Comparisons of Brewer-Dobson Circulations Diagnosed from Reanalyses

A comparison is made of the stratospheric mean-meridional circulations, Brewer-Dobson (B-D) circulations, diagnosed from the reanalyses, JRA-25, ERA-40, ERA-Interim, NCEP/NCAR and NCEP/DOE. The reanalyses coincidently exhibit seasonality of B-D circulation, although considerable discrepancy among the reanalyses is found particularly in low-latitudes. Meridional overturning circulation at 100 hPa in the northern-hemisphere is maximal in winter, while that in the southern hemisphere is maximal in fall and significantly smaller than the northern hemispheric one. Interannual variability of B-D circulation in winter is coincident among the reanalyses, because they may reasonably represent wave-mean flow interactions of planetary waves which drive mean-meridional circulation. Yearly trends are not reliably observed due to large diversity among the reanalyses. Zonal mean vertical velocity becomes very noisy owing to inconsistency between the observation and global numerical weather prediction (NWP) model used in assimilation, except for JRA-25 and ERA-Interim. Further efforts are desired to improve reanalyses mainly through reduction of systematic errors of NWP model and implementation of advanced data assimilation schemes.