Climatic Role of the Southerly Flow from the Gulf of Mexico in Interannual Variations in Summer Rainfall in the Central United States*

Abstract

This study continues the investigation of causes of the interannual variations in summer rainfall in the central United States. A previous study by the authors showed that the ENSO teleconnection significantly affected the interannual variations in summer rainfall in the central United States in two epochs, 1871‐1916 and 1949‐78. The teleconnection effect weakened in the epochs 1917‐48 and 1979‐97. The current study partially answers the question: What affected the interannual summer rainfall variations in the two epochs when the ENSO teleconnection weakened? Its results showed that the low-level southerly flow from the Gulf of Mexico was another source of interannual summer rainfall variations. The southerly flow possessed significant interannual variations independent of the ENSO variation. In the epochs when the ENSO teleconnection broke down, the variations of the southerly flow amplified. In the meantime, the circulation anomalies in the lower troposphere in the central United States favored a convergence and an unstable thermal profile. They helped to engage the variations in the southerly flow and the summer rainfall variation in the central United States and to maintain the interannual summer rainfall variation. A coherent variation of this source and ENSO teleconnection in different epochs sustained the observed interannual variations of the summer rainfall in the central United States. The coherent variation of the role of these two different sources was in accord with the multidecadal variation in SST in the mid- and high-latitude North Pacific Ocean, supporting the notion that the multidecadal variation in the SST may have facilitated the coherent variation.