Understanding the response of tropical atmospheric and oceanic circulation to increasing greenhouse gases is a central question in climate change research, since changes in the tropics can have far-reaching effects. We explore the impact of CO2-induced reduction in the intensity of large-scale atmospheric circulation, resulting from global energetic constraints, on the tropical Pacific atmospheric and oceanic structure and circulation. We examine the projected 21st Century response of the tropical circulation using a set of 22 climate model experiments performed for the IPCC-AR4. In all models there is a robust decrease in the strength of the atmospheric overturning circulation decreases as the climate warms; the circulation weakens in a manner consistent with simple thermodynamic arguments. The weakening occurs preferentially in the zonally-asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation, and results in a weakening the near-equatorial easterlies in the Pacific Ocean. These wind changes induce substantial changes to the thermal structure and circulation of the tropical oceans. Although many aspects of the model changes in both the atmospheric and ocean circulation resemble "El Nino-like" conditions, the mechanisms are shown to be distinct from those of El Nino are reproduced in both mixed-layer and full ocean dynamics coupled climate models. The projected changes in circulation result in changes to Atlantic vertical wind shear, which may be relevant to projections of changes in tropical cyclone activity under global warming.