Transport in the Middle Atmosphere

Shepherd, Theodore G.

doi:10.2151/jmsj.85b.165

Cited by 169 publications

(141 citation statements)

References 120 publications

(127 reference statements)

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“…However, Eyring et al (2006) compared transport properties between different models and they came to the result that there is little difference in key transport diagnostics between models with spectral and flux-form advection schemes (Shepherd, 2007). Moreover, the choice of the vertical coordinate (pressure or potential temperature) may influence the AoA pattern (e.g., Mahowald et al, 2002;Hoppe et al, 2016).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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Abstract. Mean age of air (AoA) is a widely used metric to describe the transport along the Brewer-Dobson circulation. We seek to untangle the effects of different processes on the simulation of AoA, using the chemistry-climate model EMAC (ECHAM/MESSy Atmospheric Chemistry) and the Chemical Lagrangian Model of the Stratosphere (CLaMS). Here, the effects of residual transport and two-way mixing on AoA are calculated. To do so, we calculate the residual circulation transit time (RCTT). The difference of AoA and RCTT is defined as aging by mixing. However, as diffusion is also included in this difference, we further use a method to directly calculate aging by mixing on resolved scales. Comparing these two methods of calculating aging by mixing allows for separating the effect of unresolved aging by mixing (which we term "aging by diffusion" in the following) in EMAC and CLaMS. We find that diffusion impacts AoA by making air older, but its contribution plays a minor role (order of 10 %) in all simulations. However, due to the different advection schemes of the two models, aging by diffusion has a larger effect on AoA and mixing efficiency in EMAC, compared to CLaMS. Regarding the trends in AoA, in CLaMS the AoA trend is negative throughout the stratosphere except in the Northern Hemisphere middle stratosphere, consistent with observations. This slight positive trend is neither reproduced in a free-running nor in a nudged simulation with EMAC -in both simulations the AoA trend is negative throughout the stratosphere. Trends in AoA are mainly driven by the contributions of RCTT and aging by mixing, whereas the contribution of aging by diffusion plays a minor role.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2017

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“…Together, both mechanisms constitute the total or Brewer-Dobson circulation (BDC), generally having an upward component at low latitudes and a return, downward component at the extratropical latitudes of both the Northern Hemisphere and the Southern Hemisphere (e.g., Butchart, 2014;Plumb, 2007;Haynes et al, 1991). The dissipation of planetary (or Rossby) waves and gravity wave forcings tends to accelerate the BDC and to enhance mixing in the winter hemisphere (e.g., Solomon et al, 1986;Plumb, 2007;Shepherd, 2007;Okamoto et al, 2011). Thus, it is likely that there is some asymmetry for the BDC in the Northern Hemisphere versus the Southern Hemisphere stratosphere because winter wave forcing and mixing processes are more pronounced and frequent in the Northern Hemisphere (Shepherd, 2007).…”

Section: Introductionmentioning

confidence: 99%

“…The dissipation of planetary (or Rossby) waves and gravity wave forcings tends to accelerate the BDC and to enhance mixing in the winter hemisphere (e.g., Solomon et al, 1986;Plumb, 2007;Shepherd, 2007;Okamoto et al, 2011). Thus, it is likely that there is some asymmetry for the BDC in the Northern Hemisphere versus the Southern Hemisphere stratosphere because winter wave forcing and mixing processes are more pronounced and frequent in the Northern Hemisphere (Shepherd, 2007).…”

Section: Introductionmentioning

confidence: 99%

Methane as a diagnostic tracer of changes in the Brewer–Dobson circulation of the stratosphere

Remsberg

2015

Atmos. Chem. Phys.

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Abstract. This study makes use of time series of methane (CH 4 ) data from the Halogen Occultation Experiment (HALOE) to detect whether there were any statistically significant changes of the Brewer-Dobson circulation (BDC) within the stratosphere during 1992-2005. The HALOE CH 4 profiles are in terms of mixing ratio versus pressure altitude and are binned into latitude zones within the Southern Hemisphere and the Northern Hemisphere. Their separate time series are then analyzed using multiple linear regression (MLR) techniques. The CH 4 trend terms for the Northern Hemisphere are significant and positive at 10 • N from 50 to 7 hPa and larger than the tropospheric CH 4 trends of about 3 % decade −1 from 20 to 7 hPa. At 60 • N the trends are clearly negative from 20 to 7 hPa. Their combined trends indicate an acceleration of the BDC in the middle stratosphere of the Northern Hemisphere during those years, most likely due to changes from the effects of wave activity. No similar significant BDC acceleration is found for the Southern Hemisphere. Trends from HALOE H 2 O are analyzed for consistency. Their mutual trends with CH 4 are anti-correlated qualitatively in the middle and upper stratosphere, where CH 4 is chemically oxidized to H 2 O. Conversely, their mutual trends in the lower stratosphere are dominated by their trends upon entry to the tropical stratosphere. Time series residuals for CH 4 in the lower mesosphere also exhibit structures that are anti-correlated in some instances with those of the tracer-like species HCl. Their occasional aperiodic structures indicate the effects of transport following episodic, wintertime wave activity. It is concluded that observed multi-year, zonally averaged distributions of CH 4 can be used to diagnose major instances of wave-induced transport in the middle atmosphere and to detect changes in the stratospheric BDC.

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“…Overall this circulation is described by tropical upwelling, poleward flow, and extratropical downwelling (e.g. Shepherd, 2007). The bulk of this circulation is driven by breaking extratropical planetary waves -often referred to as the extratropical pump (Holton et al, 1995).…”

Section: Introductionmentioning

confidence: 99%

Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

2011

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Abstract. Transport into the extratropical lowermost stratosphere (LMS) can be divided into a slow part (time-scale of several months to years) associated with the globalscale stratospheric residual circulation and a fast part (timescale of days to a few months) associated with (mostly quasi-horizontal) mixing (i.e. two-way irreversible transport, including extratropical stratosphere-troposphere exchange). The stratospheric residual circulation may be considered to consist of two branches: a deep branch more strongly associated with planetary waves breaking in the middle to upper stratosphere, and a shallow branch associated with synoptic and planetary scale waves breaking in the subtropical lower stratosphere. In this study the contribution due to the stratospheric residual circulation alone to transport into the LMS is quantified using residual circulation trajectories, i.e. trajectories driven by the (time-dependent) residual mean meridional and vertical velocities. This contribution represents the advective part of the overall transport into the LMS and can be viewed as providing a background onto which the effect of mixing has to be added. Residual mean velocities are obtained from a comprehensive chemistry-climate model as well as from reanalysis data. Transit times of air traveling from the tropical tropopause to the LMS along the residual circulation streamfunction are evaluated and compared to recent mean age of air estimates. A time-scale separation with much smaller transit times into the mid-latitudinal LMS than into polar LMS is found that is indicative of a separation of the shallow from the deep branch of the residual circulation. This separation between the shallow and the deep circulation branch is further manifested in a distinction in the aspect ratio Correspondence to: T. Birner (thomas@atmos.colostate.edu) of the vertical to meridional extent of the trajectories, the integrated mass flux along the residual circulation trajectories, as well as the stratospheric entry latitude of the trajectories. The residual transit time distribution reproduces qualitatively the observed seasonal cycle of youngest air in the extratropical LMS in fall and oldest air in spring.

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Transport in the Middle Atmosphere

Cited by 169 publications

References 120 publications

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

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

Methane as a diagnostic tracer of changes in the Brewer–Dobson circulation of the stratosphere

Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

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