Warming Seas

2015-2016 El Niño Provided ‘Natural Experiment’ on the Effects of Warming Seas

Joe Atkinson
NASA Langley Research Center

A marked increase in central Pacific Ocean sea surface temperatures during the strong 2015-2016 El Niño lofted abnormal amounts of cloud ice and water vapor unusually high up into the atmosphere, creating conditions similar to what could happen on a larger scale in a warming world.

The findings were part of a recent study led by Melody Avery, a researcher at NASA’s Langley Research Center in Hampton, Virginia. Researchers at NOAA’s Earth Science Research Laboratory, the University of Colorado at Boulder and Texas A&M University also contributed.

“In a warming world we expect sea surface temperatures to rise. And what we were able to see from a natural climate cycle with our sophisticated sensors is the impact of rising sea surface temperatures on really high-altitude convection,” said Avery. “It just makes the storms really tall, and it puts a lot of ice way up high in the atmosphere. And we also observed an unusual amount of water vapor in the stratosphere.”

Under typical conditions, storms don’t push so high into the atmosphere — most are contained in the troposphere, the lowest part of the atmosphere where we live and have weather. In fact, the anvil-shaped appearance of tall storm clouds is the result of those clouds flattening out as they reach the tropopause, ordinarily a boundary between the troposphere and stratosphere.

When water vapor reaches the stratosphere, though, like it did during the 2015-2016 El Niño, it acts as a potent greenhouse gas. Ice at that altitude can also pass from a solid state and become water vapor, further exacerbating the situation.

“This process is a self-reinforcing cycle, so it’s very important for us to understand both the process and whether or not it might change with future climate change,” said Sean Davis of NOAA’s Earth Science Research Laboratory. “This study really highlights the need for sustained measurements of Earth’s atmosphere from space. That is really the only way we can observe and monitor these events on a global scale.”

During El Niño events, surface waters in the central and eastern Pacific Ocean become warmer than usual. This warm condition affects everything from the local aquatic environment to weather in other parts of the world. The 2015-2016 El Niño was one of the three strongest on record since 1950.

Avery’s study focused on conditions in the atmosphere directly above the unusually warm 2015-2016 El Niño currents. She used measurements from NASA’s Aura satellite and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, or CALIPSO, a joint effort between NASA and France’s National Centre for Space Studies. Aura and CALIPSO are both part of the A-train, a constellation of six Earth-observation satellites that closely follow one after another along the same orbital track.

Instruments on those satellites allowed Avery and her colleagues to see the high-altitude cloud ice and water vapor with unprecedented clarity and develop model results that accounted for those variables.

According to Andrew Dessler, a professor of atmospheric sciences at Texas A&M University who contributed to the study, not accounting for those variables is like estimating daytime temperatures based solely on the time of day.

“That works OK, because daytime is warmer than night, but sometimes you’ll get a particularly cool daytime temperature that disagrees with your expectation,” he said. “You can investigate those times and you might find it was raining during those anomalously cool times. This leads you to come up with a more realistic model that incorporates time of day and meteorological conditions.”

Like Dessler, Avery believes the observations she and her fellow researchers made could prove useful if factored into future climate models.

“In order for models to be correct they have to be based on processes that are accurately represented,” she said. “And here was a chance to see this kind of process — nature provided the opportunity to see it.”