location and relative enclosure. Sea surface temperatures (SST) range from 27–29°C throughout the year, with minimal seasonal variation typical of tropical seas. A pronounced mixed layer 30–50 m deep overlies a sharp thermocline at approximately 50–100 m depth, below which temperatures drop rapidly to 10°C or less at 500 m. This thermal structure has implications for submarine operations, sonar performance, and the distribution of pelagic fish species. Salinity is reduced relative to the open Indian Ocean — typically 30–32 ppt in the northern Andaman Sea — due to the enormous freshwater input from the Irrawaddy River (Myanmar), which discharges approximately 430 km³ of freshwater annually through a complex delta into the northeastern Andaman Sea. The Salween and Sittaung rivers provide additional freshwater input. This riverine influence creates a significant surface salinity gradient and drives a density-driven circulation that influences both fishery productivity and the dispersal of pollutants and floating debris.

Tidal patterns in the Andaman Sea are predominantly semi-diurnal (two high and two low waters per day) but with significant diurnal inequality — a characteristic of the Indian Ocean tidal regime. Tidal ranges are modest: 2–3 m in the northern Andaman Sea near the Myanmar coast, decreasing to 1–2 m in the central sea, and 1 m or less in the southern and eastern sectors. Tidal currents are generally weak in the open sea (0.5–1.0 knots) but accelerate significantly in the narrow channels of the Mergui Archipelago, the approaches to the Malacca Strait, and the passages between the Andaman and Nicobar Islands, where streams of 2–4 knots are encountered at spring tides.

3. Marine Ecology & Biodiversity

The Andaman Sea is one of the most biologically rich marine environments in the Indian Ocean, supporting extraordinary coral reef ecosystems, high densities of megafauna, and a diversity of habitats ranging from fringing coral reefs and mangrove forests to deep-sea benthic communities in the central basin. Its relative isolation — particularly the Mergui Archipelago of Myanmar, long inaccessible to international research — has helped preserve ecological communities that have been severely degraded elsewhere in Southeast Asia.

The Similan Islands (Mu Ko Similan National Park, Thailand) and the Surin Islands (Mu Ko Surin National Park, Thailand) — both administered as national parks by Thailand's Department of National Parks, Wildlife and Plant Conservation — are recognised as among the finest coral reef diving destinations in the world. The reefs support over 200 species of hard coral, 300 species of reef fish, sea turtles, leopard sharks (Stegostoma fasciatum), and resident manta ray (Mobula alfredi) populations at predictable seasonal aggregation sites. The waters around the Similan Islands are closed to all visitors from May to October annually to allow reef recovery during the SW Monsoon.

Whale sharks (Rhincodon typus — the world's largest fish, reaching up to 12 m in length) make seasonal appearances in the Andaman Sea, typically concentrated around the Similan Islands and the northern Andaman Sea between February and April, when phytoplankton blooms associated with the inter-monsoon transition provide abundant food. Manta rays(Mobula alfredi, the reef manta ray) aggregate at cleaning stations and in areas of concentrated plankton throughout the NE Monsoon season. Hawksbill sea turtles(Eretmochelys imbricata) — a critically endangered species — nest on beaches throughout the Andaman and Nicobar Islands and the Mergui Archipelago. The Andaman Islands support important nesting populations and the Indian government maintains conservation programmes including beach patrolling and hatchery management.

Dugongs (Dugong dugon) — large, slow-moving herbivorous marine mammals that are the sole surviving members of the family Dugongidae — are present in the Andaman Sea, grazing on seagrass beds along the shallow coastal margins of the Myanmar coast, the Andaman Islands, and the northern Malay Peninsula. Dugongs are vulnerable to boat-strike, entanglement in fishing nets, and the destruction of their seagrass food source by trawling and runoff-related turbidity. Populations in the Andaman Sea are thought to be small and declining. The Irrawaddy dolphin (Orcaella brevirostris) — a distinctive blunt-snouted dolphin specially adapted to brackish and inshore environments — occurs in the river mouths and estuaries of the northeastern Andaman Sea, particularly the Irrawaddy Delta and the mouths of the Salween and Tenasserim rivers in Myanmar, where it occupies an ecological niche similar to that of the South Asian river dolphin.

