The unstable connection between Atlantic Multidecadal Oscillation and Indian Summer Monsoon in CESM-LE

Instrumental records since the mid-1990s show that the correlation between the Atlantic Multidecadal Oscillation (AMO) and the Indian Summer Monsoon (ISM) becomes non-significant or even negative. This seems to disagree with that the AMO is positively correlated with the ISM illustrated by most of previous studies, and implies an unstable connection between AMO and ISM. Given the limitation of the length of instrumental records and the substantial human impacts, what causes this instability, specially whether it originates from natural variability in climate system, is unclear. Here the pre-industrial and historical simulations from a Community Earth System Model Large Ensemble (CESM-LE) project are used to investigate this issue. The results suggest that the AMO-ISM connection variability is predominated by internal climatic processes, and an unstable connection occurs in the 1800-year pre-industrial simulation, which is positive in some decades but negative or neutral in some other decades. A regression analysis based on the selected decades when the AMO-ISM connection is positive and when it is negative suggests the substantial role of natural SST variability in the subtropical western North Pacific in modulating the linkage of the AMO with the ISM. The warm SST anomalies lead to higher pressure and easterly anomalies over the western subtropical Pacific, and then a convergence over South Asia and subsequently more precipitation in India, thus a significant positive AMO-ISM correlation; opposite anomalies are true for the significant negative correlation periods. This study provides a new insight into the AMO-ISM connection, in that the connection itself is unstable in the natural climate system and subjective to the modulation of subtropical western North Pacific SST.