Washington: A Scripps Institution of Oceanography, UC San Diego-led study has found a crucial connection that could drastically improve the ability of forecasters to reliably predict Asian monsoon a few months in advance.
Yu Kosaka and Shang-Ping Xie from Scripps and colleagues from NOAA found that a winter appearance of the climate phenomenon called El Nino in the Pacific Ocean could leave its mark on monsoon formation in the Indian Ocean a full six months later. In between is an atmospheric phenomenon called the Pacific-Japan pattern that provides the teleconnection between the two ocean basins and further poleward to East Asia.
"It has long been a mystery that climate anomalies in the region correlate better with El Nino in the preceding winter than with the one developing in the concurrent summer," said Xie, a climate scientist and inaugural holder of the Scripps Roger Revelle Chair in Environmental Science.
"The new paper shows that Indian Ocean temperature and atmospheric anomalies in the western Pacific are physically coupled, and their interactions amplify each other. We demonstrated that this new mode of coupled ocean-atmospheric anomalies is predictable a season ahead. Such predictions have tremendous benefits to society," Xie noted.
El Nino is a climate phenomenon coupling the ocean and atmosphere that includes a shift in the distribution of warm water in the equatorial Pacific Ocean. El Nino years are characterized by unusual weather and storm activity globally.
The summer after a major El Nino features above-average sea-surface temperatures in the Indian Ocean. El Nino exerts its influence via the Pacific-Japan pattern, which can bring to East Asia cool, wet weather in the subsequent summer, while La Nina leads to dry, hot weather.
The violent storm activity associated with El Nino takes place in the eastern Pacific Ocean, but the chain of events the researchers describe ultimately ends up being detected in the western Pacific Ocean.
Xie likened it to an echo effect, saying that El Nino serves to pull clouds and convection eastward toward the International Date Line, which means those clouds are not available over the western Pacific to keep ocean surface temperatures cool. It also weakens winds in the northern Indian Ocean and the effects of those weakened winds travel back eastward to the Pacific Ocean.
"The last sound El Nino makes is in the western Pacific Ocean," Kosaka said, "because the positive feedback between the Indian Ocean and Pacific-Japan pattern we found amplifies climate anomalies in this region."
The last echoes of El Nino have devastating consequences to the region.
Kosaka cautioned, however, that there is much more work to be done to make prediction of the Asian monsoon reliable. El Nino is just one factor; other regional patterns complicate the sequence that ultimately produces monsoon rains, Kosaka said.
But the paper does establish that El Nino influences the monsoon and describes the means by which it does so, she said.
The study recently appeared online in Proceedings of the National Academy of Sciences.