Fidelity of CMIP5-simulated teleconnection between Atlantic multidecadal oscillation and Indian summer monsoon rainfall

Abstract

The present study aims to provide a relevant ground for attaining deeper perception about the teleconnection between the Atlantic multidecadal oscillation (AMO) and the Indian summer monsoon rainfall (ISMR) in observations as well as in 30 models from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project Phase 5 (CMIP5). Approximately 73% of models reproduce the internal natural variability allied with AMO, but mostly all underestimate the variance. Amongst these, very few replicate the explicit comma-shaped AMO sea surface temperature (SST) pattern, whereas rest illustrates warm SSTs over the sub-polar region and very weak or non-existent AMO’s signature over the sub-tropical North Atlantic. However, only 53% of models emulate the observed AMO–ISMR relationship. The observational analysis bestows the compelling evidence that the AMO influences ISMR through two physical processes: firstly by modulating the El Niño related anomalous Walker and regional Hadley circulations asymmetrically and secondly through the tropospheric response allied with the Rossby wave train. The models that fail to reproduce the AMO–ISMR teleconnection are incompetent in capturing the first physical mechanism correctly, whereas in general all models show limitations in simulating the second physical mechanism. The results divulge a moderate relationship between the quality of reproducing the AMO pattern and the AMO–ISMR teleconnection in models, particularly with respect to the tropical–extratropical Pacific SST gradients during AMO phases. The models, which do show the observed rainfall response over India, also simulate the large-scale features allied with AMO like the cross-equatorial flow, the tropical easterly jets, the anomalous divergence/convergence over the Indian sub-continent at upper/lower levels, the Webster and Yang Monsoon index, and the Monsoon Hadley Circulation index.