Springtime ENSO phase evolution and its relation to rainfall in the continental U.S.

Springtime El Nino–Southern Oscillation (ENSO) phase evolution and associated U.S. rainfall variability are explored by performing composite analysis of observational data. Although the tropical Pacific ENSO sea surface temperature anomalies are weaker and less coherent in boreal spring compared to those in winter, there are unique and significant patterns of U.S. rainfall anomalies frequently appearing during the onset and decay phases of ENSO. In early spring of a decaying El Nino, the atmospheric jet stream and associated storm track shift southward, causing more frequent wet conditions across the southern U.S. and dry conditions in a belt south and east of the Ohio River. In late spring of a developing El Nino, the synoptic activity over the U.S. reduces overall and the southwesterly low-level winds that carry moist air from the Gulf of Mexico to the U.S. shift westward, causing a similar dipole of rainfall anomalies between the southern U.S. and the Ohio Valley.