The effects of mixing on age of air Garny, H.; Birner, T.; Bönisch, H. ...
Journal of geophysical research. Atmospheres,
27 June 2014, Volume:
119, Issue:
12
Journal Article
Peer reviewed
Open access
Mean age of air (AoA) measures the mean transit time of air parcels along the Brewer‐Dobson circulation (BDC) starting from their entry into the stratosphere. AoA is determined both by transport ...along the residual circulation and by two‐way mass exchange (mixing). The relative roles of residual circulation transport and two‐way mixing for AoA, and for projected AoA changes are not well understood. Here effects of mixing on AoA are quantified by contrasting AoA with the transit time of hypothetical transport solely by the residual circulation. Based on climate model simulations, we find additional aging by mixing throughout most of the lower stratosphere, except in the extratropical lowermost stratosphere where mixing reduces AoA. We use a simple Lagrangian model to reconstruct the distribution of AoA in the GCM and to illustrate the effects of mixing at different locations in the stratosphere. Predicted future reduction in AoA associated with an intensified BDC is equally due to faster transport along the residual circulation as well as reduced aging by mixing. A tropical leaky pipe model is used to derive a mixing efficiency, measured by the ratio of the two‐way mixing mass flux and the net (residual) mass flux across the subtropical boundary. The mixing efficiency remains close to constant in a future climate, suggesting that the strength of two‐way mixing is tightly coupled to the strength of the residual circulation in the lower stratosphere. This implies that mixing generally amplifies changes in AoA due to uniform changes in the residual circulation.
Key Points
A method is introduced to quantify the effects of mixing on age of air
Mixing mostly leads to additional aging of air, due to recirculation
The mixing strength is tightly coupled to the residual circulation strength
This study presents a classification of stratospheric extreme events during northern winter into events with or without a consistent downward propagation of anomalies to the troposphere. Anomalous ...strong and weak stratospheric polar vortex events are detected from daily time series of the polar cap averaged (60°–90°N) geopotential height anomaly. The method is applied to chemistry‐climate model data (E39CA and WACCM3.5) and reanalyses data (ERA40). The analyses show that in about 80% of all events no significant tropospheric response can be detected. The stratospheric perturbation of both weak and strong events with a significant tropospheric response persists significantly longer throughout the stratosphere compared to the events without a tropospheric response. The strength of the stratospheric perturbation determines the strength of the tropospheric response only to a small degree. Results are consistent across all three data sets.
Key Points
A new classification of stratospheric extreme events according to the significance of their impact on the troposphere is presented
Only few (20%) of all stratospheric extreme events have a significant impact on the troposphere
The impact on the troposphere does not depend on the strength but on the persistence of the stratospheric perturbation
The stratospheric circulation is an important element of climate as it determines the concentration of radiatively active species like water vapor and aerosol above the tropopause. Climate models ...predict that increasing greenhouse gas levels speed up the stratospheric circulation. However, these results have been challenged by observational estimates of the circulation strength, constituting an uncertainty in current climate simulations. Here, we quantify the effect of volcanic aerosol on the stratospheric circulation focusing on the Mount Pinatubo eruption and discussing further the minor extratropical volcanic eruptions after 2008. We show that the observed pattern of decadal circulation change over the past decades is substantially driven by volcanic aerosol injections. Thus, climate model simulations need to realistically take into account the effect of volcanic eruptions, including the minor eruptions after 2008, for a reliable reproduction of observed stratospheric circulation changes.
Plain Language Summary
The upper atmospheric circulation is an important element in the climate system as it determines the distributions and lifetimes of key greenhouse gases and impacts the Earth's radiation budget and surface climate. Current climate models rather uniformly predict that increasing greenhouse gas levels speed up the upper atmospheric circulation. However, these results contrast with observations, constituting a major uncertainty in current climate simulations. Our paper shows that the observed pattern of decadal circulation change over the past few decades is substantially driven by volcanic aerosol injections. The recently observed slowdown of the stratospheric circulation in the Northern Hemisphere is by 50% attributable to stratospheric aerosol from minor volcanic eruptions after 2008, which should no longer be neglected in climate simulations.
Key Points
We demonstrate that the volcanic aerosol effect is the primary signal dominating the pattern of decadal variability in the BD circulation
Main difference between observations and climate models concerns changes in the depth and strength of the aerosol effect on BD circulation
We demonstrate that Northern Hemisphere slowdown is partly driven by minor volcanic eruptions after 2008
The impact of stratospheric ozone on the tropospheric general circulation of the Southern Hemisphere (SH) is examined with a set of chemistry‐climate models participating in the Stratospheric ...Processes and their Role in Climate (SPARC)/Chemistry‐Climate Model Validation project phase 2 (CCMVal‐2). Model integrations of both the past and future climates reveal the crucial role of stratospheric ozone in driving SH circulation change: stronger ozone depletion in late spring generally leads to greater poleward displacement and intensification of the tropospheric midlatitude jet, and greater expansion of the SH Hadley cell in the summer. These circulation changes are systematic as poleward displacement of the jet is typically accompanied by intensification of the jet and expansion of the Hadley cell. Overall results are compared with coupled models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), and possible mechanisms are discussed. While the tropospheric circulation response appears quasi‐linearly related to stratospheric ozone changes, the quantitative response to a given forcing varies considerably from one model to another. This scatter partly results from differences in model climatology. It is shown that poleward intensification of the westerly jet is generally stronger in models whose climatological jet is biased toward lower latitudes. This result is discussed in the context of quasi‐geostrophic zonal mean dynamics.
The Asian summer monsoon is associated with strong upward transport of tropospheric source gases and isolation of air within the upper tropospheric anticyclone, with a high degree of dynamical ...variability. Here we study the anticyclone in terms of potential vorticity (PV) as derived from reanalysis data. The strength of the anticyclone, as measured by low PV area, varies on subseasonal time scales (periods of 30–40 days), driven by variability in convection. The convective forcing of low PV areas is associated with heating in the middle troposphere and divergent motion in the upper troposphere, and we find that upper level divergence is a good predictor of the anticyclone strength. Low PV air is often observed to propagate from the forcing region to the west, and occasionally to the east. Carbon monoxide (CO) measured by the Aura Microwave Limb Sounder is used to study the covariability of chemical tracers with the anticyclone strength and location. Concentrations of CO maximize within the upper tropospheric anticyclone, and enhanced CO is well correlated with the spatial distribution of low PV. Time variations of CO concentrations in the upper troposphere (around 360 K) are not strongly correlated with anticyclone strength, probably because CO transport also involves coupling with surface CO sources (unlike PV). Temporal correlations with PV are stronger for CO at higher levels (380–400 K), suggesting that advective upward transport is important for tracer evolution at these levels.
Key Points
Variability of the Asian Monsoon Anticyclone is investigated with PV diagnostics
Divergence is found to be a good predictor of anticyclone strength
Carbon monoxide concentrations and the anticyclone are spatially well correlated
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