Results from a large ensemble of climate model simulations over the period 1940--2080 suggest that the observed strengthening of the westerly winds over the North Atlantic during the past decades is not due to the enhanced greenhouse effect but is largely an expression of a random, internal climate variation driven by increased precipitation over the tropical Indian Ocean. Instead, the enhanced greenhouse effect drives a change in the extra-tropical winter circulation through intensified precipitation over the tropical West Pacific. This change is characterized by a wave train encompassing the whole Northern hemisphere, a pattern known as the Circumglobal Waveguide Pattern.