The 2004 Indian Ocean Tsunami inflicted severe damage on the coral reef ecosystems of the Andaman Sea. Physical damage from the tsunami waves and the associated sediment resuspension caused widespread coral breakage and smothering, particularly in the shallow fringing reef systems of the Andaman and Nicobar Islands. Post-tsunami surveys documented 50–90% loss of live coral cover at some sites close to the epicentre. Recovery has been partial and ongoing over the subsequent two decades, complicated by successive coral bleaching events driven by elevated sea surface temperatures in 2010, 2016, and 2023. The 2016 bleaching event — associated with a strong El Niño and above-normal Andaman Sea SSTs of 31–33°C sustained for several weeks — caused mass bleaching across Thai and Andaman Islands reef systems, with some sites recording mortality of 40–60% of coral colonies.

4. Maritime Trade Routes & Shipping

The Andaman Sea occupies a position of extraordinary importance in the global maritime trade network, functioning as the primary western approach lane to the Strait of Malacca — the principal chokepoint linking the Indian Ocean with the Pacific. According to the United Nations Conference on Trade and Development (UNCTAD), approximately 90,000–100,000 vessels transit the Malacca Strait annually, representing approximately 25–30% of all world maritime trade by volume. Of particular significance is the flow of energy: an estimated 15–16 million barrels of crude oil per day transit the Malacca Strait, making it the world's most critical petroleum chokepoint — more than the Suez Canal and Bab el-Mandeb combined. All westbound traffic departing the Malacca Strait enters the Andaman Sea; all eastbound traffic approaching Malacca must cross Andaman waters.

The principal trade route of the Andaman Sea runs on a northeast-southwest axis: from the Ten Degree Channel (the deep-water passage between the Bay of Bengal and the Andaman Sea at 10°N) southwestward through the central Andaman Sea to the northern approach of the Malacca Strait at approximately 5°N 100°E. This route is used by virtually all large commercial vessels transiting between the Indian Ocean and East Asia. The Singapore–Kolkata route, serving traffic between the Bay of Bengal ports (Kolkata, Haldia, Chennai, Visakhapatnam, Chittagong) and Singapore, traverses the full length of the Andaman Sea. Typical transit times for a laden VLCC at 13 knots through the Andaman Sea from the Ten Degree Channel to the Malacca Strait entrance are approximately 36–48 hours.

Liquefied natural gas (LNG) is an increasingly significant commodity in Andaman Sea shipping. Myanmar operates extensive offshore gas fields in the Gulf of Martaban and off the Tenasserim coast, with production piped to the Thai grid via the Yadana and Yetagun pipeline systems as well as exported by LNG carrier. Thailand's PTTEP operates the largest fields. LNG carrier traffic through the Andaman Sea to Thailand, Singapore, and beyond has grown substantially since the commissioning of the Map Ta Phut LNG import terminal in Thailand (2011) and the Pengerang LNG facility in Malaysia. Larger LNG carriers of the Q-flex and Q-max class (up to 266,000 m³) require careful draft management in the shallow southern approaches to the Malacca Strait but can transit the deep central Andaman Sea without constraint.

Container traffic through the Andaman Sea is dominated by feeder services linking the Malacca Strait hub ports (Singapore, Port Klang, Tanjung Pelepas) with the smaller regional ports of Myanmar, Thailand, India (Andaman Islands), and the Bay of Bengal. Mainlane container services between Europe/India and East Asia transit the Andaman Sea at speed (17–20 knots) without port calls, while feeder services zigzag between Yangon, Phuket, Penang, and smaller coastal destinations. Myanmar's offshore oil and gas fields generate periodic traffic from supply vessels and work boats operating out of Yangon, Myeik, and chartered anchorages.

