The upward branch of the Brewer-Dobson circulation quantified by tropical stratospheric water vapor and carbon monoxide measurements from the Aura Microwave Limb Sounder

The vertical distributions of water vapor (H2O) and carbon monoxide (CO) in the tropical lower stratosphere are controlled largely by their mixing ratios near the tropopause and by ascending motions as part of the Brewer-Dobson circulation (BDC). The upward propagation of seasonal variations imprinted on H2O and CO vertical profiles, often referred to as the tropical tape recorder, can be used to derive the mean vertical velocity, (w) over bar*, in this region of the lower stratosphere where quasi-horizontal mixing is not strong enough to erase the seasonal tape recorder signals. We used Aura Microwave Limb Sounder observations of the tropical tape recorders from 2004 to 2014 to derive values of (w) over bar* at pressures between 90 and 16hPa (about 18 to 28km altitude). Mean vertical profiles of (w) over bar* are consistent with calculated velocities derived from net radiative heating rates based on observed temperature, humidity, cloud, and trace gas amounts. Temporal variations in (w) over bar* are dominated by a quasi-biennial oscillation (QBO) and seasonal cycles, with maximum upwelling coinciding with easterly phases of the QBO in zonal wind shear and during the November-December period of the seasonal cycle. Both the QBO and annual modes emphasize the importance of wave phenomena in modulating the strength of tropical upwelling in the BDC. Interannual anomalies in (w) over bar* are correlated with variations in the El Nino-Southern Oscillation (ENSO), with enhanced stratospheric upwelling during El Nino phases and reduced upwelling during La Nina. A small decreasing linear trend (similar to 6%/decade) in (w) over bar* is observed from 2005 to 2014, although confidence is low in identifying such a trend as part of a long-term change due to the influence of ENSO over this period.