Articles | Volume 17, issue 2
Atmos. Chem. Phys., 17, 1313–1327, 2017
Atmos. Chem. Phys., 17, 1313–1327, 2017

Research article 30 Jan 2017

Research article | 30 Jan 2017

Time-varying changes in the simulated structure of the Brewer–Dobson Circulation

Chaim I. Garfinkel1, Valentina Aquila2,3,4, Darryn W. Waugh2, and Luke D. Oman4 Chaim I. Garfinkel et al.
  • 1The Fredy and Nadine Herrmann Institute of Earth Sciences, Hebrew University, Jerusalem, Israel
  • 2Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD, USA
  • 3Goddard Earth Science Technology and Research, Greenbelt, MD, USA
  • 4NASA Goddard Space Flight Center, Greenbelt, MD, USA

Abstract. A series of simulations using the NASA Goddard Earth Observing System Chemistry Climate Model are analyzed in order to assess changes in the Brewer–Dobson Circulation (BDC) over the past 55 years. When trends are computed over the past 55 years, the BDC accelerates throughout the stratosphere, consistent with previous modeling results. However, over the second half of the simulations (i.e., since the late 1980s), the model simulates structural changes in the BDC as the temporal evolution of the BDC varies between regions in the stratosphere. In the mid-stratosphere in the midlatitude Northern Hemisphere, the BDC does not accelerate in the ensemble mean of our simulations despite increases in greenhouse gas concentrations and warming sea surface temperatures, and it even decelerates in one ensemble member. This deceleration is reminiscent of changes inferred from satellite instruments and in situ measurements. In contrast, the BDC in the lower stratosphere continues to accelerate. The main forcing agents for the recent slowdown in the mid-stratosphere appear to be declining ozone-depleting substance (ODS) concentrations and the timing of volcanic eruptions. Changes in both mean age of air and the tropical upwelling of the residual circulation indicate a lack of recent acceleration. We therefore clarify that the statement that is often made that climate models simulate a decreasing age throughout the stratosphere only applies over long time periods and is not necessarily the case for the past 25 years, when most tracer measurements were taken.

Short summary
Previous work has noted a discrepancy between models and observations in trends of the large-scale overturning circulation in the stratosphere. Here, we show that a model can simulate trends that are reminiscent of those observed, including space- and time-varying trends in different regions of the stratosphere. We therefore clarify that the statement that is often made that models simulate an accelerated circulation only applies over long time periods and is not true for the past 25 years.
Final-revised paper