The potential development of a Kra Canal — a proposed canal cutting through the Kra Isthmus of southern Thailand that would bypass the Malacca Strait — has been periodically debated since the 17th century and most recently advanced as a Chinese-backed infrastructure proposal under the Belt and Road Initiative framework. A Kra Canal would reduce the voyage distance between the Indian Ocean and the South China Sea by approximately 1,200 km and cut transit time by 2–3 days, potentially transforming Andaman Sea shipping patterns by creating a direct passage from the western Andaman Sea to the Gulf of Thailand. As of 2026, no formal construction decision has been made by Thailand, and the project faces substantial environmental, geopolitical, and financial objections.

5. Key Ports & Harbours

The Andaman Sea is served by a diverse range of ports reflecting the economic and political diversity of its coastal nations — from the well-equipped commercial facilities of Thailand and Malaysia to the more restricted and variable-quality infrastructure of Myanmar, and the island-logistics-focused facilities of India's Andaman and Nicobar Union Territory.

Port Blair (IN PBL) — Administrative Capital, Andaman Islands

Port Blair, situated on the southeastern coast of South Andaman Island at approximately 11°40'N 92°45'E, is the headquarters of the Andaman and Nicobar Islands Administration of India. The port is administered by the Andaman and Nicobar Islands Integrated Development Corporation (ANIIDCO) and the Directorate of Shipping Services. Port Blair handles inter-island passenger and cargo ferry services connecting the 38 inhabited islands of the Andaman group, plus infrequent mainland connections to Chennai (Madras) and Kolkata. The main port area — Haddo Wharf and Phoenix Bay Jetty — can accommodate vessels up to approximately 150 m LOA with a maximum draft of around 7.5 m. Deep-draft vessels and occasional larger supply ships anchor in the Port Blair anchorage in Corbyn's Cove or off Ross Island. The Indian Navy maintains a significant base (INS Jarawa) at Port Blair, and all foreign naval vessels require prior diplomatic clearance. VHF working channel is Ch 16 (calling) and Ch 12 (Port Control). The port is classified as a restricted port for foreign nationals, requiring Restricted Area Permits (RAP) for access beyond the settlement.

Phuket (TH PHK) — Thailand's Andaman Hub

Phuket, Thailand's largest island (543 km²), connected by bridge to the Malay Peninsula, hosts multiple port facilities on its eastern coast facing the Andaman Sea. The principal commercial port — Tha Rua — handles petroleum product tankers, supply vessels for offshore operations, and general cargo. The Ao Makham Bay area houses major petroleum storage and handling facilities supplying fuel to the extensive maritime traffic of the northern Malacca Strait approaches. Phuket is a major base for offshore supply vessels servicing the Thai-owned gas fields of the western Gulf of Thailand and the PTTEP Andaman Sea concession blocks. The port coordinates traffic on VHF Ch 16 and Ch 12 (Phuket Port Control). The nearby Boat Lagoonand Royal Phuket Marina — at Ao Po on the northeast coast — serve the large and growing superyacht and cruising yacht community that uses the Andaman Sea as a cruising ground during the NE Monsoon season (November–April). Phuket International Airport (IATA: HKT) provides international crew change capability.

Penang (MY PGN) — Northern Gateway to Malaysia

The Port of Penang (Pulau Pinang), operated by Penang Port Sdn Bhd, is located at the northern entrance of the Strait of Malacca at approximately 5°24'N 100°21'E, effectively straddling the boundary between the Andaman Sea and the Malacca Strait. Comprising two terminals — the North Butterworth Container Terminal (NBCT) and Swettenham Pier Cruise Terminal — plus the conventional cargo berths at Butterworth on the mainland, Penang handles approximately 1.5 million TEU of container cargo annually. The port also handles bulk cargo (palm oil, rice, sugar) and petroleum products. The approach channel to Penang is buoyed and dredged to approximately 12 m LAT. VHF Port Control operates on Ch 12 and 14. Penang is significant as the northernmost major port on the Malaysian side of the Malacca Strait and serves as the primary maritime gateway for northern Malaysia (Kedah, Perlis) and southern Thailand (Hat Yai). Pilotage is compulsory for vessels above 50 GT.

