Picture a seesaw: one end goes up, the other goes down. In the South China Sea, a similar seesaw operates underwater, but instead of height, it swings heat. When the northern part of the sea warms up, the southern part cools down at the same time.
Scientists looked at depths from 50 to 150 meters and noticed that this seesaw kicks in every summer. When the summer monsoon is strong, the north cools while the south warms. When the wind is weak, it's the opposite. The wind is controlled by El Niño, the warm current in the Pacific Ocean that arrives every few years.
Previously, no one understood why underwater temperatures varied from year to year. Now it's clear: it's a seesaw that the monsoon rocks. This knowledge helps provide early warnings about hurricanes and underwater heat waves, which kill corals and fish.
The subsurface dipole is invisible at the surface: during strong monsoon years, the entire basin cools at the surface due to evaporation and upwelling, while the dipole hides below 50 m.