Analysis of Tropical Stratospheric Upward Mass Fluxes: A Comparison of Models and Observations

We quantify the vertical velocity and upward mass flux of the Brewer-Dobson circulation (BDC) in the tropical lower stratosphere based on accurate radiative heating rate calculations. Heating rate calculations are computed by using data from eight years of SHADOZ balloon-borne measurements of temperature, ozone and water vapor in the tropics (-20S-10N). Cloud distributions from ISCCP are used to examine the cloud forcing of tropical upwelling. We find a constant vertical upward mass flux with a magnitude of ~ 1.03 kg/m2/day from 20 km to 25 km, which is consistent with the idea that BDC is driven by wave breaking in the extratropics. A strong seasonal cycle exists in both vertical velocity and upward mass flux of BDC, with a maximum in boreal winter and a minimum in boreal summer. The upward mass fluxes also show large inter-annual variability, and a positive trend from 1998 through 2005. We compare the vertical mass fluxes from the NCAR and GFDL GCMs with those based on observational analysis in terms of annual mean, seasonal variation, and inter-annual variations.