Predictability of winter climate over the North Atlantic European region during ENSO events

The predictability of winter climate over the North Atlantic - European (NAE) region during ENSO events is investigated. Rather than employing traditional composite analyses, the authors focus on the impacts of six individual events: three El Nino events and three La Nina events. The investigation is based on the analysis of ensemble simulations with an atmospheric GCM forced with prescribed sea surface temperatures (SST) for the period December 1985 - May 2001, and on observations. Model experiments are used to separate the respective roles of SST anomalies in the Indo-Pacific basin and in the Atlantic basin. A significant ( potentially predictable) climate signal is found in the NAE region for all six ENSO events. However, there are notable differences in the impacts of individual El Nino and La Nina events. These differences arise not simply from atmospheric internal variability but also because the atmosphere is sensitive to specific features of the SST anomaly fields that characterize the individual events. The different impacts arise partly from differences in Indo-Pacific SST and partly from differences in Atlantic SST. SST anomalies in both ocean basins can influence tropical convection and excite a Rossby wave response over the North Atlantic. The evidence presented here for the importance of Atlantic Ocean conditions argues that, in the development of systems for seasonal forecasting, attention should not be focused too narrowly on the tropical Pacific Ocean.