Space-time spectra of the atmospheric intraseasonal variability in the extratropics and their dependency on the El Nino Southern Oscillation phenomenon: model versus observation

By comparing the results obtained from two sets of simulations with the ECH AM3 and the ECHAM4 atmospheric general circulation models with results deri ved from the ECMWF re-analyses, we not only investigate the models' capabil ity to reproduce aspects of the intraseasonal variability in the extratropi cs realistically, but also evaluate the impact of the changes between the t wo different versions of the ECHAM model. Moreover, we assess the impact of the marked variations of sea surface temperatures in the tropical Pacific associated with the El Nino/Southern Oscillation (ENSO) phenomenon on the c haracteristics of the intraseasonal variability in the midlatitudes. Both m odels realistically reproduce many aspects of the intraseasonal variability in the extratropics, i.e. the partition of the variability into the contri butions of the transient cell and of the stationary and transient eddies an d its seasonal variation, and also the spectral distribution of the contrib ution of the transient waves to the intraseasonal variability. The most sev ere deficiency of the models is a considerable underestimation of the contr ibutions of the transient waves to the intraseasonal variability, mainly in the low-frequency part of the spectrum. In the recent version of the ECHAM model (ECHAM4) some of the model's shortcomings in simulating the intrasea sonal variability realistically, in particular those in the Southern Hemisp here, are noticeably reduced compared to the previous version (ECHAM3). Yet some aspects are more realistically captured by ECHAM3. Both the ECMWF re- analyses and the two sets of simulations with the ECHAM models reveal a dis tinct impact of the ENSO phenomenon on the characteristics of the intraseas onal variability within the extratropics in boreal winter. In the Northern Hemisphere the most prominent effect is that the activity of the stationary waves is enhanced during El Nino events at the expense of the transient wa ves. In the Southern Hemisphere, on the other hand, all the different contr ibutions to the variance on intraseasonal time scales (transient cell, tran sient and stationary eddies) are stronger during El Nino than during La Nin a events. Concerning the transient waves, this mainly reflects changes in t he low-frequency part of the spectrum associated with the activity of ultra -long planetary waves.