A spectral view on the atmospheric intraseasonal variability in the extratropics: ECMWF re-analyses versus operational analyses

Abstract

Various aspects of the atmospheric intraseasonal variability in the extratropics as given by theECMWF re-analyses are investigated. By comparing these results with those obtained from theoriginal operational analyses by ECMWF, we assess the differences between these two datasets, considering both the long-term mean values and the temporal evolution of these differences.The atmospheric intraseasonal variability in the extratropics is larger in the re-analyses thanin the operational analyses, to a small extent in the Northern and to a large extent in theSouthern Hemisphere. These differences are mainly related to the contributions of the transientfluctuations to the intraseasonal variability. Their magnitude depends, however, on the spatialand temporal scales of the transient disturbances with the largest differences associated withthe longest (zonal wavenumbers 1–3) and the smallest differences associated with the shortestplanetary scale waves (zonal wavenumbers 6–9). Over the period of investigation the magnitudeof the differences between the two data sets varies only slightly in the Northern but considerablyin the Southern Hemisphere with small differences between the re-analyses and the operationalanalyses in the beginning of the re-analyses and after 1985 but large ones in the early 1980’s.These temporal variations of the differences can be assigned to both changes in the data coverageand changes in the analysis-forecasting-system at ECMWF.