Stratospheric Mean Age Research Articles

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

Abstract. This paper investigates the global stratospheric Brewer–Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS (Chemical Lagrangian Model of the Stratosphere), driven by reanalysis winds and total diabatic heating rates. ERA5-based results are compared to results based on the preceding ERA-Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA-Interim, manifesting in weaker diabatic heating rates and higher age of air. In the tropical lower stratosphere, heating rates are 30 %–40 % weaker in ERA5, likely correcting a bias in ERA-Interim. At 20 km and in the Northern Hemisphere (NH) stratosphere, ERA5 age values are around the upper margin of the uncertainty range from historical tracer observations, indicating a somewhat slow–biased BDC. The age trend in ERA5 over the 1989–2018 period is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear but steplike, potentially caused by multi-annual variability or changes in the observations included in the assimilation. During the 2002–2012 period, the ERA5 age shows a similar hemispheric dipole trend pattern as ERA-Interim, with age increasing in the NH and decreasing in the Southern Hemisphere (SH). Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates in both reanalyses, whereas the deep branch accelerates in ERA5 and decelerates in ERA-Interim.

GISS Model E2.2: A Climate Model Optimized for the Middle Atmosphere—2. Validation of Large‐Scale Transport and Evaluation of Climate Response

AbstractHere we examine the large‐scale transport characteristics of the new “Middle Atmosphere” NASA Goddard Institute for Space Studies (GISS) climate model (E2.2). First, we evaluate the stratospheric transport circulation in historical atmosphere‐only simulations integrated with interactive trace gas and aerosol chemistry. Compared to lower vertical resolution model versions, E2.2 exhibits improved tropical ascent and older stratospheric mean ages that are more consistent with observed values. In the troposphere, poleward transport to the Arctic and interhemispheric mean ages in E2.2 are comparable to models participating in the Chemistry Climate Modeling Initiative. In addition to validating E2.2, we also assess its “transport sensitivity” using the coupled atmosphere‐ocean abrupt 4xCO2 and transient 1%CO2 simulations submitted to the Coupled Model Intercomparison Project, Phase 6, along with a 2xCO2 simulation used to evaluate the linearity of the transport circulation's response to increased CO2. We show that decreases (increases) in a stratospheric mean age (idealized surface loss) tracer scale linearly with increased lower stratospheric upwelling, which also increases linearly with warming tropical sea surface temperatures (SSTs). Abrupt 2xCO2 and 4xCO2 experiments constrained with (fixed) pre‐industrial SSTs are also used to quantify the relative importance of rapid adjustments versus SST feedbacks to the transport circulation responses in the model. Finally, sensitivity experiments are presented to illustrate the impact of changes in the convective parameterization on stratospheric transport.

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

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.

The influence of mixing on stratospheric age of air changes in the 21st century.

Climate models consistently predict an acceleration of the Brewer-Dobson circulation (BDC) due to climate change in the 21st century. However, the strength of this acceleration varies considerably among individual models, which constitutes a notable source of uncertainty for future climate projections. To shed more light upon the magnitude of this uncertainty and on its causes, we analyze the stratospheric mean age of air (AoA) of 10 climate projection simulations from the Chemistry Climate Model Initiative phase 1 (CCMI-I), covering the period between 1960 and 2100. In agreement with previous multi-model studies, we find a large model spread in the magnitude of the AoA trend over the simulation period. Differences between future and past AoA are found to be predominantly due to differences in mixing (reduced aging by mixing and recirculation) rather than differences in residual mean transport. We furthermore analyze the mixing efficiency, a measure of the relative strength of mixing for given residual mean transport, which was previously hypothesized to be a model constant. Here, the mixing efficiency is found to vary not only across models, but also over time in all models. Changes in mixing efficiency are shown to be closely related to changes in AoA and quantified to roughly contribute 10% to the long-term AoA decrease over the 21st century. Additionally, mixing efficiency variations are shown to considerably enhance model spread in AoA changes. To understand these mixing efficiency variations, we also present a consistent dynamical framework based on diffusive closure, which highlights the role of basic state potential vorticity gradients in controlling mixing efficiency and therefore aging by mixing.

