ORIGINS OF EXTREME CLIMATE STATES DURING THE 1982-83 ENSO WINTER

The origin of extreme climate states during the exceptional 1982-83 El Ni (n) over tilde o event has continued to be a source of controversy and debate. On the one hand, empirical analyses of extratropical clim ate patterns during past El Ni (n) over tilde o events suggests the ob served anomalies during 1982-83 were consistent with tropical forcing. On the other hand, the large amplitude of those anomalies have not be en replicated in atmospheric general circulation model (AGCM) simulati ons for that period performed as part of the Atmospheric Model Interco mparison Project (AMIP). It has recently become apparent, however, tha t the sea surface boundary conditions used to drive the multitude of A MIP simulations were deficient, in that at least 30% of available trop ical Pacific SST observations were discarded in the analysis cycle due to excessive trimming constraints. It is shown from a reanalysis of t he sea surface temperatures that the observed east equatorial Pacific waters were 1.5 degrees C warmer than original estimates. In order to address the extent to which simulations of the extratropical climate o f 1982-83 are sensitive to different SST analyses of that period, a pa rallel suite of AGCM simulations using two SST prescriptions is perfor med. One set is based on the blended satellite-in situ data used also in the AMIP runs, whereas the other is based on the optimum interpolat ion (OI) reanalysis. A nine-member ensemble of such simulations is per formed, and this is compared with observations. The model response usi ng the original blended SSTs is shown to severely underestimate the tr opical rainfall anomalies, and this contributes to the simulation of a weak extratropical response as reported earlier in the AMIP experimen ts. A larger, more realistic response during 1982-83 is shown to occur in an identical set of runs based on che OI SST boundary conditions, and most aspects of the observed pattern and strength of the Pacific-N orth American climate anomalies during that winter are reproduced in t he model's ensemble mean response. Further analysis of the models' int ersample variability are performed to ascertain the extent to which th e observed anomalies may have been influenced by atmospheric initial c onditions. It is shown from the OI runs that the observed tropical Pac ific rainfall anomalies and the Southern Oscillation index were phenom ena causally determined by the El Ni (n) over tilde o. Even over the P acific-North American region, the spatial pattern of the anomalies in individual runs was highly reproducible, and several members of the OI runs achieved climate anomalies exceeding in amplitude those observed . The findings strongly indicate the important role of El Ni (n) over tilde o in determining the climate state over the Pacific-North Americ an region during 1882-83, and various competing hypotheses are critiqu ed in light of these new model results.