Abstract
Abstract Northern Hemisphere land monsoon (NHLM) precipitation exhibits multidecadal variability, decreasing over the second half of the twentieth century and increasing after the 1980s. We use a novel combination of CMIP6 simulations and several large ensembles to assess the relative roles of drivers of monsoon precipitation trends, analyzing the effects of anthropogenic aerosol (AA), greenhouse gas (GHG) emissions, and natural forcing. We decomposed summer global monsoon precipitation anomalies into dynamic and thermodynamic terms to assess the drivers of precipitation trends. We show that the drying trends are likely to be mainly due to increased AA emissions, which cause shifts of the atmospheric circulation and a decrease in moisture advection. Increases in GHG emissions cause monsoon precipitation to increase due to strengthened moisture advection. The uncertainty in summer monsoon precipitation trends is explored using three initial-condition large ensembles. AA emissions have strong controls on monsoon precipitation trends, exceeding the effects of internal climate variability. However, uncertainties in the effects of external forcings on monsoon precipitation are high for specific periods and monsoon domains, resulting from differences in how models simulate shifts in atmospheric circulation. The effect of AA emissions is uncertain over the northern African monsoon domain due to differences among climate models in simulating the effects of AA emissions on net shortwave radiation over the North Atlantic Ocean.
Highlights
Tropical precipitation variability directly impacts billions of people, with the Northern Hemisphere land monsoon (NHLM) regions containing around 60% of the world’s population (Wang et al 2017)
We explore the Northern Hemisphere land monsoon precipitation trends over two periods: 1950-1980, when global emissions of anthropogenic aerosols rapidly increased, and 19802014, when US and western European emissions decreased while Asian emissions continued to increase
While the physical complexity of climate models has increased since CMIP5, Coupled Model Intercomparison Project Phase 6 (CMIP6) models are not appreciably different to CMIP5 models in their simulation of tropical precipitation on different temporal and spatial scales (Fiedler et al 2020)
Summary
Tropical precipitation variability directly impacts billions of people, with the Northern Hemisphere land monsoon (NHLM) regions containing around 60% of the world’s population (Wang et al 2017). Differences between observed and simulated monsoon precipitation trends can be due to effects of internal climate variability and to model biases.
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