New research from the University of Washington and NOAA’s Pacific Marine Environmental Laboratory (PMEL) has found that climate models systematically misrepresent how clouds behave during El Niño events, limiting forecast accuracy. The study was published 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 Michael McPhaden, PMEL’s former senior scientist. “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 Niño events occur in two forms distinguished by the location of ocean warming. Eastern Pacific El Niños, including the event currently underway, produce the strongest warming in the eastern equatorial Pacific and off South America’s coast. Central Pacific El Niños concentrate warming thousands of miles west near the International Dateline, leaving waters off South America near average.
Clouds influence ocean-atmosphere heat exchange by either blocking sunlight or trapping heat: tall, deep clouds shade and cool the ocean, while low, thick clouds allow more sunlight through as they dissipate with rising sea surface temperatures. How these cloud types behave during El Niño-Southern Oscillation events has historically been difficult to predict, according to McPhaden.
Analyzing decades of ocean and atmospheric measurements, McPhaden, University of Washington researcher Robert Wood, and lead author Aakash Manapat found that stratiform low cloud cover near South America actually increases during Central Pacific El Niños – contradicting the conventional assumption that low clouds dissipate as sea surface temperatures rise. This unexpected persistence of cloud cover keeps the ocean cooler than expected, dampening the overall intensity of the El Niño in 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 Niño further in advance.”
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