Hoerling, M. P., and A. Kumar, 1997: Why do North American climate anomalies differ from one El Niño event to another? Geophys. Res. Lett., 24, 1059-1062.


This study explores the cause of the appreciable variations in North American climate from one El Niño event to another. Two competing sources of inter-El Niño variations are examined; one associated with differences in sea surface temperature (SST) forcing between El Niño events, and the other associated with purely internal atmospheric processes that operate independently of El Niño. The analysis is based upon a large ensemble of atmospheric general circulation model (AGCM) simulations that are forced with the observed variations in global SSTs during 1950-1994. From investigation of the seven strongest warm events in this record, the AGCM is shown to posses only one dominant large-scale pattern over the Pacific-North American region. The case-to-case variability seen in nature is thus claimed to be primarily due to atmospheric processes, and the inter-El Niño differences in SST appear to be of secondary importance. The significance of these results for an atmospheric-ocean forecasting system are discussed.