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

Abstract

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 H2 O and CO vertical profiles, often referred to as the tropical “tape recorder,” can be used to derive the mean vertical velocity, w*, 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* at pressures between 90 and 16 hPa (about 18 to 28 km altitude). Mean vertical profiles of w* 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* 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* are correlated with variations in the El Niño–Southern Oscillation (ENSO), with enhanced stratospheric upwelling during El Niño phases and reduced upwelling during La Niña. A small decreasing linear trend (~6%/decade) in w* 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.