Yangon/Rangoon (MM RGN) — Myanmar's Main Commercial Port

The Port of Yangon (Rangoon), administered by Myanmar Ports Authority (MPA), is the largest and most important port in Myanmar, handling approximately 90% of the country's seaborne trade. Located approximately 30 km inland from the Gulf of Martaban (northeast Andaman Sea) on the Rangoon River, the port is approached via a 30 km dredged channel (the Rangoon/Yangon River approach) maintained to approximately 8.5 m LAT — a constraint that excludes most panamax and larger vessels, limiting Yangon to general cargo, container feeder, tanker, and bulk vessels of moderate size. The port handles container cargo (via Asia World Port Terminal and Myanmar Industrial Port), general cargo, petroleum products, and LPG. Pilotage is compulsory from the Foul Island Pilot Station seaward. VHF coordination is on Ch 16 and 14. Political instability following the February 2021 military coup has introduced significant uncertainty into port operations, labour availability, and financial clearing procedures at Yangon; mariners and operators should consult current advisories before calling.

Tavoy/Dawei (MM TVY) — Southern Myanmar

Tavoy (Dawei) in Tanintharyi Region, southern Myanmar, is a small coastal port approximately 580 km south of Yangon at 14°05'N 98°12'E. The port handles timber, agricultural products, and coastal cargo. The proposed Dawei Deep Sea Port and Special Economic Zoneproject — an ambitious industrial and port development that would include a deep-sea port capable of accommodating Panamax vessels, an industrial estate, and a road/pipeline link to Thailand — has been under development since 2008 with various international partners including Thailand, Japan, and Italian-Thai Development. As of 2026, the project remains significantly delayed due to political and financing obstacles. If completed, Dawei would provide Myanmar with direct access to the Andaman Sea for large vessels, bypassing the congested Malacca Strait for cargo destined for Thailand and western markets.

Mergui/Myeik (MM MEI) — Archipelago Gateway

Myeik (Mergui) at 12°26'N 98°35'E is the largest settlement of the Tanintharyi Region coast and serves as the primary access point for the Mergui Archipelago. The port handles coastal cargo, fishing vessels, and inter-island services. Facilities for international shipping are limited, and the approach channels require careful navigation using large-scale charts. The port is the administrative gateway for any foreign vessel seeking to enter the restricted waters of the Mergui Archipelago; permits from Myanmar Ports Authority and the Ministry of Hotels and Tourism are required. The surrounding waters support one of Southeast Asia's largest pearl farming industries as well as substantial artisanal and semi-industrial fishing operations targeting prawns, grouper, and high-value reef fish for the Thai market.

6. Historical & Strategic Significance

The Andaman Sea has been a maritime highway for at least 2,000 years, carrying the seaborne trade of the ancient “Maritime Silk Road” between South Asia, Southeast Asia, and China. Arab, Indian, Malay, and Chinese traders navigated its waters, exploiting the seasonal monsoon winds with remarkable sophistication — the SW Monsoon to carry dhows and junks eastward from India toward the Malay Peninsula and China, the NE Monsoon to return westward. The Srivijaya Empire (7th–13th centuries CE), centred at Palembang in Sumatra, controlled the Malacca Strait and much of the Andaman Sea trade routes, extracting tribute from passing vessels in a pattern of maritime political economy that has modern echoes in contemporary port and passage dues.

British colonialism fundamentally transformed the Andaman Sea from the late 18th century onward. The East India Company established the first British settlement on the Andaman Islands in 1789 at Port Cornwallis (present-day Port Blair), primarily as a penal colony and strategic base. The colony was abandoned in 1796 but re-established in 1858 following the Indian Rebellion of 1857, when the Andaman Islands became one of the British Empire's most feared penal settlements — the infamous Cellular Jail (Kālā Pānī, “Black Waters”), built 1896–1906, housed thousands of Indian independence activists in solitary confinement. Burma (Myanmar) was progressively annexed by Britain between 1824 and 1885 through three Anglo-Burmese Wars, incorporating the Tenasserim Coast (and the Mergui Archipelago) in 1826, giving the British Empire full control of the eastern Andaman Sea coastline and, critically, secure access to the Malacca Strait.

