Analysis of Changes in Large-scale Atmospheric Circulation and their Implication on Climate Sensitivity

A robust feature across climate models is a weakening of the atmospheric circulation in a warming world. Our Earth has been warming at a rate of 0.19 K/decade since 1979 i.e. there has been a roughly 0.76 K increase in global-mean temperature in the past four decades alone. This raises important questions like has there been a detectable change in large-scale atmospheric circulations and if so, can these changes be attributed to human activities? Consistent with earlier models, the recent CMIP6 models simulate a robust weakening of atmospheric circulation during the satellite era. We also find observations-based evidence for a weakening of the atmospheric circulation with warming despite a strengthening of the Pacific Walker Circulation in the past few decades. This weakening with warming is found to be a response to anthropogenic forcing and is highly unlikely to be caused by natural variability alone. In the deep tropics, this weakening coincides with a northward shift of the Intertropical Convergence Zone, which is found to be a combined response to a reduction in anthropogenic aerosols over Northern hemisphere midlatitudes and an increase in greenhouse gases. Satellite measurements of water vapor radiances also reveal a similar magnitude of increase in global-mean upper tropospheric water vapor between observations and climate models. However, the longer satellite observations from the HIRS instrument exhibit a stronger northward interhemispheric gradient in upper tropospheric moistening trend than simulated by the CMIP6 climate models. Furthermore, based on the strong correlation between mid-tropospheric vertical pressure velocity (ω500) and upper tropospheric relative humidity (UTH) in climate models, we construct a proxy ω500 dataset using a growing record of global satellite observations of UTH from both infrared and microwave channels. This 44+ years of UTH record from the infrared channel and the proxy ω500 derived from it provide us with a long temporal record to test the robustness of atmospheric circulation changes.