The Effect of the 1997/98 El Niño on Individual Large-Scale Weather Events

Can an individual weather event be attributed to El Nino? This question is addressed quantitatively using ensembles of medium-range weather forecasts made with and without tropical sea surface temperature anomalies. The National Centers for Environmental Prediction (NCEP) operational medium-range forecast model is used. It is found that anomalous tropical forcing affects forecast skill in midlatitudes as early as the fifth day of the forecast. The effect of the anomalous sea surface temperatures in the medium range is defined as the synoptic El Nino signal. The synoptic El Nino signal over North America is found to vary from case to case and sometimes can depart dramatically from the pattern classically associated with El Nino. This method of parallel ensembles of medium-range forecasts provides information about the changing impacts of El Nino on timescales of a week or two that is not available from conventional seasonal forecasts. Knowledge of the synoptic El Nino signal can be used to attribute aspect...