The Second World War brought catastrophic disruption to the Andaman Sea. Japan occupied the Andaman and Nicobar Islands in March 1942 — within weeks of the fall of Singapore — establishing a forward naval and air base from which to threaten British India and interdict Allied shipping in the Bay of Bengal and Indian Ocean. The Japanese garrison at Port Blair (which they renamed Andoman/Nikobar) held the islands until formal Japanese surrender in September 1945. During the occupation, the Cellular Jail was used by Japanese military authorities to detain and execute suspected Allied sympathisers among the local population. The Andaman and Nicobar Islands reverted to British administration in 1945 and subsequently became part of the Republic of India at independence in 1947, as an administered Union Territory.

The 2004 Indian Ocean Tsunami of 26 December stands as the defining modern catastrophe of the Andaman Sea region. The Mw 9.1–9.3 earthquake ruptured a 1,600 km section of the Sunda megathrust subduction zone off the northern tip of Sumatra, generating tsunami waves that devastated coastlines across 14 countries within hours. In the Andaman and Nicobar Islands — located just 150 km from the epicentre — over 7,000 people were killed and entire island communities were destroyed. Car Nicobar Island, hosting an Indian Air Force base, was particularly severely affected. In Thailand, the tourist beaches of Khao Lak (Phang Nga Province) — struck by waves reaching 15–20 m — suffered over 4,000 deaths, and the resorts of Phuket's Patong Beach were heavily damaged. The disaster prompted immediate regional action to establish the Indian Ocean Tsunami Warning System (IOTWS) under UNESCO-IOC coordination, now operational through an integrated network of seismometers, deep-ocean DART buoys, and tide gauge stations providing warnings within 10–15 minutes of a triggering earthquake.

The Malacca Strait piracy corridor — which encompasses the southern Andaman Sea approaches — was historically one of the world's most serious maritime security concerns. The International Maritime Bureau (IMB) recorded over 200 incidents per year in the Malacca Strait in the early 2000s. Coordinated regional action by Indonesia, Malaysia, Singapore, and Thailand — including the MALSINDO Coordinated Patrol (2004), the Eye in the Sky initiative (2005), and information sharing through the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP, established 2006) — dramatically reduced piracy in the Malacca Strait, with incidents falling to single digits by the 2010s. The Andaman Sea itself has seen relatively few piracy incidents, with the highest-risk area remaining the southern Malacca Strait and the approaches to the Singapore Strait rather than the open sea.

7. Navigation Safety & Hazards for Mariners

The Andaman Sea falls within NAVAREA VIII (Indian Ocean), coordinated by the National Hydrographic Office of India (NHO Dehradun). NAVAREA VIII covers the Indian Ocean from approximately 10°E to 100°E longitude, including the waters of India, Sri Lanka, the Maldives, and the Andaman and Nicobar Islands, as well as the Myanmar, Thai, and Malaysian portions of the Andaman Sea. Navigational warnings are broadcast on NAVTEX(518 kHz) from transmitters at Mumbai, Goa, Chennai, and Port Blair (callsign “A”). Urgent warnings may also be promulgated via SafetyNET on Inmarsat-C and through the IMD meteorological services. Mariners transiting the Andaman Sea should register all NAVTEX transmitters applicable to their track and maintain a continuous receiver watch.

