The advective Brewer-Dobson circulation in the ERA5 reanalysis: variability and trends

Abstract

Abstract. The stratospheric Brewer-Dobson circulation (BDC) is an important element of climate as it determines the transport and distributions of key radiatively active atmospheric trace gases, which affect the Earth’s radiation budget and surface climate. Here, we evaluate the inter-annual variability and trends of the BDC in the ERA5 reanalysis and inter-compare with the ERA-Interim reanalysis for the 1979–2018 period. We also assess the modulation of the circulation by the Quasi-Biennial Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO), and the forcings of the circulation by the planetary and gravity wave drag. A comparison of ERA5 and ERA-Interim reanalyses shows a very good agreement in the morphology of the BDC and in its structural modulations by the natural variability related to QBO and ENSO. Despite the good agreement in the spatial structure, there are substantial differences in the strength of the BDC and of the natural variability impacts on the BDC between the two reanalyses, particularly in the upper troposphere and lower stratosphere (UTLS), and in the upper stratosphere. Throughout most regions of the stratosphere, the variability and trends of the advective BDC are stronger in the ERA5 reanalysis due to stronger planetary and gravity wave forcings, except in the UTLS below 20 km where the tropical upwelling is about 40 % weaker due to a weaker gravity wave forcings at the equatorial flank of the subtropical jet. In the extra-tropics, the large-scale downwelling is stronger in ERA5 than in ERA-Interim linked to significant differences in planetary and gravity wave forcings. Analysis of the BDC trend shows a global acceleration of the annual mean residual circulation with an acceleration rate of about 1.5 % per decade at 70 hPa due to the long-term intensification in gravity and planetary wave breaking, consistent with observed and future climate model predicted BDC changes.

The stratospheric Brewer-Dobson circulation inferred from age of air in the ERA5 reanalysis

“…It is so far unclear whether the representation of the BDC in ERA5 is improved compared to the older reanalyses. In the tropical lower stratosphere, the weaker heating rates in ERA5 are consistent with slower residual circulation upwelling found by Diallo et al (2020) and appear to correct the 30-40% high bias in ERA-Interim heating rates in the tropical tropopause layer (TTL) found in previous studies (e.g., Ploeger et al, 2012;Schoeberl et al, 2012). On the other hand, the minimum in tropical upwelling around the level of zero radiative heating (around 350 K) is much lower for ERA5 than for ERA-Interim with heating rates even showing a gap in annual mean upwelling in a shallow layer, similar to the case in MERRA-2 reanalysis (e.g., compare Fig.…”

“…In a recent study, Diallo et al (2020) compared the residual circulation in ERA5 and ERA-Interim based on residual circulation vertical velocity w * and stream function. They showed, using these standard circulation metrics, that the BDC in ERA5 is significantly slower than in ERA-Interim, consistent with the findings here based on age of air and the diabatic circulation (heating rate based).…”

“…Minor differences between ERA5 and ERA-Interim concern downward velocities in the summer hemisphere and a stronger upwelling in the tropical lower stratosphere in ERA-Interim. Diallo et al (2020) showed in their Fig. 2 that tropical upwelling differences in w * in the tropical lower stratosphere amount to about 40% at around 15 km and decrease above (zero difference at about 22 km).…”

“…2). Diallo et al (2020) analyzed the ERA5 BDC based on TEM residual circulation vertical velocity, a common measure for the strength of the BDC (e.g., Andrews et al, 1987, Eq. 3.5.1b),…”

“…Case studies on stratospheric and tropospheric transport indicate improvements in the representation of physical processes, like convection, in ERA5 (e.g., Li et al, 2020). Recently, Diallo et al (2020) have analyzed the ERA5 residual mean mass circulation of the BDC, its variability and trend, based on Transformed Eulerian mean (TEM) calculations. The present paper can be viewed complementary as it investigates the representation of the BDC in ERA5 in terms of stratospheric age of air from transport model simulations, and compares results to the previous ERA-Interim reanalysis.…”