Evaluating the advective Brewer‐Dobson circulation in three reanalyses for the period 1979–2012

Abstract

Most chemistry-climate models show an intensification of the Brewer-Dobson circulation (BDC) in the stratosphere associated with increasing greenhouse gas emissions and ozone depletion in the last decades, but this trend remains to be confirmed in observational data. In this work the evolution of the advective BDC for the period 1979-2012 is evaluated and compared in three modern reanalyses (ERA-Interim, MERRA, and JRA-55). Three different estimates of the BDC are computed for each reanalysis, one based on the definition of the residual circulation and two indirect estimates derived from momentum and thermodynamic balances. The comparison among the nine estimates shows substantial uncertainty in the mean magnitude (∼40%) but significant common variability. The tropical upwelling series show variability linked to the stratospheric quasi-biennial oscillation and to El Niño-Southern Oscillation (ENSO) and also reflect extreme events such as major sudden stratospheric warmings and volcanic eruptions. The trend analysis suggests a strengthening of tropical upwelling of around 2-5%/decade throughout the layer 100-10 hPa. The global spatial structure of the BDC trends provides evidence of an overall acceleration of the circulation in both hemispheres, with qualitative agreement among the estimates. The global BDC trends are mainly linked to changes in the boreal winter season and can be tracked to long-term increases in the resolved wave drag in both hemispheres.

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…After 1995 the ERA-Interim data show a negative trend in the tropical upward mass flux (at 70/74 hPa), which is not captured by the model simulations. As shown by Seviour et al (2012), this negative trend contains large uncertainties, and does not occur in other reanalysis systems or when using different estimates of upwelling in ERA-Interim (Abalos et al, 2015). The nudged simulations, especially the simulations RC1SD-base-09/10 without global mean temperature nudging, are closer to ERA-Interim data than the RC1-simulations without nudging.…”

Effects of mixing on resolved and unresolved scales on stratospheric age of air

“…AoA trends of EMAC-RC1SD and CLaMS-ERAI show considerable differences, despite being nudged to the same dynamics. However, as recently discussed by Abalos et al (2015), the residual circulation calculated from reanalysis data differs strongly, if different estimates are used as it is the case in EMAC and CLaMS. Therefore, model simulations driven by or nudged by reanalysis have to be interpreted with care with respect to circulation and circulation changes.…”

“…However, while CLaMS is directly driven by ERA-Interim data using diabatic heating rates as vertical velocities, the EMAC-RC1SD simulation is nudged to ERA-Interim horizontal winds and temperatures. As recently pointed out by Abalos et al (2015), the residual circulation calculated from reanalysis data using different estimates (from the direct TEM residual velocities, from the momentum balance, and from the thermodynamic balance, i.e., the diabatic circulation) differs strongly in mean magnitude (up to 40 %), likely due to data assimilation. The fact that direct and momentum-based estimates are different in ERA-Interim is confirmed in Fig.…”

“…4a). Furthermore, it also differs from the diabatic circulation in ERA-Interim (see Abalos et al, 2015, their Fig. 6), which is used in CLaMS.…”

Seasonal and inter-annual variability of lower stratospheric age of air spectra

“…Garny et al, 2011) and in situ observations Ray et al, 2010Ray et al, , 2014. Reanalyses indicate a long-term strengthening of the residual circulation (Abalos et al, 2015), which causes decreasing mean age. However, strong decadal variations and, in particular, changes in eddy mixing have been found to mask these long-term residual circulation driven age changes (Ploeger et al, 2015a).…”