In summer, the South China Sea can show a strange pattern: the water in the north warms while the south cools at the same time. A year later, it's the opposite. This isn't random; it's a steady seesaw that the ocean rocks every summer.
Scientists analyzed temperatures at depths from 50 to 150 meters over 26 years and found the dominant pattern: a north-south contrast with a boundary around 11°N. When the summer monsoon is strong, the north cools and the south warms. When the monsoon is weak, it's the reverse. This seesaw is linked to the ocean heat content and depends on the phase of El Niño.
Meanwhile, a hidden circulation works between north and south: warm water flows toward the cooler region and vice versa. It's like a seesaw where the two ends constantly trade height, but here the traded quantity is heat.
Previously, it was unclear how subsurface temperature varied from year to year and what controlled it. Now it's clear: the main switch is the summer monsoon, with El Niño behind it. This finding already helps forecast tropical cyclones and subsurface heatwaves that threaten corals and fisheries.
The ocean's seesaw isn't visible from the surface, but it determines how much heat is stored in the upper water column. And when the monsoon rocks it again, we'll know what to expect.
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.