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

Abstract

Driven by the increase in CO 2 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 CO 2 (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 CO 2 forcing and other internal processes including albedo and water vapor feedbacks, oceanic heat storage, and sensible heat flux are found to be minor. 摘要 本文基于第六次国际耦合模式比较计划共18个模式的工业革命前实验和CO 2 浓度突然四倍实验, 发现在CO 2 四倍强迫下, 南亚夏季风环流呈显著减弱趋势, 但减弱强度存在较大模式间差异. 利用Webster-Yang指数和经向哈得莱环流指数的下降趋势表征SASM减弱强度, 发现该下降趋势与欧亚大陆-印度洋之间对流层上层经向温度梯度的变化值(EUTT-IUTT)高度相关. 进一步利用气候反馈-响应分析方法进行分析, 发现EUTT-IUTT变化的模式间差异主要来自于大气动力过程, 其次是云的短波辐射效应的贡献. 地表潜热通量和云的长波辐射效应缩小了EUTT-IUTT变化的模式间差异.