Relationships between Interdecadal and Interannual Climatic Variations and Their Effect on Pennsylvania Climate

Abstract The variability of Pennsylvania winter climate is shown to be the result of mechanisms working on two distinctly different time scales: interdecadal and interannual. Interdecadal variability is associated with hemispheric changes in large-scale atmospheric circulation regimes. During periods of zonal flow, Pennsylvania temperatures are above normal, while meridional flow is associated with below-normal temperatures. Precipitation is near normal during zonal regimes, but can be either very dry (as it was in the 1960s) or abnormally wet (like the 1970s) with meridional flow, depending on the exact placement of the deepened trough over the eastern United States. Imbedded in the interdecadal fluctuations is interannual variability which is related to two important Northern Hemisphere teleconnection patterns: the Pacific/North American (PNA) and North Atlantic Oscillation (NAO) patterns. Interannual changes in the atmospheric circulation, as expressed by indices of the two teleconnections, explain 50 percent of the variance in Pennsylvania winter temperature, but only 14 percent of the precipitation variance. It is argued that knowledge of such relationships provides a basis for refining and better interpreting models of greenhouse gas-induced climatic change. Key Words: climatic variabilityatmospheric circulationteleconnectionssynoptic climatology