A River Through the Ocean
The Makassar Strait, a deep channel between the islands of Borneo and Sulawesi, is an important chokepoint for global ocean circulation. It is the main conduit for the Indonesian Throughflow (ITF), a massive current that moves water from the Pacific Ocean to the Indian Ocean. The volume of this flow is immense, averaging between 12 and 15 Sverdrups. One Sverdrup represents a flow of one million cubic meters per second; the entire ITF is equivalent to about 15,000 large rivers.
This powerful, warm current does not flow unimpeded. As the ITF surges south, it is forced over and through the complex underwater topography of the Indonesian archipelago. The main source for the strait's internal waves is the Labani Channel, a 45-kilometer-wide and 2,000-meter-deep constriction in the strait. Tidal forces push the stratified layers of ocean water—warmer, less dense water on top and colder, denser water below—over these submarine ridges. This disturbance at the density interface gives birth to enormous internal waves. These are not surface waves, but disturbances that travel deep within the ocean.
Silent Giants
The internal waves generated in the Makassar Strait are some of the most powerful in the world. They are technically known as internal solitary waves, or solitons, because they can travel for great distances while maintaining their shape. These waves can have vertical amplitudes exceeding 100 meters, meaning the boundary layer between warm and cold water is pushed down by the height of a 30-story building as the wave passes.
Though they cause only a few centimeters of change in sea level, their passage is detectable from space. Satellites equipped with synthetic aperture radar can see the surface effects of these deep waves. The wave's internal currents interact with the surface, creating alternating bands of smooth and rough water that are visible to radar instruments. In-situ studies rely on moored instruments that measure changes in temperature and current, providing a detailed picture of the waves' structure and power. These waves drive ocean mixing, churning nutrients up from the depths and redistributing heat, which has significant effects on both local ecosystems and global climate models. The intense, often sudden currents are also a known hazard for submarines and other underwater engineering projects.