Monsoons may cause heavy rainfall associated with flooding and mudslides, but sometimes they provide a vital source of water needed for growing crops. Accurate forecast of monsoons and prediction of climate changes that drive these events could help communities to better prepare and make plans.

A team of researchers from the Chinese Academy of Sciences have conducted a series of model perturbation experiments, producing datasets that can help improve these predictions. The model design, experiments and datasets from the simulations are described in a paper published in Advances in Atmospheric Sciences on December 10, 2019.

A monsoon is a seasonal change in atmospheric circulation or prevailing wind direction associated with corresponding changes in precipitation resulting from uneven heating of sea and land surfaces. Monsoons blow from cold regions to warm regions, and are responsible for wet and dry seasons in the tropics.

However, external factors such as the location of land masses and oceans can influence regional wind and rainfall patterns, thus the characteristics and behavior of monsoons vary from region to region. The South Asian monsoon, for example, is particularly strong as the Himalayas and Tibetan-Iranian Plateau prevent dry air from the north flowing to the humid monsoon region in India and southern Asia. 

The Chinese Academy of Sciences (CAS) Flexible Global Ocean-Atmosphere-Land System (FGOALS-f3-L) model datasets provide a valuable tool to assess sea surface temperature trends and its influence on monsoon circulation and precipitation patterns, as well as a clearer understanding of how topography can affect the global monsoon system as it passes over landscapes with high altitudes.

"These datasets are useful especially for understanding the changes of sub-seasonal climate signals forced by the Tibetan-Iranian Plateau," said lead author, HE Bian, a research scientist at the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences.

The global monsoon system is made up of several sub-monsoon systems, each with their own unique characteristics and behaviors relative to when and where they occur. These differences have proved challenging for current climate models, primarily because we do not yet fully understand the complex atmosphere-ocean-land interactions that drive monsoon systems, which in turn are influenced by external forces and internal variabilities.

 The Tibetan Plateau plays an important role in regulating synoptical and climate variations over East Asia. (Image by LI Jinxiao)  

Topography can affect weather, for example, by forcing air upwards which can cause disturbances in the weather system. As the air rises, changes in pressure and temperature can result in precipitation. Topography can influence monsoons, there is still much debate regarding the direct impact that global highlands have on monsoon circulation and precipitation.

"We provided three ensemble simulations of long-term changes of the global monsoon under observed Sea Surface Temperature (SST) and Sea ice forcing to reduce uncertainty from the initial method," said HE, "We also provided high time frequency outputs in GMMIP Tier-3 experiments for better understanding the role of the Tibetan Plateau in the global monsoon system by transient processes."

"Our next step is to consider air-sea interactions in the simulation, as this is also an important factor in understanding global monsoons and associated topographical effects," said HE. "Our ultimate goal is to improve model simulations on monsoon behavior in order to more accurately forecast monsoons."