Understanding uncertainties in projected summer precipitation changes over the Tibetan plateau

The Tibetan Plateau (TP), a complex high-elevation region with an average height of 4000 meters, is widely recognized as the "Asian Water Tower" and referred to as "the third pole." Changes in precipitation over the TP significantly impact the water cycle in the surrounding areas, directly and indirectly affecting the lives of millions of people and ecosystems. Despite extensive efforts to project future precipitation changes over the TP due to global warming, there remains a considerable spread in the magnitude of existing projections. The underlying physics causing this inter-model spread of precipitation projections over the TP remains unclear. Therefore, gaining insights into the precipitation response to global warming and identifying sources of uncertainty are crucial for enhancing the reliability of these projections.

A recent study, published on February 1 in Geophysical Research Letters, highlighted the persistent increase in precipitation throughout the 21st century, with the most significant changes occurring along the southern edge of the TP. However, the study noted a substantial spread among models in precipitation projections, emphasizing that model uncertainty dominates the overall uncertainty in the mid- and long-term.

Led by Prof. Tianjun Zhou from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, the study employed inter-model empirical orthogonal function analysis of projected precipitation changes under a scenario with very high Greenhouse Gases (GHG) emissions, termed as "SSP5-8.5" in climate modeling. The analysis revealed that the leading principal component explains over 40%, and even 80%, of the total variance at regional scales. Moisture budget analysis indicated that the increase in precipitation is primarily driven by enhanced vertical thermodynamic (TH) responses to the increased water-holding capacity of the atmosphere, with a weak effect from the vertical dynamical (DY) term. However, both vertical DY and TH components contribute to the leading mode of inter-model spread in precipitation projections.

Hui Qiu, the first author of the study and a Ph.D. student from the University of the Chinese Academy of Sciences, explained, "The vertical TH component is significantly related to the climate sensitivity among the models involved in the phase-6 of Coupled Model Intercomparison Project, suggesting that models projecting a warmer climate also tend to project a stronger TH term." 

The study further revealed that the inter-model spread of the dynamic component is influenced by the equatorial Pacific warming pattern through the Walker Circulation change, controlling diabatic heating over the Marine continent and leading to atmospheric circulation changes affecting northward moisture transport to southern TPs.

"Both model weighting technique and selection of high skill models with better performance of historical climate simulation were traditionally used to increase the robustness of climate projection in previous studies. " Prof. Tianjun Zhou, the corresponding author, emphasized, "Our results enrich the research by highlighting that the diversity in CMIP6 models projecting precipitation changes over the TP is not solely related to local model performance but is influenced by the overall performance of climate models in the context of climate sensitivity and the response of equatorial Pacific Sea surface temperature to global warming."

The study also examined the relationship between the thermodynamic term and climate sensitivity under scenarios with low (termed as "SSP1-2.6") and intermediate (termed as "SSP2-4.5") GHG emissions, finding similar results to scenarios with very high GHG emissions. The authors call for further research on the inter-model spread of the equatorial Pacific Sea Surface Temperature response to global warming.