The approach to the Malacca Strait Traffic Separation Scheme (TSS) via the Andaman Sea requires careful passage planning. The Malacca Strait TSS — adopted by IMO and in force since 1998, revised in 2011 — operates three Traffic Lanes along the length of the Strait, with One-Way Traffic Lanes in the Singapore Strait approach (COLREG Rule 10 applies in full). Vessels approaching the Malacca Strait from the northwest — from the Ten Degree Channel or the Bay of Bengal — should plan their approach to join the appropriate inbound (eastbound) lane. The northbound/inbound lane is navigated keeping to starboard in the conventional sense of right-hand traffic flow. Mariners must familiarise themselves with the full extent of the TSS before arrival; departure from an incorrect lane in the Malacca Strait has resulted in collisions and has been prosecuted by Malaysia, Indonesia, and Singapore under their respective maritime jurisdictions.

Myanmar territorial waters and restricted areas present a significant navigation management challenge in the eastern Andaman Sea. Myanmar claims a 12-nautical-mile territorial sea and a 200-nm Exclusive Economic Zone (EEZ) from its Tenasserim coast and the baseline drawn around the Mergui Archipelago. Transit through Myanmar's territorial sea — particularly through the channels of the Mergui Archipelago — requires prior notification to Myanmar Ports Authority and, in many cases, explicit permission. Foreign warships must give advance diplomatic notice through official channels. The charting of Mergui Archipelago waters is less comprehensive than major trade route areas; mariners should use British Admiralty charts BA 839, 840, 841, and 842 (Mergui Archipelago series) as well as Thai Hydrographic Department charts, and should navigate at reduced speed with a dedicated lookout posted and echo sounder actively monitored.

Cyclone routing is a seasonal obligation for mariners operating in the Andaman Sea during April–May and October–November. Bay of Bengal cyclones that enter the Andaman Sea typically move west-northwestward or northwestward, threatening the Myanmar coast and potentially the Andaman and Nicobar Islands. The primary routing guidance for cyclone avoidance in the Indian Ocean/Andaman Sea is the UK Meteorological Office and IMD's seasonal guidance and the IMD Cyclone Warning Division advisories. Vessels should maintain a 250-nautical-mile avoidance radius from active tropical cyclones and be prepared to deviate substantially from planned routes. Port Blair and the outlying Andaman Islands anchorages offer limited shelter during severe cyclone passage; the preferred action for most commercial vessels is to move away from the projected cyclone track rather than seek shelter in the archipelago.

Illegal, Unreported, and Unregulated (IUU) fishing is prevalent throughout the Andaman Sea, with large numbers of unregistered or improperly documented fishing vessels from Thailand, Myanmar, and other regional nations operating in overlapping EEZ zones. Commercial vessels may encounter fishing fleets — including boats operating without proper lights at night — anywhere in the Andaman Sea, and the risk of collision with small, poorly lit fishing vessels is particularly acute during hours of darkness and in poor visibility. Radar watch should be maintained in fishing areas and speed should be moderated consistent with COLREG Rule 8 (action to avoid collision). The ReCAAP Information Sharing Centre (ISC) in Singapore publishes regular reports on incidents in the Andaman Sea and Malacca Strait approaches, including piracy, robbery, and stowaways; mariners should consult these reports prior to and during transit.

Shoal water and reef navigation is a primary concern for vessels operating near the island groups. The Andaman and Nicobar Islands, the Similan Islands, and the Mergui Archipelago are surrounded by extensive coral reef systems, isolated shoals, and submerged rocks. Landfall approaches to Port Blair should be made in daylight where possible, using the leading lights on Ross Island and South Point. Approaches to the Similan Islands and Surin Islands — used by dive liveaboard vessels and yachts — require careful pilotage using large-scale charts and GPS cross-checks. Anchoring on or near coral reefs is prohibited within Thai national park waters and is environmentally damaging everywhere; vessels should use designated mooring buoys where provided.

8. Environmental Issues

The Andaman Sea faces an array of converging environmental pressures that threaten both its ecological integrity and the long-term sustainability of maritime activities that depend on a healthy marine environment. These pressures are particularly acute given the sea's role as a biological reservoir for the Indo-Pacific region and the socioeconomic dependence of coastal communities on artisanal fisheries, dive tourism, and reef-related ecosystem services.

