Comparison of mean age of air in five reanalyses using the BASCOE transport model

Abstract

We present a consistent intercomparison of the mean age of air (AoA) according to five modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), the Japanese Meteorological Agency's Japanese 55-year Reanalysis (JRA-55), the National Centers for Environmental Prediction Climate Forecast System Reanalysis (CFSR) and the National Aeronautics and Space Administration's Modern Era Retrospective analysis for Research and Applications version 1 (MERRA) and version 2 (MERRA-2). The modeling tool is a kinematic transport model driven only by the surface pressure and wind fields. It is validated for ERA-I through a comparison with the AoA computed by another transport model.The five reanalyses deliver AoA which differs in the worst case by 1 year in the tropical lower stratosphere and more than 2 years in the upper stratosphere. At all latitudes and altitudes, MERRA-2 and MERRA provide the oldest values ( ∼ 5-6 years in midstratosphere at midlatitudes), while JRA-55 and CFSR provide the youngest values (∼ 4 years) and ERA-I delivers intermediate results. The spread of AoA at 50 hPa is as large as the spread obtained in a comparison of chemistry-climate models. The differences between tropical and midlatitude AoA are in better agreement except for MERRA-2. Compared with in situ observations, they indicate that the upwelling is too fast in the tropical lower stratosphere. The spread between the five simulations in the northern midlatitudes is as large as the observational uncertainties in a multidecadal time series of balloon observations, i.e., ap-proximately 2 years. No global impact of the Pinatubo eruption can be found in our simulations of AoA, contrary to a recent study which used a diabatic transport model driven by ERA-I and JRA-55 winds and heating rates.The time variations are also analyzed through multiple linear regression analyses taking into account the seasonal cycles, the quasi-biennial oscillation and the linear trends over four time periods. The amplitudes of AoA seasonal variations in the lower stratosphere are significantly larger when using MERRA and MERRA-2 than with the other reanalyses. The linear trends of AoA using ERA-I confirm those found by earlier model studies, especially for the period 2002-2012, where the dipole structure of the latitudeheight distribution (positive in the northern midstratosphere and negative in the southern midstratosphere) also matches trends derived from satellite observations of SF 6 . Yet the linear trends vary substantially depending on the considered period. Over 2002-2015, the ERA-I results still show a dipole structure with positive trends in the Northern Hemisphere reaching up to 0.3 yr dec −1 . No reanalysis other than ERA-

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“…In contrast, the NH trends have opposite sign in both reanalyses. This discrepancy in AoA trends is consistent with the recent results of Chabrillat et al () for 1989–2001, despite the different periods and AoA calculations (based on diabatic transport model here and on kinematic transport model in Chabrillat et al, ). Here we show that, despite large discrepancies in BDC trends among reanalyses pointed out in previous works, the trends over the region and period of the Antarctic ozone hole are robust.…”

“…However, this is not necessarily the case for other reanalyses (e.g., Diallo et al, , Ploeger et al, ). The recent paper by Chabrillat et al () compares the AoA in five modern reanalyses and finds substantial differences in the linear trends for the period 1989–2001. Hence, uncertainties in BDC trends derived from reanalysis are large, and this issue will be further addressed in section .…”

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“…The consistency in the magnitude and sign of the upwelling changes between the reanalyses is encouraging and shows that the changes exhibited by MERRA‐2 (and M2GMI) are also exhibited by another reanalysis product. We stress that this result is not obvious given that the TEM circulation can exhibit large differences among different reanalysis products depending on the pressures and latitudes sampled (e.g., Abalos et al, ; Chabrillat et al, ; Ploeger et al, ; Seviour et al, ; see Figure S4). Those studies, however, focused on different regions (e.g., tropical lower stratosphere and high‐latitude middle stratosphere), so our results indicate that MERRA‐2 and ERA‐I are overall consistent, at least with respect to upwelling changes over the NH subtropics.…”

How robust are stratospheric age of air trends from different reanalyses?

“…The results of the present paper here are based on a diabatic transport model (for further details see Sect. 2) and therefore complement the study by Chabrillat et al (2018) regarding the representation of vertical transport. The main goal of our paper is to assess the robustness of the climatology and seasonality of the BDC as well as its trends in current generation reanalyses, as imprinted on stratospheric age of air.…”

“…A very recent study by Chabrillat et al (2018) compares the BDC in various reanalyses by using a kinematic transport model (Belgian Assimilation System for Chemical ObsErvations, BASCOE) within the scope of the S-RIP project. The results of the present paper here are based on a diabatic transport model (for further details see Sect.…”