How NASA Spotted El Niño Changing the Saltiness of Coastal Waters

At other times, the opposite was found: Areas with higher-than-normal rainfall over land saw increased river discharge, reducing salinity near those coasts.

“We’re able to show coastal salinity responding to ENSO on a global scale,” said lead author Severine Fournier, an ocean physicist at JPL.

The team found that salinity is at least 30 times more variable in these dynamic zones near coasts than in the open ocean. The link between rain, rivers, and salt is especially pronounced at the mouths of large river systems such as the Mississippi and Amazon, where freshwater plumes can be mapped from space as they gush into the ocean.

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Salt as Signal

With global warming, researchers have been observing changes in the water cycle, including increases in extreme precipitation events and runoff. At the intersection of land and sea, coastal waters may be where the impacts are most detectable.

“Given the sensitivity to rainfall and runoff, coastal salinity could serve as a kind of bellwether, indicating other changes unfolding in the water cycle,” Fournier said.

She noted that some of the world’s coastal waters are not well studied, despite the fact that about 40% of the human population lives within about 60 miles (100 kilometers) of a coastline. One reason is that river gauges and other on-site monitors can be costly to maintain and cannot provide coverage of the whole planet, especially in more remote regions.

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