The tropical heating impact on the NAO is examined and the evolution process of the influence is explored by numerical experiments using a primitive equation atmospheric model forced by atmospheric heating perturbations. Results from both the reanalysis data and numerical experiments suggest that the tropical Indian Ocean atmospheric heating appears to be a driving forcing for the NAO variability. The atmospheric response to the tropical forcing involves the combined effects of Rossby wave dispersion, normal mode instability and transient eddy feedback. The influence of the forcing on the NAO tends to be organized and achieved by the circumglobal teleconnection pattern. By contrast, the influence of the tropical American-Atlantic heating on the NAO appears to be weak. The linkage between the NAO and the tropical American-Atlantic heating is likely through the anomalously meridional atmospheric circulation over the Atlantic Ocean.