Observations from the tropical Pacific Ocean identify an abrupt climate shift in 1976 with surface temperatures changing from cooler than normal to warmer than normal in the span of about 1 year. Model results show that this climate shift originates with subsurface temperature anomalies in the south tropical Pacific Ocean which propagate first to the western boundary, then northward to the equator, and finally eastward along the equator to 140¿ W where they rise to the surface. The results suggest that changes in the North Pacific respond to changes in the tropical Pacific.