Regional pressure fluctuations the key to unraveling monsoon mysteries
By Summit Voice
FRISCO — Hawaii-based scientists say that tracking hemispheric climate patterns can help develop more accurate forecasts for the critical Asian monsoon season, which is critical to the agriculture, economy, and people in the region.
Better monsoon forecasts have been a sort of Holy Grail for meteorologists, but season seasonal predictions of these two types of weather phenomena are still poor. But the research done at the International Pacific Research Center, University of Hawaii at Manoa, shows the strength of the East Asian summer monsoon and storm activity in the western North Pacific depend on fluctuations in the western Pacific Subtropical High, a major atmospheric circulation system in the global subtropics centered over the Philippine Sea.
When this system is strong in summer, then monsoon rainfall tends to be greater than normal over East Asia, and in the western North Pacific there tend to be fewer tropical storms that make landfall.
“Our findings create a promising way for predicting monsoon rainfall and tropical storm days during the East Asian summer,” said Bin Wang, meteorology professor at the University of Hawaii at Manoa and faculty at the International Pacific Research Center. “As a first step, we use global general circulation models to predict the fluctuations in the WPSH, and then in a second step, we use this forecast to predict rainfall and storm days in regional analyses,” Wang said.
With the help of computer modeling experiments, the scientists found that fluctuations in the western Pacific Subtropical High are more than 65 percent predictable in spring. When the Indo-Pacific warm pool shows a dipolar sea surface temperature anomaly (that is, an unusually warm Indian Ocean together with an unusually cool western North Pacific) or the central Pacific tends to cool in spring, then the WPSH will be strong and stable with ensuing greater summer monsoon rainfall over the East Asian monsoon front and the Ganges River Valley in India, but fewer tropical storms will affect East Asian coastal areas and the western subtropical Pacific.
The team traced the rainfall and storm variability in the Asian monsoon region to the feedback occurring between the WPSH and the underlying Indo-Pacific warm-pool ocean.
“We have done hindcasts from 1979 to 2009 using this approach and have found substantially improved skills over the use of dynamical climate models in predicting the East Asian Summer Monsoon rainfall and tropical storm activity,” Wang said.
The study was published in the January 21 Proceedings of the National Academy of Sciences.