The signature of ENSO in the Northern Hemisphere midlatitude seasonal meanflow and high-frequency intraseasonal variability

The impact of pronounced positive and negative sea surface temperature (SST ) anomalies in the tropical Pacific associated with the Fl Nino/Southern Os cillation (ENSO) phenomenon on the atmospheric circulation in the Northern Hemisphere extratropics during the boreal winter season is investigated. Th is includes both the impact on the seasonal mean flow and on the intraseaso nal variability on synoptic time scales. Moreover, the interaction between the transient fluctuations on these times scales and the mean circulation i s examined. Both data from an ensemble of five simulations with the ECHAM3 atmospheric general circulation model at a horizontal resolution of T42 eac h covering the period from 1979 through 1992 and operational analyses from ECMWF for the corresponding period are examined. in each of the simulations observed SSTs for the period of investigation are given as lower boundary forcing, but different atmospheric initial conditions are prescribed. The simulations with ECHAM3 reveal a distinct impact of the pronounced SST- anomalies in the tropical Pacific on the atmospheric circulation in the Nor thern Hemisphere extratropics during Fl Nino as well as during La Nina even ts. These changes in the atmospheric circulation, which are found to be hig hly significant in the Pacific/North American as well as in the Atlantic/Eu ropean region, are consistent with the essential results obtained from the analyses. The pronounced SST-anomalies in the tropical Pacific lead to chan ges in the mean circulation, which are characterized by typical circulation patterns. These changes in the mean circulation are accompanied by marked variations of the activity of the transient fluctuations on synoptic time s cales, that are changes in both the kinetic energy on these time scales and the atmospheric transports of momentum and heat accomplished by the short baroclinic waves. The synoptic disturbances, on the other hand, play also a n important role in controlling the changes in the mean circulation associa ted with the ENSO phenomenon. They maintain these typical circulation patte rns via barotropic, but counteract them via baroclinic processes. The hypothesis of an impact of the ENSO phenomenon in the Atlantic/European region can be supported. As the determining factor the intensification (re duction) of the Aleutian low and the simultaneous reduction (intensificatio n) of the Icelandic low during El Nino and during La Nina events, respectiv ely, is identified. The changes in the intensity of the Aleutian low during the ENSO-events are accompanied by an alteration of the transport of momen tum caused by the short baroclinic waves over the North American continent in such a way that the changes in the intensity of the Icelandic low during Fl Nino as well as during La Nina events are maintained.