CISL-powered climate analysis of Southeast Asia reveals fascinating historical insights

A recent study, published September 2024 in Climate Dynamics, leveraged NSF NCAR computing and data resources to explore extreme rainfall changes—associated with both droughts and pluvials—are primarily driven by internal climate variability in the Pacific and Indian Oceans. Internal variability refers to natural fluctuations in ocean temperatures and atmospheric patterns. The article, authored by a team of scientists from four institutions, is entitled "Quantifying the Internal and External Drivers of Southeast Asian Rainfall Extremes on Decadal Timescales."

Led by Shawn (Shouyi) Wang and Caroline C. Ummenhofer (pictured), of the Woods Hole Oceanographic Institution (WHOI) and the MIT–WHOI Joint Program in Oceanography, the research team used a Data Analysis project allocation to access NSF NCAR-curated datasets and use CISL's Casper analysis cluster to investigate climate patterns in Southeast Asia. 

Southeast Asia, a region dependent on monsoon rains, has historically experienced extreme shifts in rainfall. To further understand these shifts, the authors examined both effects stemming from internal climate factors—that is, natural climate variations, such as El Niño and La Niña events—and effects originating from external forcing factors, such as volcanic eruptions and changes in the sun’s irradiance. 

The authors highlight that several of the lengthy wet and dry interludes identified here coincide with periods associated with “societal change and civil unrest throughout Asia” in previous studies, spotlighting the following historical events within the context of climate:

• In the late 14th to 15th century, droughts, punctuated by severe flooding, contributed to Angkor Wat’s collapse as the capital city of the Khmer empire.

• The prolonged Ming Dynasty Drought from 1637–1643 (which coincided with a 1641 volcanic eruption) played a major role in the fall of the Ming Dynasty in 1644. 

• In 1756–1768, the unprecedented Strange Parallels Drought hit numerous societies hard in South and East Asia. The authors partially attribute this drought to a 1761 volcanic eruption.

The research team used the Community Earth System Model Last Millennium Ensemble (CESM-LME) to simulate climate over the last thousand years. The dataset is developed and made available by the CESM Paleoclimate Working Group, and is housed within CISL. The CESM-LME allowed the team to differentiate between internal and external climate factors. According to the authors:

“The CESM-LME consists of multiple realizations of global climate under identical external forcings which allows for the separation of internal and external drivers... Additionally, each LME simulation spans over 1000 model years (850–2005 CE), thus providing a sufficiently large sample size for robust probabilistic analysis.” 

The CESM-LME simulations showed that in Southeast Asia, most droughts and pluvials—extremely dry or wet rainfall events—are primarily driven by internal climate variability, meaning natural fluctuations in ocean temperatures and atmospheric patterns, especially in the Pacific and Indian Oceans. These internal variations affect how air circulates and moisture is transported from ocean to land as part of the monsoons, leading to periods of drought or excessive rain.

However, the study also found that external factors, particularly volcanic eruptions, can play a role in some extreme rainfall events, especially droughts. Volcanic eruptions release particles into the atmosphere that can cool the planet and disrupt rainfall patterns. The simulations suggested that volcanic activity may have contributed to some historical droughts, like the one that coincided with the fall of the Ming Dynasty in China. Interestingly, droughts seemed more susceptible than periods of excessive rain to external forcings.

The research highlights the interaction between internal climate variability and external forces in shaping rainfall patterns in Southeast Asia. While natural climate fluctuations are the main drivers of extreme rainfall events, external factors like volcanic eruptions can sometimes tip the balance, particularly towards drought. 

Understanding these different influences, a feat made possible by CISL data science resources, is crucial for predicting future changes in rainfall and managing water resources in this region.

Wang, S., Ummenhofer, C.C., Murty, S.A. et al. Quantifying the internal and external drivers of Southeast Asian rainfall extremes on decadal timescales. Clim Dyn 62, 9821–9841 (2024). https://doi.org/10.1007/s00382-024-07412-x.

This study was supported by the U.S. National Science Foundation under AGS-2002083, AGS-2001949, OCE-2303513, and AGS-2302668.

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