Andaman Sea

Under the Andaman Sea, the sea floor has tall underwater mountains and deep holes.

One island here is an active volcano.

The floor of the Andaman Sea is very uneven. Around the Andaman and Nicobar Islands, the sea floor rises quickly from about 3,000 meters deep to only 200 meters, then drops again into the Andaman Sea. There are shallow ridges, reefs and underwater banks.

The area is volcanically and tectonically active, so there are mountains on the sea floor. Barren Island, in the Andaman Islands, is an active volcano: in March 2007, a satellite photographed it giving off puffs of steam.

NASA describes a basin of rugged hidden topography: the Indian Ocean floor rises rapidly from about 3,000 m to 200 m around the Andaman and Nicobar Islands and then drops again into the Andaman Sea, with shallow ridges, reefs and submarine banks. Because the region is volcanically and tectonically active, seamounts rise from the floor.

The Advanced Land Imager on Earth Observing-1 photographed the active Barren Island volcano emitting puffs of steam on 6 March 2007. All this topography alters the flow of water sloshing into and out of the basin, which NASA gives as the main reason the sea is known for large internal waves.

The sea has layers. Cold, salty water sits at the bottom. Warmer water floats on top.

When tides push the water over underwater hills, big waves roll between the layers.

The ocean is layered: deeper water is usually colder, denser and saltier, and shallower water is warmer, lighter and fresher. When tides and currents push these layers over ridges on the sea floor, waves form inside the sea, between the layers. They are called internal waves.

Internal waves can be hundreds of meters tall and tens to hundreds of kilometers long, yet they move along tens to hundreds of meters below the surface. From space, they show up as long stripes of smooth and rough water.

In the 19th century, oceanographer Matthew Fontaine Maury described 'ripplings' in the Andaman Sea that beat against ships; scientists now call them internal waves, or solitons, studied from ships since the 1960s and now seen on a grand scale by satellites. They form because density and salinity differences make ocean layers behave like separate fluids; tides, currents, gravity and Earth's rotation drive these layers over ridges and canyons.

At the surface they appear as alternating bands of calm water over crests and rough water over troughs, which sunglint reveals in Landsat 8 images from 29 November 2019. NASA notes they can be hundreds of meters tall and tens to hundreds of kilometers long, last many hours, and focus and dissipate energy in the ocean.

Many kinds of dolphins and whales swim around the Andaman Islands.

Dugongs graze on seagrass in quiet bays.

Scientists have recorded 16 kinds of marine mammals in the waters of South Andaman: 15 kinds of whales and dolphins, plus the dugong. Indo-Pacific bottlenose dolphins live near the shore, have been seen there with calves and hunt fish such as Indian mackerel around the reefs.

A small group of dugongs grazes on seagrass meadows in sheltered bays and channels. Dugongs are listed as Vulnerable on the IUCN Red List, and across the Andaman and Nicobar Islands the places they use are estimated to have shrunk by about 60% over 20 years.

The Important Marine Mammal Area (IMMA) factsheet for the Southern Andaman Islands lists 16 marine mammal species, 15 cetaceans and the dugong, recorded from vessel, ferry and opportunistic surveys. Only Indo-Pacific bottlenose dolphins (Tursiops aduncus) are nearshore residents, seen with calves and feeding on Indian mackerel around reefs; Omura's whale, recorded twice, marks the species' northern record in the Indian Ocean.

A small, Vulnerable dugong population uses seagrass meadows of Halophila and Halodule; D'Souza and colleagues (2013) estimated that dugong occupancy across the archipelago declined by 60% over 20 years, leaving them mostly in sheltered bays and channels.

On a reef, never touch or stand on the coral.

Watch dolphins and dugongs from far away, and never feed them.

NOAA, the United States ocean agency, asks people to watch dolphins and other sea animals from a safe, respectful distance, never feed them, and never try to touch them or swim with them. Snorkelers and divers should not touch, kick or stand on corals.

At the beach, NOAA warns about rip currents. If one catches you, don't fight it: swim parallel to the shore, then back to land at an angle.

NOAA Fisheries advises viewing marine mammals from a safe, respectful distance (at least 50 yards, about 46 m, from dolphins by boat), never feeding them, limiting viewing to 30 minutes or less, and not touching, kicking or standing on corals. These are US guidelines, but the reasons behind them apply to any sea, especially where a small dugong population depends on a few seagrass meadows.

NOAA's National Ocean Service notes that rip currents can move at up to eight feet (about 2.4 m) per second; swim parallel to the shore and return at an angle rather than fighting one.