Abstract
Abstract. An accelerating Brewer–Dobson circulation (BDC) is a robust signal of climate change in model predictions but has been questioned by trace gas observations. We analyse the stratospheric mean age of air and the full age spectrum as measures for the BDC and its trend. Age of air is calculated using the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA-Interim, JRA-55 and MERRA-2 reanalysis data to assess the robustness of the representation of the BDC in current generation meteorological reanalyses. We find that the climatological mean age significantly depends on the reanalysis, with JRA-55 showing the youngest and MERRA-2 the oldest mean age. Consideration of the age spectrum indicates that the older air for MERRA-2 is related to a stronger spectrum tail, which is likely associated with weaker tropical upwelling and stronger recirculation. Seasonality of stratospheric transport is robustly represented in reanalyses, with similar mean age variations and age spectrum peaks. Long-term changes from 1989 to 2015 turn out to be similar for the reanalyses with mainly decreasing mean age accompanied by a shift of the age spectrum peak towards shorter transit times, resembling the forced response in climate model simulations to increasing greenhouse gas concentrations. For the shorter periods, 1989–2001 and 2002–2015, the age of air changes are less robust. Only ERA-Interim shows the hemispheric dipole pattern in age changes from 2002 to 2015 as viewed by recent satellite observations. Consequently, the representation of decadal variability of the BDC in current generation reanalyses appears less robust and is a major uncertainty of modelling the BDC.
Highlights
The global circulation of the stratosphere, known as the Brewer–Dobson circulation (Brewer, 1949; Holton et al, 1995), is a crucial factor controlling the lower stratospheric composition of radiatively active trace gases; it plays an important role in the Earth’s radiation budget and in climate
Studies based on reanalysis meteorology have provided a diverse picture of mean age trends (e.g. Diallo et al, 2012; Monge-Sanz et al, 2012; Ploeger et al, 2015a) which is partly related to the different trend periods considered
We compared stratospheric mean age and the full age spectrum from simulations with the diabatic Chemical Lagrangian Model of the Stratosphere (CLaMS) model driven by different reanalyses (ERA-Interim, JRA-55 and MERRA-2) to investigate the robustness in the representation of the climatology, seasonality and trends of the Brewer– Dobson circulation in current generation reanalysis data sets
Summary
The global circulation of the stratosphere, known as the Brewer–Dobson circulation (Brewer, 1949; Holton et al, 1995), is a crucial factor controlling the lower stratospheric composition of radiatively active trace gases; it plays an important role in the Earth’s radiation budget and in climate. Current climate models and observations disagree regarding long-term changes of the BDC Climate models simulate a strengthening and accelerating circulation which is not evident from observations, representing a major uncertainty in current model predictions (see Butchart, 2014, for a recent review). The BDC is characterised by the slow upwelling in the tropics from the troposphere across the tropical tropopause layer (TTL) into the stratosphere, followed by poleward motion in the stratosphere and downwelling at middle and high latitudes. The residual mean mass circulation may be separated into two different
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
