A Comparative Study of Wave Forcing Derived from Era-40 and Era-Interim

This study examines the wave forcing estimated from the ERA-40 and ERA-Interim reanalyses during northern winter. The discrepancies in the wave forcing are most significant at high latitudes and are marked by vertically alternating positive-negative anomalies of the Eliassen-Palm (E-P) flux divergence. In the stratosphere, the discrepancies intensify with altitude and expand equatorwards in the upper stratosphere. They are mainly associated with the vertical E-P fluxes and due primarily to differences in the climatology. The magnitude of discrepancies is about 20-40% of the climatology derived from ERA-Interim and can be greater than the interannual variability of the E-P flux divergence in certain regions. The dynamical implications of these discrepancies on the Brewer-Dobson circulation are also studied. A sudden drop in the 100hPa poleward eddy heat flux is detected in ERA-40 over subtropical oceans after the 1990/1991 northern winter. It is likely related to the bias corrections applied to the infrared radiances from the NOAA-12 High-resolution Infrared Radiation Sounder, which was known to be contaminated by volcanic aerosol from the eruption of Mt. Pinatubo. A sudden increase in 10hPa poleward eddy heat flux is detected in ERA-Interim at high latitudes after the 1997/1998 winter, likely due in part to the use of uncorrected radiances from the NOAA-15 Advanced Microwave Sounding Units. These sudden changes induced artificial trends in stratospheric wave forcing in the affected reanalysis.