Tropical Atmospheric Forcing of the Wintertime North Atlantic Oscillation

The relationship between the interannual wintertime variability of the North Atlantic Oscillation (NAO) and tropical heating anomalies is examined using the NCEP-NCAR reanalysis and observation-based sea surface temperature (SST) and precipitation data for the period from 1980 to 2011. The NAO is found to be significantly correlated with the precipitation anomalies in the tropical Indian Ocean and tropical American-Atlantic region, but not with the underlying SST anomalies. 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 the reanalysis data and numerical experiments suggest that the atmospheric heating in the tropical Indian Ocean appears to be a driving force for the NAO variability. The atmospheric response to the tropical heating involves the combined effects of Rossby wave dispersion, normal mode instability, and transient eddy feedback. The remote forcing influence 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 is weak. The linkage between the NAO and the tropical American-Atlantic heating is likely through the anomalously meridional atmospheric circulation over the Atlantic Ocean.