Uncertainty in projections of the South Asian summer monsoon under global warming by CMIP6 models : Role of tropospheric meridional thermal contrast

Driven by the increase in CO₂ concentration, climate models reach a consensus that the large-scale circulation of the South Asian summer monsoon (SASM) becomes weakened but with different magnitudes. This study investigates the major uncertainty sources of the SASM response to an abrupt quadrupling of CO₂ (abrupt-4×CO₂) in 18 models of phase 6 of the Coupled Model Intercomparison Project. The projected weakening of the SASM indicated by both zonal and meridional monsoon circulation indices is closely linked to decreases in the meridional gradient of upper-tropospheric temperature between Eurasia and the Indian Ocean (EUTT-IUTT). A climate feedback–response analysis method is applied to linearly decompose the uncertainty of changes in EUTT-IUTT into the partial changes due to external forcing and internal processes of the earth–atmosphere column. Results show that the uncertainty of changes in EUTT-IUTT is contributed positively by the dominant atmospheric dynamic process, followed by the cloud shortwave radiative effect, and negatively by the surface latent heat flux and cloud longwave radiative effect. Contributions from CO₂ forcing and other internal processes including albedo and water vapor feedbacks, oceanic heat storage, and sensible heat flux are found to be minor. 

本文基於第六次國際耦合模式比較計畫共18個模式的工業革命前實驗和CO₂ 濃度突然四倍實驗, 發現在CO₂四倍強迫下, 南亞夏季風環流呈顯著減弱趨勢, 但減弱強度存在較大模式間差異. 利用Webster-Yang指數和經向哈得萊環流指數的下降趨勢表徵SASM減弱強度, 發現該下降趨勢與歐亞大陸-印度洋之間對流層上層經向溫度梯度的變化值(EUTT-IUTT)高度相關. 進一步利用氣候反饋-響應分析方法進行分析, 發現EUTT-IUTT變化的模式間差異主要來自於大氣動力過程, 其次是雲的短波輻射效應的貢獻. 地表潛熱通量和雲的長波輻射效應縮小了EUTT-IUTT變化的模式間差異.