Long‐Term Variation in the Mixing Fraction of Tropospheric and Stratospheric Air Masses in the Upper Tropical Tropopause Layer

AbstractObservational records of the stratospheric mean age of air (AoA), the average transit time of an air parcel since entering the stratosphere, must reflect changes in the in‐mixing from the extratropical lower stratosphere to the tropical tropopause layer (TTL). Focusing on the mixing fraction of tropospheric and stratospheric air masses in the upper TTL, the impact of mixing processes that occur prior to air masses entering the stratosphere separately from those that occur within the stratosphere is investigated. Using trajectory analysis, the mixing fraction during 1980–2016 is evaluated, and AoA and water vapor mixing ratios in the upper TTL are reconstructed. The interannual variation in the reconstructed water vapor shows good agreement with observations in the tropics. Furthermore, the reconstructed AoA exhibits long‐term variation with a positive trend between 1980 and 1999. To compare the reconstructed AoA with the observed AoA in the midlatitude stratosphere, the transit time spectra are estimated for the 1989–2016 period, and the AoA in the midlatitude stratosphere is estimated by integration of the spectrum together with the AoA in the upper TTL. As a result, although the explicitly calculated transit time shows a clear underestimation compared with previous studies, it is suggested that the “aging” effects are 3.1–4.2 years for transit time in the stratosphere, 0.2–0.7 years for in‐mixing prior to entering the stratosphere, 0.1 years for transit time from the troposphere to upper TTL, and 0.0–1.6 years for subgrid‐scale mixing in the stratosphere.

Effects of Greenhouse Gas Increase and Stratospheric Ozone Depletion on Stratospheric Mean Age of Air in 1960–2010

AbstractThe relative impacts of greenhouse gas (GHG) increase and stratospheric ozone depletion on stratospheric mean age of air in the 1960–2010 period are quantified using the Goddard Earth Observing System Chemistry‐Climate Model. The experiment compares controlled simulations using a coupled atmosphere‐ocean version of the Goddard Earth Observing System Chemistry‐Climate Model, in which either GHGs or ozone depleting substances, or both factors evolve over time. The model results show that GHGs and ozone depleting substances have about equal contributions to the simulated mean age decrease, but GHG increases account for about two thirds of the enhanced strength of the lower stratospheric residual circulation. It is also found that both the acceleration of the diabatic circulation and the decrease of the mean age difference between downwelling and upwelling regions are mainly caused by GHG forcing. The results show that ozone depletion causes an increase in the mean age of air in the Antarctic summer lower stratosphere through two processes: (1) a seasonal delay in the Antarctic polar vortex breakup that inhibits young midlatitude air from mixing with the older air inside the vortex, and (2) enhanced Antarctic downwelling that brings older air from middle and upper stratosphere into the lower stratosphere.

Large‐Scale Atmospheric Transport in GEOS Replay Simulations

AbstractOffline chemical transport models (CTMs) have traditionally been used to perform studies of atmospheric chemistry in a fixed dynamical environment. An alternative to using CTMs is to constrain the flow in a general circulation model using winds from meteorological analyses. The Goddard Earth Observing System (GEOS) “replay” approach involves reading in analyzed fields every 6 h and recomputing the analysis increments, which are applied as a forcing to the meteorology at every model time step. Unlike in CTM, all of the subgrid‐scale processes are recalculated online so that they are consistent with the large‐scale analysis fields, similar in spirit to “nudged” simulations, in which the online meteorology is relaxed to the analysis. Here we compare the transport of idealized tracers in different replay simulations constrained with meteorological fields taken from The Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2). We show that there are substantial differences in their large‐scale stratospheric transport, depending on whether analysis fields or assimilated fields are used. Replay simulations constrained with the instantaneous analysis fields produce stratospheric mean age values that are up to 30% too young relative to observations; by comparison, simulations constrained with the time‐averaged assimilated fields produce more credible stratospheric transport. Our study indicates that care should be taken to correctly configure the model when the replay technique is used to simulate stratospheric composition.

Quantifying the effects of mixing and residual circulation on trends of stratospheric mean age of air

AbstractIt is an outstanding issue to what degree trends in stratospheric mean age of air reflect changes in the (slow) residual circulation and how they are affected by (fast) eddy mixing. We present a method to quantify the effects of mixing and residual circulation on mean age trends, based on simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by ERA‐Interim reanalysis and on the integrated tracer continuity equation. During 1990–2013, mean age decreases throughout most of the stratosphere, qualitatively consistent with results based on climate model simulations. During 2002–2012, age changes show a clear hemispheric asymmetry in agreement with satellite observations. We find that changes in the residual circulation transit time cannot explain the mean age trends, and including the integrated effect of mixing is crucial. Above about 550 K (about 22 km), trends in the mixing effect on mean age appear to be coupled to residual circulation changes.

