The Indonesian Throughflow

The key to understanding why the diving in Bali is so good and so potentially challenging, particularly on the east coast, are Sverdrups and the Indonesian Throughflow.

An excellent explanation of Sverdrups and the Indonesian Throughflow can be found on page 130 & 131 of David Pickell & Wally Siagian’s excellent book Diving Bali (ISBN 962-593-323-9) but let me provide the Readers Digest version. To the northwest of the Indonesian archipelago lies the Pacific Ocean where the sea level is 150 mm (6 ins) above average, whilst to the south lies the Indian Ocean where the sea level is 150mm below average.

This disparity is caused by the trade winds and associated currents that act in opposite directions in the northern & southern hemispheres, but the overall result is a massive flow of water from the Pacific to the Indian Ocean. So huge is the volume 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 his book, if you think of a river 100m wide, 10m deep and flowing at 4 knots. Then imagine 500 similar rivers - that’s one Sverdrup!

It is estimated that the total amount of seawater that passes through the Indonesian Throughflow is 20-22 Sverdrups, or 10,000 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.

There are a limited number of channels between the islands of the Lesser Sundas and of these the 35km wide Lombok Strait, between Bali and its neighboring island Lombok, offers the most direct path to the Indian Ocean. It is estimated that about 20% of the shallow water flow of the Indonesian Throughflow passes through the Lombok Strait, which in terms of rivers means 1500 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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