Coral bleaching is now a recurring crisis in the Andaman Sea. The 2010 bleaching event — associated with a positive Indian Ocean Dipole and elevated SSTs — caused significant mortality at reefs across the Andaman and Nicobar Islands and the Thai national park islands. The more severe 2016 event, driven by a combination of the 2015–16 El Niño (the strongest since 1997–98) and long-term warming of the Andaman Sea, saw sustained SSTs of 31–33°C across much of the sea for weeks, triggering mass bleaching from the Similan Islands south to the Straits of Malacca. Scientists from Chulalongkorn University and the Department of Marine and Coastal Resources documented mortality rates of 40–60% of coral cover at the most severely affected sites. The 2023 bleaching event — the fourth major event in less than two decades — further confirmed that bleaching is transitioning from an episodic to a near-chronic stressor on Andaman Sea reef systems. The Intergovernmental Panel on Climate Change (IPCC) projects that coral reefs globally will experience annual bleaching conditions by 2050 under a 1.5°C warming scenario — a threshold that the Andaman Sea is on track to exceed within this timeframe.

Mangrove destruction has eliminated vast areas of coastal habitat throughout the Andaman Sea margin. Thailand's mangrove coverage declined from an estimated 367,900 ha in 1961 to approximately 170,000 ha by the early 2000s — a loss of more than 50% in 40 years — driven overwhelmingly by conversion to shrimp aquaculture ponds and coastal development. Although rates of mangrove loss have slowed following national legislation and international attention to the issue, rehabilitation of degraded mangrove areas has been slow and incomplete. In Myanmar, mangroves along the Irrawaddy Delta and Tenasserim coast have been similarly cleared for aquaculture, though remote sensing data suggests that Myanmar retains larger intact mangrove areas than Thailand due to limited coastal development in some regions. Mangrove loss reduces the sea's resilience to storm surge, eliminates nursery habitat for commercially important fish and crustaceans, and releases substantial quantities of stored “blue carbon” as CO₂ when cleared.

Plastic pollution from the Malacca Strait and regional river systems — particularly the Irrawaddy, Salween, and Sittaung in Myanmar, and the rivers draining Thailand's Andaman coast — accumulates in the Andaman Sea, transported by monsoon-driven currents and concentrated in surface gyres. Ocean Conservancy and other organisations estimate that Southeast Asia is responsible for a disproportionate share of global ocean plastic pollution relative to its GDP. Ghost fishing gear — lost or abandoned nets, longlines, and fish traps — entangles and kills marine megafauna including sea turtles, dolphins, and sharks throughout the Andaman Sea.

Offshore gas extraction in the Gulf of Martaban and adjacent Andaman Sea blocks operated by PTTEP (Thailand's national oil company), Total, and MOGE (Myanmar Oil and Gas Enterprise) presents chronic low-level pollution risks from platform discharges and pipeline leakage, as well as acute blowout risk. The Montara Well Head Platform blowout in the Timor Sea in August 2009 — which released an estimated 400 barrels of oil per day for 74 days before being capped — demonstrated the consequences of a major offshore blowout in a remote tropical sea environment. Emergency response capability in the Andaman Sea and surrounding region is less developed than in North Sea or Gulf of Mexico operating areas.

Climate change and sea level rise present long-term existential threats to low-lying coastal communities and island ecosystems throughout the Andaman Sea. IPCC AR6 projections indicate 20–40 cm of sea level rise by 2100 under intermediate emissions scenarios, with higher-end projections reaching 70–100 cm or more under high-emissions pathways. Many of the low-lying coral islands and atoll-like formations of the Andaman and Nicobar archipelago — as well as the mangrove-fringed coastal strips of Myanmar and Thailand — are acutely vulnerable to even modest increases in mean sea level, as storm surge and wave overwash events already affecting these areas will become more frequent and severe. The warming of the Andaman Sea at approximately 0.5–0.8°C per decade (as measured by satellite SST records since 1982) is reducing the thermal tolerance margin of reef-building corals and is associated with northward range shifts of several warm-water fish species into previously cooler waters.