Internal waves in the South China Sea

The 250-km-wide Luzon Strait divides the Philippine island of Luzon from Taiwan to the north. To the east of the strait lies the Pacific Ocean; to the west, the South China Sea. When tides drive water back and forth through the strait, the sloshing of the vertically stratified water generates internal gravity waves. Of all the internal waves in Earth’s oceans, the ones that propagate westward through the shallow South China Sea are the largest. Now, thanks to a marine field campaign that involved measurements from ships, autonomous underwater vehicles, buoys, and spacecraft, those waves have been comprehensively characterized. Known as the Internal Waves in Straits Experiment (IWISE), the 25-institution campaign began as a pilot in 2010 and ran in earnest in 2011. Analysis of the data gathered, combined with numerical modeling, has revealed that internal tides in the Luzon Strait generate 24 GW of wave power, of which 60% propagates away and 40% dissipates locally. Internal waves in the South China Sea can top 200 meters in height. When they break on China’s continental shelf, they engender levels of turbulence that exceed those found in the open ocean by a factor of 10 000. Derived from a simulation, the accompanying figure shows the waves’ amplitude at a depth of 200 meters. The findings from IWISE will improve climate models and elucidate the waves’ role in the transport of nutrients, sediment, and pollutants. (M. H. Alford et al., Nature 521, 65, 2015, doi:10.1038/nature14399 .)