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

Wednesday, 17 December 2014
Hua Lu, Tom Bracegirdle, Tony Phillips and John Turner, NERC British Antarctic Survey, Cambridge, United Kingdom
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.