by Emily Benson

Sea levels are rising worldwide, but not at a constant rate. Higher sea levels mean floods can reach further inland and storms can cause more damage.
Credit: U.S. National Park Service, via Flickr.

Patterns of sea level changes in the Pacific may be a better way to monitor global temperatures than measuring ocean temperatures at the sea surface, new research finds. Those changes in sea level can explain observed global temperature trends and even predict how much temperatures will change during the current El Niño event, according to the researchers.

Because water expands when it heats up, warming ocean water – regardless of whether it’s deep or shallow – will cause the sea level to rise, said Cheryl Peyser, a geoscience graduate student at the University of Arizona in Tucson, Arizona. Sea surface temperatures, on the other hand, might not reveal changes buried beneath the waves.

“Sea level is rising, but that’s not uniform across the globe,” Peyser said. Her research shows that differences in sea surface height can indicate how much heat is trapped in the ocean.

Peyser and a team of researchers from the University of Arizona and NASA’s Jet Propulsion Laboratory in Pasadena, California, used satellite data to measure sea levels in western and eastern sections of the Pacific Ocean. They then created a “seesaw index” that quantifies the difference in sea level height – and therefore ocean heat – between these two areas.

In an ensemble of climate models, changes in the seesaw index correspond with global temperature trends, Peyser reported at the 2015 American Geophysical Union Fall Meeting in San Francisco. The seesaw index was a better predictor of global temperature trends than sea surface temperature alone, Peyser said.

Between 1998 and 2012, global temperatures continued to rise in response to anthropogenic carbon emissions, but the rate of warming slowed down, Peyser said. During that time, the seesaw index skewed toward higher sea levels in the western Pacific relative to the eastern Pacific. (Some scientists suspect this “warming hiatus” may be an artifact of sampling methods, but the issue is far from resolved.)

“She’s showing there was movement of heat in the Pacific,” said Sang-Ki Lee, an oceanographer at the University of Miami in Miami, Florida, who was not involved in the study. “Heat actually moved from east to west and piled up there.”

That makes sense, Lee said, because heat entering the ocean instead of the atmosphere could explain the warming hiatus. A direct proxy of ocean heat content, like the seesaw index, may be more useful than simply relying on sea surface temperatures, he added.

The researchers think that the correlation between warmer water in the western Pacific and the slowdown in rising global temperatures is due to atmosphere-ocean interactions driven by winds. The researchers hope to investigate the mechanisms behind the link between seesawing seawater and global temperature in future studies.

– Emily Benson is a science communication graduate student at UC Santa Cruz. Visit her website at erbenson.com and follow her on Twitter at @erbenson1.