Understanding changes in the stratospheric circulation from observations and simulations

Abstract

This study investigates the variation of the Brewer-Dobson circulation (BDC) in the stratosphere on interannual and decadal timescales from both observations and numerical simulations. Lower stratospheric temperature has been employed to indicate changes of the BDC over the past few decades. Based on regression analysis and radiative calculations, the observed lower stratospheric temperature trends are separated into a dynamical contribution resulting from changes of the BDC and a radiative contribution resulting from concentration changes of ozone and greenhouse gases (GHGs). We found that the observed BDC since 1979 shows a strong acceleration in its southern cell during austral winter and spring and also in its northern cell during boreal winter, but a deceleration in its northern cell during boreal spring. The interannual variation of the BDC is partly contributed by variations in tropical SST anomalies through the mediation of waves. In the Southern Hemisphere spring season, the stratospheric planetary wave activity is coupled with tropical SST anomalies primarily through two modes. The first mode shows an El Nino-Southern Oscillation (ENSO) -like SST anomaly pattern, and the second mode shows a central-Pacific ENSO-like pattern with strong anomalies over the equatorial central Pacific. Stronger stratospheric wave activity, and hence a stronger BDC, is associated with La Nina-like SST anomalies for mode-1, and central Pacific El Nino-like SST anomalies for mode-2. The simulated BDC in Chemistry Climate Models (CCMs) are diagnosed using the Transformed Eulerian Mean (TEM) formulation. We found robust BDC strengthening from 1960 to the end of the 21st century in these models. We divided the BDC into transition, stratospheric shallow and deep branches based on its vertical extent. Models consistently simulate the acceleration in all three BDC branches over the 140 years, but the acceleration rate of the deep branches is much smaller. The acceleration of the BDC shallow and transition branches can be understood in terms of GHGs-induced warming in the tropical upper troposphere and the enhancement of the subtropical jets. The acceleration of the deep branch is also a response to the increase of greenhouse gas concentrations but is modulated by the changes in ozone concentrations. The effect of ozone changes is particularly prominent in the southern deep branch during austral summer: almost all models simulated strong significant acceleration during the ozone depletion era, weak deceleration during the ozone recovery era and near-zero trends during the stable ozone era. However, the ozone effect is less evident in other seasons and in the other branches. Although both observations and model simulations indicate overall strengthening of the BDC over the past few decades, the BDC changes simulated by CCMs and Atmosphere Ocean General Circulation Models (AOGCMs) show a different character from the observations. The strong BDC-related lower stratospheric temperature trend patterns are not simulated in AOGCMs or CCMs. On the other hand, the strong BDC acceleration during austral summer that is consistently simulated by CCMs is not supported by observations. The discrepancy between the model simulations and the observations suggests that further research efforts are required to improve our understanding of the stratospheric circulations.