A recent study published in Environmental Research Letters by a team of researchers from the Institute of Atmospheric Physics (IAP) at the Chinese Academy of Science (CAS) finds that the rainy season of northern Central Asia, which occurs in May-July in present-day, will shift to March-May at the end of the 21st century.

Central Asia is the geographic core region of the Silk Road Economic Belt, characterizing by scarce precipitation and high evaporation. Mismatching the deficiency of water resources, Central Asian economies still rely on primary industries, including agriculture. People’s livelihoods and the fragile ecosystem are highly sensitive to the changes in local precipitation.

"This region is one of the hot spots of global warming, where a stronger than global mean warming trend is projected in the coming century. But the future precipitation changes are less clear," said JIANG Jie from IAP, the paper's first author. "Besides the amount of precipitation, the changes in the phase of precipitation also have important effects especially for agriculture and need a scientific answer."

To give a comprehensive picture of the future changes in precipitation over Central Asia, the researchers adopted the multimodel simulations and projections of 15 models from the new phase of the Coupled Model Intercomparison Project (CMIP6). They also use projections under four combined scenarios of the Shared Socioeconomic Pathways and the Representative Concentration Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5). These scenarios reflect a set of alternative futures of social development and greenhouse gas emission.

The researchers found robust increase of annual mean precipitation at the end of the 21st century under all scenarios. The increasing rate is about 14.41% for the highest emission scenario (SSP5-8.5) and near 4.23% for the lowest emission scenario (SSP1-2.6) relative to present-day. The model spread is larger for higher emission scenarios, which can be partly attributed to the larger uncertainty of the surface air temperature. Further analysis reveals a wetting trend in spring and a drying trend in summer, which will result in enhanced seasonality of precipitation over Central Asia and a shift of the rainy season in the northern Central Asia. The increase of precipitation in spring is balanced by increasing evaporation, while the decrease in summer is mainly due to the changes in vertical moisture advection.

Pamirs Plateau over Kyrgyzstan (73°47′，39°40′) (Image by CHEN Feng)

"Central Asia is dominated by descending motion and low-level divergence in summer. The increase of atmospheric water vapor due to rapid warming can result in the decrease of vertical moisture advection and contributes to the drying trend in summer, which is the so-call dry-getting-drier mechanism," JIANG explained. "In addition, the changes in the location and strength of subtropical westerly jet due to anthropogenic activity can also affect the annual cycle of precipitation."

The researcher also discussed the changes in drought condition over this drought-prone region. Based on the Surface Wetness Index (SWI), they found drought may become even severer in the coming century though increasing precipitation is projected. "Adaptation measure for future climate changes should consider both the shift of rainy season and increased aridity over this region." JIANG emphasized.
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This work was supported by the Chinese Academy of Sciences and National Natural Science Foundation of China.

Jiang Jie, Tianjun Zhou, Xiaolong Chen, and Lixia Zhang, 2020: Future changes in precipitation over Central Asia based on CMIP6 projections. Environ. Res. Lett. 15 (2020) 054009. https://doi.org/10.1088/1748-9326/ab7d03