Variability of stratospheric mean age of air and of the local effects of residual circulation and eddy mixing

AbstractWe analyze the variability of mean age of air (AoA) and of the local effects of the stratospheric residual circulation and eddy mixing on AoA within the framework of the isentropic zonal mean continuity equation. AoA for the period 1988–2013 has been simulated with the Lagrangian chemistry transport model CLaMS driven by ERA‐Interim winds and diabatic heating rates. Model simulated AoA in the lower stratosphere shows good agreement with both in situ observations and satellite observations from Michelson Interferometer for Passive Atmospheric Sounding, even regarding interannual variability and changes during the last decade. The interannual variability throughout the lower stratosphere is largely affected by the quasi‐biennial‐oscillation‐induced circulation and mixing anomalies, with year‐to‐year AoA changes of about 0.5 years. The decadal 2002–2012 change shows decreasing AoA in the lowest stratosphere, below about 450 K. Above, AoA increases in the Northern Hemisphere and decreases in the Southern Hemisphere. Mixing appears to be crucial for understanding AoA variability, with local AoA changes resulting from a close balance between residual circulation and mixing effects. Locally, mixing increases AoA at low latitudes (40°S–40°N) and decreases AoA at higher latitudes. Strongest mixing occurs below about 500 K, consistent with the separation between shallow and deep circulation branches. The effect of mixing integrated along the air parcel path, however, significantly increases AoA globally, except in the polar lower stratosphere. Changes of local effects of residual circulation and mixing during the last decade are supportive of a strengthening shallow circulation branch in the lowest stratosphere and a southward shifting circulation pattern above.

Net influence of an internally generated quasi-biennial oscillation on modelled stratospheric climate and chemistry

Abstract. A Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) simulation with strong tropical non-orographic gravity wave drag (GWD) is compared to an otherwise identical simulation with near-zero tropical non-orographic GWD. The GEOSCCM generates a quasi-biennial oscillation (QBO) zonal wind signal in response to a tropical peak in GWD that resembles the zonal and climatological mean precipitation field. The modelled QBO has a frequency and amplitude that closely resembles observations. As expected, the modelled QBO improves the simulation of tropical zonal winds and enhances tropical and subtropical stratospheric variability. Also, inclusion of the QBO slows the meridional overturning circulation, resulting in a generally older stratospheric mean age of air. Slowing of the overturning circulation, changes in stratospheric temperature and enhanced subtropical mixing all affect the annual mean distributions of ozone, methane and nitrous oxide. Furthermore, the modelled QBO enhances polar stratospheric variability in winter. Because tropical zonal winds are easterly in the simulation without a QBO, there is a relative increase in tropical zonal winds in the simulation with a QBO. Extratropical differences between the simulations with and without a QBO thus reflect the westerly shift in tropical zonal winds: a relative strengthening of the polar stratospheric jet, polar stratospheric cooling and a weak reduction in Arctic lower stratospheric ozone.

Evidence for changes in stratospheric transport and mixing over the past three decades based on multiple data sets and tropical leaky pipe analysis

Variability in the strength of the stratospheric Lagrangian mean meridional or Brewer‐Dobson circulation and horizontal mixing into the tropics over the past three decades are examined using observations of stratospheric mean age of air and ozone. We use a simple representation of the stratosphere, the tropical leaky pipe (TLP) model, guided by mean meridional circulation and horizontal mixing changes in several reanalyses data sets and chemistry climate model (CCM) simulations, to help elucidate reasons for the observed changes in stratospheric mean age and ozone. We find that the TLP model is able to accurately simulate multiyear variability in ozone following recent major volcanic eruptions and the early 2000s sea surface temperature changes, as well as the lasting impact on mean age of relatively short‐term circulation perturbations. We also find that the best quantitative agreement with the observed mean age and ozone trends over the past three decades is found assuming a small strengthening of the mean circulation in the lower stratosphere, a moderate weakening of the mean circulation in the middle and upper stratosphere, and a moderate increase in the horizontal mixing into the tropics. The mean age trends are strongly sensitive to trends in the horizontal mixing into the tropics, and the uncertainty in the mixing trends causes uncertainty in the mean circulation trends. Comparisons of the mean circulation and mixing changes suggested by the measurements with those from a recent suite of CCM runs reveal significant differences that may have important implications on the accurate simulation of future stratospheric climate.

