Variability in United States Cloudiness and its Relation to El Ni&ntilde

Abstract

Based on linear regression, during 1950?85 the year-average, United States cloudiness (defined here as an average of cloud amount and sunshine duration) increased by a significant 0.8% per decade, with the increase greatest in the South Central region (1.3% per decade). During 1970?85, however, the United States cloudiness increased by only 0.4% per decade, a nonsignificant amount. Over the 36-year interval, most of the cloudiness increase was in autumn (2.1% per decade for the contiguous United States), but during 1970?85 the cloudiness increase was similar (about 0.4% per decade) in all four seasons. For the United States as a whole, cloudiness has been above average at the time of strong El Niños. such as those in 1972?73 and 1982?83, but often below average at the time of weaker El Niños As a consequence, over the 36-year interval the maximum correlation between seasonal SST (in the region 0°?10°S, 180°?80°W) and United States cloudiness is only 0.26 (cloudiness leading this SST by about one season), just significant at the 5% level taking account of serial correlations. The correlation (r) has been largest in southwestern (r = 0.26) and north central (r = 0.30) regions of the United States and least in the northeastern region (r = 0.09).