The physical mechanism for the Tropical Atlantic influence on African and Indian Monsoon and its representation in the AR4 models in the 20th century

In recent studies we have shown that the tropical Atlantic SSTs significantly impact not only African rainfall, but also on the Indian summer monsoon. The atmospheric teleconnection between the tropical Atlantic and the Indian basin is indeed responsible for modulating the interannual variability of the Indian monsoon rainfall. In this work, the observational basis for this teleconnection is examined in details using five different data-sets and the physical mechanism responsible for bridging sea surface temperatures in the Atlantic and precipitation over India is investigated with idealized atmospheric general circulation model (AGCM) experiments. A simple humid Gill-Matsuno-type quadrupole response is proposed to explain the teleconnection between the tropical Atlantic and the Indian basin, with an enforcement of the eastward response due to nonlinear interactions with the mean monsoon circulation. The simplicity of this mechanism suggests reproducibility with a broad range of GCMs. We compare five of the AR4 coupled GCMs (GFDL2.1, MPI-ECHAM5, MRI, HADCM3 and NCAR) for the XX century and we investigate how well the wave response is reproduced. Our results have important implications for seasonal forecast efforts.