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The science behind massive undersea waves Indonesian navy officials have suggested could have been responsible for dragging their submarine to the ocean floor.

May 3, 2021

Indonesian navy officials have suggested a massive underwater wave could have been responsible for dragging their submarine to the ocean floor.
All 53 crew on board the KRI Nanggala 402 are dead after it went missing off Bali last week. The remains of the submarine were found broken into at least three pieces on the ocean floor at a depth of 838 metres.
Indonesian navy officials have said images taken by the Japanese weather satellite Himawari 8 appeared to show an internal solitary wave in the area at the time the submarine disappeared.
And Nasa has previously said the Lombok Strait a relatively narrow passageway between Bali and Lombok is famous for the generation of intensive internal waves.
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Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC
This image taken by a Nasa satellite in 2016 shows oceanic nonlinear internal solitary waves in the Lombok Strait.
In commentary accompanying a 2016 satellite picture of internal waves in the strait, which allows the flow of water from the Pacific to the Indian Ocean, Nasa said the bottom of the strait was complex and rough.
Because of the variation in water movement due to the complexity of the channels and ocean interface, the tides in the strait have a complex rhythm but tend to combine about every 14 days to create an exceptionally strong tidal flow, Nasa said.
It was the combination of rough topography, strong tidal currents, and stratified water from the ocean exchange that had created the straits reputation for producing such intensive internal waves.
Niwa marine physics principal scientist Dr Craig Stevens said internal waves could be hundreds of metres high under the water. They are big, but theyre slow moving.
Indonesian Navy submarine KRI Nanggala in 2014.
Internal waves were common in oceans around the world, including New Zealand, but some of the biggest were in the South China Sea and around Indonesia, Stevens said.
They were produced where the ocean water was stratified, so it was lighter near the surface and became denser as the depth increased.
That produced a layering in the ocean, and if there was some irregularity on the seafloor, or some kind of headland, or a sloping continental shelf, the layers had to be squeezed past that hurdle.
That sets off internal waves within the ocean, Stevens said. They could be seen from space, even though they were underwater, because they changed the way ripples worked on the water surface.
Internal waves at least 100m high were produced near the Poor Knights Islands because of the shape of the continental shelf in the area.
There would also be some east of Kaikura. You can see them whey you are flying down to Christchurch. You will see slicks on the water surface that are lined up.
A photo released by the Indonesian Navy showing parts of the Nanggala at the bottom of the Bali Sea.
Internal waves were a large Earth-scale process. They are required as part of the climate system. Theyre what mixes the ocean from the bottom to the top.
While there had been suggestions in the past that internal waves could be a risk to submarines, there had not been any strong evidence for that, Stevens said.
Unless something had gone wrong, submarines were able to cope with the speed internal waves were known to move at, and the submarines buoyancy could be adjusted to adapt.
That was not to say everything was known about what internal waves could do.
You could be unlucky. There could be just some sort of confluence where some waves have come together and interacted. You never say never, Stevens said.
But these processes arent rare in a general sense. Theyre happening throughout the oceans.