To What Extent Does High-Latitude Wave Forcing Drive Tropical Upwelling in the Brewer-Dobson Circulation?

The causes of the annual cycle and nonseasonal variability in the globally averaged, equator-to-pole Brewer Dobson circulation (BDC: defined here as the equatorially symmetric component of the Lagrangian-mean meridional circulation) are investigated based on zonally averaged, lower-stratospheric temperature data from satellite-borne Microwave Sounding Unit (MSU) and Advanced Microwave Sounding Unit (AMSU). Time-varying vertical velocities in the BDC are inferred from departures of the meridional temperature profiles from the respective radiative equilibrium temperature profiles. Equatorward of similar to 45 degrees N/S, the annual-mean profile of lower-stratospheric temperature and the seasonal and nonseasonal variations about it project almost exclusively onto the equatorially symmetric component. The climatological-mean annual cycle accounts for nearly 90% of the month-to-month variance of the equatorially symmetric component of the temperature field; January/February is colder than July/August equatorward of similar to 45 degrees N/S and warmer than July/August poleward of that latitude. The equator-to-subpolar temperature contrast roughly doubles from July/August to January/February, implying an approximate doubling of the strength of the BDC. The nonseasonal variability is dominated by a similar pattern. Tropical upwelling in the BDC, as inferred from of the temperature field, varies in response to variations in eddy heat fluxes at high latitudes with comparable strength on the intraseasonal and interannual time scales; it does not appear to be correlated with equatorial tropospheric planetary wave activity or with variations in wave forcing in subtropical lower stratosphere. It is concluded that high-latitude wave forcing plays an important role in modulating tropical upwelling in the BDC across a wide range of frequencies.