Step Increase in Eastern US Precipitation Linked to Indian Ocean Warming

A step increase in annual precipitation over the eastern United States in the early 1970s commenced five decades of invigorated hydroclimate, with ongoing impacts on streamflow and water resources. Despite its far-reaching impacts, the dynamical origin of this change is unknown. Here analyses of a century of atmospheric and oceanic data trace the dynamics to changes in the Indian Ocean. Increases in fall precipitation contribute most strongly to the step increase, and the associated mechanism is emergence of a pan-Pacific atmospheric wave emanating from deep convection over the warming Indian Ocean. Documentation of this fall teleconnection draws attention to projected anthropogenic increases in tropical oceanic heat content and their potential impacts on hydroclimate of the midlatitudes. Plain Language Summary Precipitation abruptly increased to higher levels over the eastern United States in the early 1970s. The explanation for this important change in the hydrologic cycle is unknown. This study traces the origin of the change to the fall season and an atmospheric wave that begins with thunderstorms over the warming Indian Ocean. The wave crosses the Pacific and changes the track and strength of storms over North America. This finding is important because it draws attention to increases in tropical oceanic heat content associated with climate change, and how these changes can impact weather in the midlatitudes.