Large-Scale Circulation and Climate Variability (Invited Presentation)

The causes of U.S. temperature and precipitation trends cannot be understood without considering the impact of changes in large-scale atmospheric circulation and assessing whether these circulation changes are externally forced or an expression of internal climate variability. Furthermore, changes in persistent atmospheric flow patterns that remain in place for an extended period of time (for example, blocking and quasi-stationary Rossby waves)—and thus affect a region with similar weather conditions like rain or clear sky for several days to weeks—can lead to changes in the frequency of occurrence and/or maginitude of extreme weather and climate events like flooding, drought, heat waves, and cold waves.

Modes of variability can affect the local-to-regional climate response to external forcing in various ways. The climate response may be altered by the forced response of these existing, recurring modes of variability and persistent flow patterns. Further, the structure and strength of regional temperature and precipitation impacts of these recurring modes of variability may be modified due to a change in the background climate. Modes of internal variability of the climate system also contribute to observed decadal and multidecadal temperature and precipitation trends on local to regional scales, masking possible systematic changes due to an anthropogenic influence. In this talk we summarize our current understanding on changes in global circulation, recurring modes of variability of the atmospheric circulation (like North Atlantic Oscillation and Pacific/North American Pattern), the coupled atmosphere–ocean system (like El Nino/Southern Oscillation), and persistent atmospheric flow pattern, and the contribution of human influence on these changes.