They found that SMAP detected a decrease in the saltiness of ocean water in the tropical Pacific Ocean near Ecuador, an area that was already experiencing above average sea surface temperatures. It was the result of heavy rain falling hundreds of miles away in the Andes mountains flowing down into the Pacific.
The SMAP team worked closely with scientists at the International Research Institute for Climate and Society at Columbia University (IRI). They were studying forecasts from their global computer models to see if they predicted these changes in ocean salinity.
"El Niño events cause global changes in rainfall and ocean temperatures," explained Eric Hackert, a research scientist with NASA's Goddard Space Flight Center and a co-author of a recent paper in Geophysical Research Letters about these findings. "In the tropical Pacific, where we are seeing these ocean salinity changes, El Niño also causes changes in ocean currents and the trade winds that steer the currents. These changes can alter the path of moisture that forms rain across the Pacific basin, including on the slopes of the Andes Mountains. The effect of these changes on rainfall are captured in the predictions from our models."
To test their models' rainfall predictions, the team focused on data from March 2015 to October 2015, when El Niño conditions were beginning to develop.
The models successfully forecasted the observed rainfall over the Pacific Ocean, as well as the movement of the associated weather patterns over the ocean. The models predicted above-average rainfall over the mountains and along the coast of Ecuador and Peru, and a change in the direction of winds, blowing in a north-easterly direction that pushed the rain further south from its usual spot over the Pacific.
"With a stronger El Niño event expected this winter, we anticipate that these changes in the water cycle are likely to be even more pronounced," Hackert said.
The SMAP team is not only watching how heavy rains affect the sea surface salinity in the tropical Pacific during El Niño, they are also watching where the fresh water goes next. Models developed by IRI showed that the salty ocean currents can carry the fresh water westward in the tropical Pacific, where rainfall is lower than normal during an El Niño. Understanding how fresh water moves around the Pacific Ocean will ultimately help the team learn how El Niño affects rainfall patterns around the world.
