The Brewer-Dobson circulation in the JRA-3Q reanalysis and the impact of changes in model physical processes

This study evaluates the climatology and trends of the Brewer-Dobson circulation (BDC) in the Japanese Reanalysis for Three Quarters of a Century (JRA-3Q) reanalysis compared with those in the Japanese 55-year reanalysis (JRA-55) and ERA5 reanalyses. The spatial structure of the BDC shows good agreement among the reanalyses. However, zonal mean tropical upwelling near the lower stratosphere in JRA-3Q is approximately 80% of that in JRA-55 throughout the reanalysis period. The long-term trend of BDC intensity for the period 1948-2022 showed a slight increase in JRA-3Q and in JRA-55, whereas that in ERA5 decreased. To investigate the cause of the difference in tropical upwelling between JRA-3Q and JRA-55, data assimilation sensitivity experiments were conducted by modifying the forecast model used in the data assimilation system. These results indicate that the difference in BDC intensity between JRA-3Q and JRA-55 can be primarily attributed to diabatic heating in the tropical troposphere produced in the forecast model. However, the difference between JRA-3Q and ERA5 cannot be accounted for by diabatic heating in the forecast model. It is also found that the implementation of the non-orographic gravity wave drag parameterisation scheme in the JRA forecast model is not the reason for the weaker BDC in JRA-3Q.