A review of previous studies revealed the following paradoxical situation: the outstanding interannual variations of the summer rainfall in the central U.S. showed no persistent correlations with known influential interannual variations in the Northern Hemisphere, e.g., the El Nino/Southern Oscillation (ENSO). This study was undertaken to identify the cause of this puzzling situation. Results of this study show a teleconnection of the ENSO with the summer rainfall in the central U.S. and the intensity of which has varied over the last 125 years. The variation is related to and regulated by multidecadal variations in the mid- and high-latitude Northern Pacific Ocean. The teleconnection was active in the two epochs 1871-1916 and 1948-76, and suppressed in the epochs 1917-47 and 1977-present. In the epochs when the ENSO teleconnection was suppressed, summer rainfall in central U.S. was in good correlation with variations in the Gulf of Mexico and the tropical western Atlantic. Both sea surface temperature anomalies and sea level pressure variations indicate distinct patterns between the opposite epochs in the teleconnection variation. Analysis of lower troposphere geopotential height and wind in the epochs of active teleconnection revealed a deformation field in wet summers favoring a stationary front and summer storms in the central U.S.
It was further found that the variations of the teleconnection were "in-phase" with variations of the average surface temperature of the Northern Hemisphere. The two weak cooling periods of surface temperature in 1871-1916 and 1948-76 were also the periods of active teleconnection. Following the "abrupt changes" (warming) of the surface temperature around 1920 and 1976, the teleconnection broke down.
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