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Bates, G. T., M. P. Hoerling, and A. Kumar, 2001: Central U.S. springtime precipitation extremes: Teleconnections and relationships with sea surface temperatures. J. Climate, 14, 3751-3766.


ABSTRACT

Dynamical methods are used to investigate atmospheric teleconnections associated with extreme seasonal precipitation anomalies over the central United States during April-June. The importance of sea surface temperature (SST) anomalies in forcing atmospheric teleconnections is specifically addressed through analyses of atmospheric general circulation model (GCM) simulations forced with the monthly varying SSTs of the years 1950-98. The results from three different models, each run in ensemble mode, are compared with observations of extreme April-June precipitation events in the central United States during the last half of the twentieth century.

Analysis of GCM simulations of April-June 1988 indicates that the atmospheric circulation anomalies associated with the 1988 drought were not forced by SST anomalies and that the coexistence of central U.S. drought and La Niña during that spring was coincidental. Likewise, composite analysis reveals no SST forcing for the teleconnections associated with extreme dry spring seasons over the central United States during the last half of the twentieth century in either observations or GCMs. Nonetheless, this characteristic teleconnection pattern of the composite analysis resembles the circulation anomalies of 1988. The results imply that such drought events and the teleconnections related with them have little SST-based predictability.

A somewhat different conclusion is drawn regarding the role of tropical SSTs in the occurrence of extreme wet spring seasons over the central United States. Simulations of the 1993 flood period exhibit skill in reproducing the seasonal circulation anomalies over the Pacific-North American region, and the ensemble mean precipitation anomalies in one GCM nearly replicate the observed strength and distribution of positive rainfall anomalies over the United States. Further composite analysis of extreme wet spring seasons over the last half of the twentieth century confirms the impression gathered from the 1993 case study, with observations and all three GCMs possessing positive tropical east Pacific SST anomalies in conjunction with extreme wet spring seasons over the central United States. Some SST-based potential predictability of extreme wet springs over the central United States consequently exists.