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

The relative contributions of El Nino-Southern Oscillation (ENSO), long-term tropical Pacific variations, and the Arctic oscillation (AO) to the explained variance of U.S. precipitation and surface air temperature are investigated. The time variability of monthly precipitation in the tropical pacific basin is separated into high-pass and low-pass filtered components. The leading EOFs of the high-pass and low-pass filtered data capture ENSO cycle-related interannual variability and ENSO-like interdecadal variability, respectively. 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 more subtle, but still significant, shifts in tropical precipitation that contribute significantly to the explained variance and to trends in the North Pacific sector, over the United States, and extending into the North Atlantic sector. Consistent with previous studies, the largest and most significant AO-related contributions are during the cold season (October-March). particularly over the eastern half of the United States, the North Atlantic sector, Eurasia, and the polar cap. The results indicate that a significant portion of the skill of climate forecast models will likely arise from an ability to forecast the temporal and spatial variability of the interdecadal shifts in tropical precipitation as well as the associated teleconnection patterns into midlatitudes. Because the AO encompasses the North Atlantic oscillation, it appears that additional increases in skill over portions of North America require forecasts of the AO.