What drives the decadal variation of global land monsoon precipitation over the past 50 years?

Abstract

Global land monsoon precipitation (GLMP) has shown quasi‐20‐year oscillations overlying a significant downwards trend over the past 50 years (1951–2004). However, what drives this decadal variation remains an open question. Based on Coupled Model Intercomparison Project Phase 5 (CMIP5) models, in this study we explored the possible drivers for the decadal variation of GLMP and quantified their contributions. A downwards trend of −0.30 mm/day (100 years) is observed in GLMP over 1951–2004 that explains 30% of total decadal variance, and it is primarily attributed to anthropogenic aerosols (AAs) external forcing. Further analyses showed that the downwards trend in GLMP mainly results from the widespread drying over Northern Hemisphere (NH) monsoon domain. Faster growth of AAs amount in NH than Southern Hemisphere (SH) over the past 50 years results in a reduced hemispheric thermal contrast (cooler NH–warmer SH) due to more AA‐forced cooling in NH. Reduced hemispheric thermal contrast facilitates a weakened NH summer monsoon circulation and finally suppresses monsoon precipitation. Besides the downwards trend, the remaining variation of GLMP exhibited quasi‐20‐year oscillations that explain 70% of total decadal variance. The decadal oscillations are primarily attributed to internal variability, in which the central tropical Pacific sea surface temperature anomaly and Arctic Oscillation together can explain 55% of its variation.