El Niño occurs every two to seven years, marked by wetter conditions in some parts of the world and drier weather in others. Its formation depends on weakening trade winds—persistent air currents that have propelled countless sailboats from east to west along the equator.
Typically, these winds push deeper, cooler eastern Pacific waters toward the surface through a process called upwelling. But when trade winds weaken, upwelling subsides, and warm surface waters become warmer. Those warmer waters promote air pressure changes that further weaken trade winds, eventually leading to the weather patterns associated with El Niño.
But existing research has overlooked the potential role of ocean salinity, or salt distribution, in shaping El Niño events, according Shineng Hu, assistant professor of climate dynamics at the Nicholas School, who oversaw the new study.
Ocean salt is not uniform—some parts of the sea are saltier, some fresher, depending on rainfall, evaporation and other factors.
“Ocean currents can transport these salty or fresh waters around and redistribute the ocean salinity,” Hu explained. “It is also likely that this salinity variability could in turn influence ocean currents and thus climate phenomena like El Niño.”
With funding from NASA, Hu and team used publicly available ocean data to identify global patterns in salinity that have preceded major El Niño events over the past 65 years.
They found compelling correlations between certain salinity patterns and El Niño, which prompted them to ask: Does salinity actually influence El Niño, and if so, how? To address those questions, the team used computer models primed to crunch climate data.
“Basically, we used the models to see if certain salinity patterns we identified could alter El Niño conditions,” said Shizuo Liu, first author of the paper and a postdoctoral associate in Hu’s lab. “For example, we examined whether adjusting salinity would make El Niño more or less likely, or more or less intense.”
The results confirmed that certain salinity patterns can fuel El Niño strength. During springtime in the western Pacific, a combination of fresher water at the equator and saltier water farther away promote eastward ocean currents. Those currents push warm surface waters east that fuel El Niño development, the authors explained.
This process can increase the intensity of El Niño by about 20% and make an extreme El Niño twice as likely, according to the study. More intense El Niño events can cause catastrophic flooding and severe droughts, among other disasters.
“The findings indicate that salinity is another factor that should be considered in future models forecasting El Niño,” Liu said.
