The key to understanding why the diving in Bali is so good and also why it is so potentially challenging, particularly on the east coast, are Sverdrups and the Indonesian Throughflow.
Excellent descriptions of both Sverdrups and the Indonesian Throughflow can be found in David Pickell & Wally Siagian’s book Diving Bali and on Wikipedia, but the Readers Digest version is that to the northwest of the Indonesian archipelago lies the Pacific Ocean where the sea level is 200 mm (8 ins) above average, whilst to the south lies the Indian Ocean where the sea level is 100mm (4 ins) below average.
This disparity is caused by the trade winds and associated oceanic currents that act in opposite directions in the northern & southern hemispheres, but the overall result is the largest movement of water on the planet which flows through the Indonesian archipelago from the Pacific Ocean to the north-east to the Indian Ocean in the south-west.
Map of the Indonesian Throughflow
So huge is the volume of water associated with the Throughflow that traditional measurements such as cubic meters & gallons are inadequate to describe it in an easily understandable way, so the Norwegian scientist Harald Sverdrup invented the Sverdrup – one million cubic meters of water per second.
David Pickell visualizes it best in Diving Bali: think of a river 100m wide, 10m deep and flowing at 2 knots – then imagine 1,000 of those rivers all combined together and that is one Sverdrup!
It is estimated that the total amount of seawater that passes through the Indonesian Throughflow is about 15 Sverdrups, or 15,000 of those rivers. A massive volume of water which has to make it’s way around the chain of islands that runs along the bottom part of the Indonesian archipelago called the Lesser Sundas which stretch from Bali in the west to Timor in the east.
Surface indications of the strength of the Indonesian Throughflow
There are a limited number of channels between the islands of the Lesser Sundas and the main ones that carry the majority of the water flow are the Lombok Strait between Bali & Lombok, the Sape Strait between Sumbawa & Komodo and the Ombai Strait between Alor & Timor.
Of these the 35km wide Lombok Strait offers the most direct path to the Indian Ocean and it is estimated that about 20% of the shallow water flow of the Indonesian Throughflow passes through that conduit, which in terms of rivers means 3000 of them.
That’s a lot of water, but what’s so important is that it carries with it the eggs & larvae of the marine life of the Indo-Pacific, an incredibly diverse area with over 4000 identified species – compared to around 1000 in the Red Sea & 400 in the Caribbean.
This helps to explain the intense biodiversity of some of Bali’s reefs & dive sites, but the other piece of the puzzle is the seasonal upwellings from the deep waters around the island.
The Indonesian archipelago’s underwater topography is incredibly complex with deep trenches, troughs & basins surrounding its 18,000 islands. Around the Lesser Sundas it is particularly complex, with the very deep Flores & Banda basins to the north and the Bali & Sunda trenches to the south.
As the Indonesian Throughflow weaves its way over & through this complex underwater landscape it creates upwellings that carry streams of nutrient rich cold water from the deep, which nourish the reefs of eastern Bali and other hot spots on the island.
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