Climate models get a key feature of El Niño wrong, undermining forecast accuracy

The El Niño Southern Oscillation is one of the most intensively studied climate drivers on the planet. During the warm El Niño phase, vast volumes of warm water release heat previously trapped in the deep ocean, altering weather patterns around the globe.

Oceanographers use climate models to understand how the rate and location of this heat transfer reorganizes the atmosphere. However, new research by scientists at the University of Washington and NOAA’s Pacific Marine Environmental Laboratory has demonstrated how climate models systematically misrepresent one of the important modulators of atmospheric-ocean heat exchange: clouds.

The study was published recently in the journal Geophysical Research Letters.

"Most coupled ocean-atmosphere models are typically not able to simulate low cloud cover increases in the southeastern tropical Pacific during El Niño," said PMEL’s former senior scientist Michael McPhaden. "In general, they also do not simulate sea surface temperatures well in this region. "These systematic errors limit the ability of climate models to accurately simulate the development of El Niño events."

El Niños come in two "flavors" that differ primarily by where the anomalous ocean warming occurs and how it affects global weather patterns. During an Eastern Pacific El Niño, like the one currently underway, warming is strongest in the eastern equatorial Pacific and off the coast of South America. During "garden variety" Central Pacific El Niños, warming is concentrated thousands of miles to the west near the International Dateline, while the waters off South America remain near average.

Though winds and ocean currents significantly influence the transfer of heat to the atmosphere, clouds also play a major role by either blocking sunlight or trapping heat. Tall, deep clouds shade and cool the ocean, while low, thick clouds that burn off as the ocean warms allow more sunlight to heat the water. How these different cloud types react during ENSO has historically been difficult to predict, McPhaden said.

But after analyzing decades of ocean and atmospheric measurements, McPhaden, along with University of Washington researchers Robert Wood and lead author Aakash Manapat, found that during Central Pacific El Niños, stratiform low cloud cover in the tropical Pacific close to South America increases. This defies the conventional understanding that low clouds dissipate as sea surface temperatures rise. Low clouds in the Southeast Pacific actually increase in extent rather than boiling off, which helps keep the ocean cooler than expected. The persistent ocean cloud cover dampens the overall intensity of the El Niño over the southeast Pacific.

"If scientists can improve the representation of clouds in climate models, it can lead to better representation of the interactions between clouds and Pacific sea surface temperatures," Manapat said. "This will likely open the door to better seasonal forecasting of El Nino further in advance."