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ABSTRACT
The Eliassen-Palm (E-P) flux divergences derived from ERA-40 and ERA-Interim show significant differences during northern winter. The discrepancies are marked by vertically alternating positive and negative anomalies at high latitudes and are manifested via a difference in the climatology. The magnitude of the discrepancies can be greater than the interannual variability in certain regions. These wave forcing discrepancies are only partially linked to differences in the residual circulation but they are evidently related to the static stability in the affected regions. Thus, the main cause of the discrepancies is most likely an imbalance of radiative heating.
Two significant sudden changes are detected in the differences between the eddy heat fluxes derived from the two reanalyses. One of the changes may be linked to the bias corrections applied to the infrared radiances from the NOAA-12 High-Resolution Infrared Radiation Sounder in ERA-40, which is known to be contaminated by volcanic aerosol from the 1991 eruption of Mt. Pinatubo. The other change may be due in part to the use of uncorrected radiances from the NOAA-15 Advanced Microwave Sounding Units by ERA-Interim since 1998. These sudden changes have the potential to alter the wave forcing trends in the affected reanalysis, suggesting that extreme care is needed when one comes to extract trends from the highly derived wave forcing quantities.
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1. Introduction
The equator to pole circulation in the winter stratosphere is primarily driven by the upward propagating waves from the troposphere. This large-scale dynamical process is called the Brewer-Dobson (B-D) circulation and can be studied using the transformed Eulerian mean (TEM) equations (Edmon et al. 1980; Andrews et al. 1987; Holton et al. 1995; Shepherd 2007; Birner and Bonisch 2011). The Eliassen-Palm (E-P) flux divergence, which represents the wave forcing that acts on the mean flow to cause wind and temperature variations, is the most important quantity in the TEM equations. Numerous studies have used the TEM equations together with the E-P flux divergence to study the interannual variation and long-term trends of the B-D circulation (Edmon et al. 1980; Seviour et al. 2012), the behavior of planetary wave activity (Hu and Tung 2002; Karpetchko and Nikulin 2004; Hu et al. 2005), the variability of the polar vortex (Waugh et al....