Contribution of Gravity-Wave Forcing to the Brewer-Dobson Circulation

Contribution of gravity wave (GW) forcing (GWF) to the Brewer-Dobson circulation (BDC) was examined using four reanalysis data (JRA-55, ERA-Interim, MERRA, and MERRA-2) based on the transformed-Eulerian mean primitive equation. The part of residual mean circulation driven by Rossby waves (RW: planetary and synoptic-scale waves) was estimated using the downward control theory from resolved Eliassen-Palm flux divergence. The GW contribution to the BDC was estimated as difference between the RW-driven circulation and the residual-mean circulation calculated by its definition. We examined climatology of the residual mean circulation for equinoctial seasons (i.e., MAM and SON) as well as solstitial seasons (i.e., DJF and JJA). Results obtained from the four reanalysis data were similar. It was shown that GW drives the summer hemispheric part of winter circulation and contributes to the lower latitude part of shallow branches of BDC for all reanalysis data as is consistent with previous studies. New findings are in the following. First, the GW contribution to the poleward circulation of the fall hemisphere is comparable to the RW contribution. Second, in the spring hemisphere, GWF acts to weaken the RW-driven poleward circulation. This means that GWF tends to advance the winter cell of the BDC in both hemispheres. Third, GWF acts to weaken the poleward circulation in the winter middle stratosphere. As a result of these characteristic GW contributions to the BDC, the upward mass flux in the middle stratosphere is maximized twice a year, namely, January and September, while a maximum is observed in January in the lower stratosphere.