In the summer of 2020, eastern Asia suffered the devastating effects of an anomalous Western Pacific Subtropical High (WPSH) that brought unprecedented rainfall and catastrophic floods. In China alone, hundreds of people died, 28,000 homes were destroyed and billions of dollars of damage was done. Although people hope this scenario will not be repeated anytime soon, Chinese scientists fear that it will become an all-too-familiar occurrence in future.
The WPSH is a key anticyclonic atmospheric circulation system that hovers over the middle and lower troposphere of the northwestern Pacific. It occupies about 20–25 percent of the Northern Hemisphere’s surface and plays a major role in the global circulation of the atmosphere and oceans. It is also closely related to Asian climate and weather, and dominates summer climate extremes in the densely populated countries of East China.
In an article published today in the Proceedings of the National Academy of Sciences, scientists from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences and the Commonwealth Scientific and Industrial Research Organization (CSIRO), find that the frequency of strong WPSH events, such as the one seen in the summer of 2020, is likely to increase as carbon emissions in the atmosphere increase. This conclusion was based on the use of 32 different climate models for predicting outcomes under current scenarios of global warming.
Each year, the intensity of the WPSH is influenced by the variability of sea surface temperature (SST) in tropical regions such as the central Pacific and Indian Oceans. For this reason, anomalous SST can affect both local and remote rainfall and atmospheric convection, which in turn modulate atmospheric circulation over the northwestern Pacific.
Under conditions of global warming, rainfall and atmospheric convection can be more sensitive to SST variability. This is because the amount of water vapor that can be carried by the air increases exponentially with increases in temperature.
In a warmer climate, the average moisture content of the atmosphere will increase and the response of tropical humidity and associated gross moist instability to SST anomalies will also be larger.
“These changes, combining with the non-uniform change in background SST, can lead to enhanced responses of atmospheric convection to the central Pacific SST, and then increased variability of atmospheric circulation, including the WPSH,” said study lead author Dr. Yang Kai.
“The increase in WPSH variability translates into an increase in frequency of strong WPSH events, which suggests that greenhouse warming is likely to increase the risk of East China flooding associated with strong WPSH events, as seen in the 2020 episode,” said study co-author Professor Huang Gang.