Atmospheric response to sea surface temperature anomalies during El Nino 1997/98 as simulated by ECHAM4

One of the strongest El-Nino events on record was observed in 1997/98, a time period characterized by many strong climate anomalies all over the globe. To study the origin of these climate anomalies, ensemble experiments have been performed with the ECHAM4 atmospheric general circulation model at T42 resolution forced by observed sea surface temperatures (SSTs). Many of the observed climate anomalies and the well known El Nino Southern Oscillation teleconnection patterns could be reproduced by the ensemble integrations. In particular, the model reproduces the observed weakening of the trade winds, although it fails to simulate the early decline of the Southern Oscillation Index in November 1997. The observed climate anomalies over the Pacific, the Americas, and the Atlantic were successfully simulated. The model also reproduces the observed splitting of the jet over the North Atlantic which caused a very mild winter in western Europe. Our results do not indicate that the intense 1997 summer rainfalls leading to severe flooding in eastern central Europe were related to El Nino. With respect to the Indian Ocean, India, Australia, and southern Africa, the results were less satisfying. For example, the model yields reduced Indian monsoon rainfall which has not been observed. Additional experiments with SST anomalies restricted to particular ocean basins reveal that most of the atmospheric climate anomalies in 1997/98 were related to SST anomalies in the tropical Pacific. In particular, Atlantic SST anomalies had only a marginal impact on the atmosphere.