The ocean's invisible giants
Beneath the seemingly calm surface of the South China Sea, some of the world's most powerful waves are moving. These are not surface waves, but internal waves, propagating along density layers deep within the ocean. The largest of these waves originate in the Luzon Strait, the deep channel between Taiwan and the Philippines. Here, strong tidal currents flow back and forth over a pair of massive underwater ridges. This interaction between the tide and the seafloor topography generates massive disturbances deep in the ocean.
As the powerful tide pushes water over these submarine ridges, it converts tidal energy into internal waves, a process known as barotropic to baroclinic energy conversion. The specific spacing of the two main ridges in the strait is almost perfectly tuned to amplify these waves. The result is a series of enormous internal waves that then propagate westwards into the South China Sea. These waves, often appearing in packets, travel for hundreds of kilometers across the deep basin.
A scale that defies models
Field observations and satellite images have confirmed the colossal scale of these phenomena. The waves can have amplitudes (the vertical distance from trough to crest) of over 200 meters. Their crests stretch for more than 200 kilometers, and they can move water vertically by 200 meters in just ten minutes. The energy flux in the Luzon Strait is immense, with estimates of lee wave generation reaching up to 19.7 gigawatts.
These waves travel at speeds of 2 to 3 meters per second. As they move west and encounter the shallow continental shelf, they steepen and break, much like surface waves on a beach. This breaking process releases a tremendous amount of energy, creating turbulence up to 10,000 times greater than that typically found in the open ocean. The turbulent mixing driven by these waves helps distribute heat and nutrients throughout the sea. The sheer scale and energy of these waves are not fully captured by current global climate models and is a major missing piece in understanding ocean dynamics.