Connections between the tropical and Arctic climates have important implications for the Northern Hemisphere climate. This study considers the connection between sea surface temperature anomalies (SSTAs) of the central tropical Pacific and the semi-circumpolar jet in summer. We find robust evidence from structurally different reanalyses that this connection has strengthened since the mid-1980s. Climate models are able to simulate such changes as well, but available evidence from models does not suggest that external forcing has played a discernable role in the observed strengthening. In contrast, our analysis suggests that the change is primarily due to low frequency natural internal variability associated with the Atlantic Multidecadal Oscillation (AMO). The warm SSTAs in the Atlantic cause a zonal wavenumber-1 planetary wave anomaly in the Arctic when the AMO is in its positive phase. This anomaly affects westerly shear near the Bering Strait that creates more favorable background conditions for poleward Rossby wave propagation into the Arctic and an enhanced troposphere polar vortex. The intensification of the polar vortex enhances the meridional pressure gradient, which in turn amplifies the intensity of the semi-circumpolar jet.