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One Hundred Plus Years of Wintertime Climate Variability in the Eastern United States*
ABSTRACT
Interannual anomalies of climate variability in the eastern United States for the past 100+ yr have been studied for their spatial EOF structure, long-term changes, and the covariability with several climate indices: the Southern Oscillation index (SOI), North Pacific index (NPI), and North Atlantic Oscillation (NAO) index. Especially for air temperature, wintertime (December-February) variability is much more pronounced than summertime (June-August). The leading principal component (PC) of wintertime air temperature, which explains 70% of the interannual variance, is significantly correlated with the NAO, while the leading PC of wintertime precipitation correlates with the SOI. The spatial structure of the leading EOFs have a similar spatial character when compared to the correlation between the data and the climate indices, suggesting that the EOFs can be thought of as proxies for mapping the effects of climate indices upon the eastern United States. The effects of the SOI and NPI are generally the same; however, these two climate indices are not independent. The longterm sensitivity of the eastern U.S. climate to the Pacific indices seems only weakly dependent with time, whereas the NAO has grown considerably in importance with time since the beginning of the twentieth century. Surrogate temperature data from New Haven, Connecticut, has been used to extend this 100+ yr analysis back into the previous century, and the apparent long-term trend in the sensitivity to the NAO completely disappeared in the latter part of the nineteenth century. If a measure of potential predictability is the degree to which interannual climate covaries with these climate indices, the recent period (post 1960) may overestimate this predictability based on the long-term changes observed in sensitivity.
1. Introduction
The goal of this endeavor is to understand the spatial patterns and temporal variability of interannual "climate" variability in the eastern half of the United States during the twentieth century. Studies examining climate variability have usually started with the relationship of climate to a given phenomenon, such as the El NinoSouthern Oscillation (ENSO) or the North Atlantic Oscillation (NAO). In particular, Higgins et al. (2000) use tropical Pacific precipitation to define a high-passed, ENSO-like index and a low-passed, North Pacific index-like (NPI) index during the...