Dominant factors influencing the seasonal predictability of US precipitation and surface air temperature

The relative contributions of El Nino-Southern Oscillation (ENSO), long-ter m tropical Pacific variations, and the Arctic oscillation (AO) to the expla ined variance of U.S. precipitation and surface air temperature are investi gated. The time variability of monthly precipitation in the tropical pacifi c basin is separated into high-pass and low-pass filtered components. The l eading EOFs of the high-pass and low-pass filtered data capture ENSO cycle- related interannual variability and ENSO-like interdecadal variability, res pectively. The dominant mode of variability in the extratropics is the AO, which has been implicated in some of the secular variability of climate in the Northern Hemisphere extratropics. ENSO produces large, reasonably reproducible spatial and temporal shifts in tropical precipitation. The tropical interdecadal variability produces mor e subtle, but still significant, shifts in tropical precipitation that cont ribute significantly to the explained variance and to trends in the North P acific sector, over the United States, and extending into the North Atlanti c sector. Consistent with previous studies, the largest and most significan t AO-related contributions are during the cold season (October-March). part icularly over the eastern half of the United States, the North Atlantic sec tor, Eurasia, and the polar cap. The results indicate that a significant portion of the skill of climate for ecast models will likely arise from an ability to forecast the temporal and spatial variability of the interdecadal shifts in tropical precipitation a s well as the associated teleconnection patterns into midlatitudes. Because the AO encompasses the North Atlantic oscillation, it appears that additio nal increases in skill over portions of North America require forecasts of the AO.