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

The impact of pronounced positive and negative sea surface temperature (SST) anomalies in the tropical Pacific associated with the E1 Niño/Southern Oscillation (ENSO) phenomenon on the atmospheric circulation in the Northern Hemisphere extratropics during the boreal winter season is investigated. This includes the impact on the mean circulation and on the intra—seasonal variability as well. Both observational data and data from an ensemble of five simulations with the ECHAM3 atmospheric general circulation model at a resolution of T42 covering the period from 1979 through 1992 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. This allows for an assessment of the external variability caused by variations of the SSTs compared to the natural variability, leading to an estimate of the potential predictability of the changes in the atmospheric circulation due to pronounced variations of the SSTs. The simulations with ECHAM3 reveal a distinct impact of the pronounced SST-anomalies in the tropical Pacific on the atmospheric circulation in the Northern Hemisphere extratropics during El Niño and La Niña events as well. The changes in the atmospheric circulation, which can be found both in the Pacific/North American and in the Atlantic/European region, are consistent with the essential results obtained from observational data. The pronounced SST—anomalies in the tropical Pacific lead to changes in the mean circulation characterized by typical circulation patterns. These circulation patterns induce changes in the activity of the high—frequency transient fluctuations or in the characteristics of the stormtracks, respectively. Due to non-linear interactions with the high—frequency transient disturbances this, in turn, leads to changes in the activity of the low—frequency fluctuations, such as blocking anticyclones and cutoff lows. The variations in the mean circulation and in the characteristics of the stormtracks during the different ENSO-events are found to be highly significant over most of the Pacific/ North American and the Atlantic/European region as well, whereas the changes in the activity of the low—frequency transient fluctuations are only in a few areas highly significant. In these areas also marked differences in the occurrence of persistent anomalies in the geopotential height field depending on the phase of ENSO are found. The changes in the mean circulation and in the stormtracks due to pronounced variations of the SSTs can also be predicted to a certain degree in the Pacific/North American region. Over the Atlantic and Europe these changes, which are roughly half as strong as over the Pacific, are generally not predictable due to the rather strong natural variations in the extratropics. The hypothesis of an ENSO-impact in the Atlantic/European region can be supported. As the determining factor the intensification (reduction) of the Aleutian low and the simultaneous reduction (intensification) of the Icelandic low during El Niño and during La Niña events, respectively, is identified