MELISSA LUNDEN NAILED IT when she explained why El Niño causes flooding without rain.
“It doesn’t mean rain, it doesn’t mean floods, it doesn’t mean anything like that,” said Lunden, the Exploratorium’s senior scientist-in-residence. “It means that the water is abnormally warm.”
El Niño is a naturally occurring phenomenon in which the equatorial zone of the Pacific Ocean is 0.5°C hotter than normal, creating a giant swollen wave, thousands of miles long.
“It tends to happen every two to seven years,” said Lunden, adding the cycle alternates between hot and cold. “A La Niña is when this same area is 0.5°C cooler than normal.”
Lunden spoke at the San Francisco science museum’s Thursday After Dark lecture series. She described the phenomenon within global, local and historical contexts.
Centuries ago, Peruvian and Ecuadorian fishermen named the ocean current El Niño de Navidad (The Christmas Boy) because the warm water typically arrived near Christmastime, she said. Prehistoric evidence is even found through markers left on coral reefs.
“The ocean isn’t flat,” Lunden said, adding it is shaped by gravity and wind and other factors. “The ocean has all kinds of shapes to it.”
El Nino is like a planetary-scale warm water worm. Scientifically, it is called a Kelvin wave. It moves across the ocean at about 6 mph.
“You can see as it moves, it pushes the water down,” she said. “You get the wave, and then you can see on the surface it’s thickening the ocean.”
Global effects
El Niño is not caused by climate change, but climate change can amplify some of its effects, including higher water levels during coastal flooding. This year, El Nino has already set off a whole raft of effects around the globe, Lunden said.
The phenomenon kicks off along the equator. Trade winds that push warm water into the Western Pacific weaken when episodically hit with opposing winds. That causes a pool of warm water to move in the opposite direction and hit the coast of South America. From there it moves north to California and south to Peru, she said.
When that warm water is where it’s supposed to be, near Indonesia, the heat that rises from the warm water is drawn into a global jet stream. But when the warm water pools off California, it just causes havoc.
“I think of it as having a pasta pot on the stove,” Lunden said. “When I move it under the exhaust fan, the fan draws all the steam away. When I move it away from the exhaust fan, suddenly my house is full of steam, and it is condensing water onto the windows and on everything.”
By moving a pot of hot water to a different place, all the conditions of the environment change. The result for California will be increased rain and snowfall.
“It’s just doing something different to the atmosphere that is our house, basically,” Lunden said. “Like flooding on sunny days.”
Growing coastal flood risks
“Sea levels in San Francisco are already eight inches higher than they were in 1900,” Lunden said. “Then we’ve got the king tides, which can raise the water levels to rise between 12 inches and 24 inches. Then we have storms.”
In the first week of January, a storm brought a surge that was 1.3 inches higher than the king tide, caused by alignment of the Earth, the sun and the moon, that happened at the same time. This month, king tides are predicted to peak on the day before Thanksgiving and the day before Christmas, as well as Jan. 20 to 23.
“With king tides, we could get another one or two feet of additional water rise,” she said. “That could be some significant flooding in areas where we normally don’t see it.”
