Secular Changes of the Decadal Relationship Between the Northern Hemisphere Land Monsoon Rainfall and Sea Surface Temperature Over the Past Millennium in Climate Model Simulations

Abstract

Understanding the decadal variability of Northern Hemisphere land monsoon rainfall (NHLMR) is crucial to social-economic development. However, due to the temporal limitation of instrumental data, the decadal relationship between NHLMR and sea surface temperature (SST) remains uncertain. The extended El Niño–Southern Oscillation (XEN) SST index and the North Atlantic–south Indian Ocean dipole (NAID) SST index can be the predictors to help us understand the decadal relationship between NHLMR and SST. In this study, using the Community Earth System Model-Last Millennium Ensemble (CESM-LME) simulations, we find a significant and stable decadal correlation between the NHLMR and XEN in the all-forcing (AF) (with natural and anthropogenic forcings, AF), control (internal variability only), and other external forcings run over the past millennium, which means that this simulated NHLMR-XEN relationship is caused by internal variability. The AF experiments show that the decadal relationship between NHLMR and NAID is practically non-existent until an abrupt and significant increase in volcanic eruptions at about 1700, which is also indicated by the Paleoclimate Modeling Intercomparison Project Phase 3 (Paleoclimate Modeling Intercomparison Project Phase 3) simulations. Using the volcanic forcing sensitivity experiments, we find that the successive strong volcanic eruptions in the Northern Hemisphere (NH) significantly strengthen the synchronous changes of NHLMR and NAID after 1700, with a periodicity of 20–40-year for NHLMR and NAID. Specifically, successive NH eruptions weaken the north-south hemispheric thermal contrast, contributing to the weakened NAID index. The cross-equator flow is thus weakened, decreasing the NHLMR, which cause a resonant behavior of NHLMR and NAID.