On the influence of anthropogenic forcings on changes in the stratospheric mean age

A common feature of stratospheric simulations of the past or future is an increase in tropical upwelling and a decrease in mean age. Possible causes of these changes include (1) increases in tropical sea surface temperatures (SSTs) driven by increases in well‐mixed greenhouse gases (WMGHGs), (2) the direct radiative effect of increases in WMGHGs, and (3) changes in ozone. Here we examine a suite of simulations from the Goddard Earth Observing System chemistry‐climate model (GEOS CCM) to isolate the relative role of these three factors. Our analysis indicates that all three factors cause changes in the mean age, but the relative impact of each factor depends on the time period analyzed. Over the past 30–40 years ozone depletion is the major factor causing the decrease in mean age, with negligible changes due to direct radiative impact of WMGHGs. However, ozone is predicted to recover back to 1970 levels during the next 50–60 years, and this causes an increase in the mean age, whereas the continued increase in SSTs from increased levels of WMGHGs and the direct radiative impact of WMGHGs will still cause a decrease in the mean age. The net impact of these factors will still result in a decreasing mean age although the rate will be smaller than that of the past. The decreases in mean age are primarily caused by increases in upwelling in the tropical lower stratosphere. The increased upwelling from both increased tropical SSTs and polar ozone loss appears to be related to changes in zonal winds and increases in wave activity propagating into the stratosphere. The different contributions of changes in SSTs, WMGHGs, and ozone to the circulation of the stratosphere may help explain the large spread in the rate of change of tropical upwelling seen in previous studies.

Middle atmosphere age of air in a globally balanced two‐dimensional model

We extend the concepts of the mean age derived from the age spectrum and the age of air derived from the time lag in the middle atmosphere by specifying the tracer source gases uniformly in the lower troposphere. The concepts are illustrated by use of both a time‐independent one‐dimensional diffusive model and a globally balanced two‐dimensional middle atmosphere model. We quantitatively examine several factors that may cause the difference between the mean age and the age of air. It is found that the mean age and the age of air can differ by any amount in general for a time‐dependent transport operator. For a time‐independent transport operator the age of air derived by integrating a model for a finite time is less than the mean age. The age of air at the stratopause is about 5 to 6 years in our globally balanced two‐dimensional middle atmosphere model. In the mesosphere the age of air changes with season significantly due to the reversal of the mesospheric meridional circulation between the summer and the winter. With a chemical lifetime of SF6 above 30 km being ∼38 years our two‐dimensional model shows no signs of overestimate of age of air for SF6 at 30 km. It is also demonstrated that stratospheric mean age is sensitive to the horizontal mixing in the troposphere. Weak horizontal mixing in the troposphere will reduce the mean age in the upper stratosphere but increase the mean age in the high‐latitude lower stratosphere. The former is caused by diminishing the recycling of air parcels between the troposphere and stratosphere, and the latter is caused by minimizing the portion of tracers directly transported upward from the high‐latitude troposphere into the stratosphere.

Sensitivity of mean age and long‐lived tracers to transport parameters in a two‐dimensional model

Recent comparisons of modeled and observed stratospheric mean age show large differences between model predictions and data as well as between individual models, indicating large variations (and deficiencies) in model transport. We explore here the sensitivity of the mean age to different components of the transport by examining the effect of varying the transport parameters within a two‐dimensional model. The mean age is shown to be sensitive to changes in advective circulation and diffusion coefficients, with the sensitivity being largest for changes in the circulation strength. In most cases the magnitudes, but not the orientation, of the mean age isopleths change. However, if the horizontal mixing is made very small within middle latitudes or large within low latitudes, large changes occur in the orientation of mean age isopleths. The effects of these transport changes on chemically active long‐lived tracers are also examined. It is found that the lower stratospheric concentrations are relatively insensitive to the transport changes if these changes do not alter the general shape of mean age isopleths. However, significant changes occur when the transport parameters are modified so as to change the orientation of the mean age (and long‐lived tracer) isopleths.