scholarly journals Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

2010 ◽  
Vol 10 (7) ◽  
pp. 16837-16860 ◽  
Author(s):  
T. Birner ◽  
H. Bönisch
Keyword(s):  
Time Scale ◽  
Climate Model ◽  
Global Scale ◽  
Reanalysis Data ◽  
Residual Circulation ◽  
Deep Circulation ◽  
Deep Branch ◽  
Tropical Tropopause ◽  
Transit Times ◽  
Part Time

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 global-scale stratospheric residual circulation and a fast part (time-scale of days to a few months) associated with (mostly quasi-horizontal) mixing (i.e. two-way irreversible transport, including stratosphere-troposphere exchange). The stratospheric residual circulation can 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 more strongly associated with synoptic-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 clear time-scale separation with much smaller transit times into the mid-latitudinal LMS than into polar LMS is found that is indicative of a clear 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 clear distinction in the aspect ratio of the vertical to meridional extent of the trajectories as well as the integrated mass flux along the residual circulation 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.

scholarly journals Residual circulation trajectories and transit times into the extratropical lowermost stratosphere

2011 ◽  
Vol 11 (2) ◽  
pp. 817-827 ◽  
Author(s):  
T. Birner ◽  
H. Bönisch
Keyword(s):  
Time Scale ◽  
Climate Model ◽  
Global Scale ◽  
Reanalysis Data ◽  
Residual Circulation ◽  
Deep Circulation ◽  
Deep Branch ◽  
Tropical Tropopause ◽  
Transit Times ◽  
Part Time

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 global-scale stratospheric residual circulation and a fast part (time-scale 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 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.

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

2019 ◽  
Author(s):  
Felix Ploeger ◽  
Bernard Legras ◽  
Edward Charlesworth ◽  
Xiaolu Yan ◽  
Mohamadou Diallo ◽  
...  
Keyword(s):  
Climate Model ◽  
Decadal Variability ◽  
Reanalysis Data ◽  
Forced Response ◽  
Lagrangian Model ◽  
Current Generation ◽  
Transit Times ◽  
Age Variations ◽  
Dipole Pattern ◽  
Climate Model Simulations

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 analyze stratospheric mean age of air and the full age spectrum as measures for the BDC and its trend. Age of air is calculated with 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 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 age for MERRA-2 is related to a stronger spectrum tail, likely related to 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 over 1989–2015 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 age of air changes are less robust. Only ERA-Interim shows the hemispheric dipole pattern in age changes during 2002–2015 as viewed by recent satellite observations. Consequently, the representation of decadal variability of the BDC in current generation reanalyses appears less robust and a major uncertainty of modelling the BDC.

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

2019 ◽  
Vol 19 (9) ◽  
pp. 6085-6105 ◽  
Author(s):  
Felix Ploeger ◽  
Bernard Legras ◽  
Edward Charlesworth ◽  
Xiaolu Yan ◽  
Mohamadou Diallo ◽  
...  
Keyword(s):  
Climate Model ◽  
Decadal Variability ◽  
Reanalysis Data ◽  
Forced Response ◽  
Lagrangian Model ◽  
Current Generation ◽  
Transit Times ◽  
Age Variations ◽  
Dipole Pattern ◽  
Climate Model Simulations

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.

scholarly journals Residual Circulation and Tropopause Structure

2010 ◽  
Vol 67 (8) ◽  
pp. 2582-2600 ◽  
Author(s):  
Thomas Birner
Keyword(s):  
Large Scale ◽  
Climate Model ◽  
Inversion Layer ◽  
Vertical Gradient ◽  
Static Stability ◽  
Reanalysis Data ◽  
Tropopause Height ◽  
Residual Circulation ◽  
Stratospheric Dynamics ◽  
Extratropical Tropopause

Abstract The effect of large-scale dynamics as represented by the residual mean meridional circulation in the transformed Eulerian sense, in particular its stratospheric part, on lower stratospheric static stability and tropopause structure is studied using a comprehensive chemistry–climate model (CCM), reanalysis data, and simple idealized modeling. Dynamical forcing of static stability as associated with the vertical structure of the residual circulation results in a dominant dipole forcing structure with negative static stability forcing just below the tropopause and positive static stability forcing just above the tropopause. This dipole forcing structure effectively sharpens the tropopause, especially during winter. Furthermore, the strong positive lowermost stratospheric static stability forcing causes a layer of strongly enhanced static stability just above the extratropical tropopause—a tropopause inversion layer (TIL)—especially in the winter midlatitudes. The strong positive static stability forcing is shown to be mainly due to the strong vertical gradient of the vertical residual velocity found just above the tropopause in the winter midlatitudes. Stratospheric radiative equilibrium (SRE) solutions are obtained using offline radiative transfer calculations for a given tropospheric climate as simulated by the CCM. The resulting tropopause height in SRE is reduced by several kilometers in the tropics but is increased by 1–2 km in the extratropics, strongly reducing the equator-to-pole contrast in tropopause height. Moreover, the TIL in winter midlatitudes disappears in the SRE solution in contrast to the polar summer TIL, which stays intact. When the SRE solution is modified to include the effect of stratospheric dynamics as represented by the stratospheric residual circulation, the TIL in winter midlatitudes is recovered, suggesting that the static stability forcing associated with the stratospheric residual circulation represents the main cause for the TIL in the winter midlatitudes whereas radiation seems dominant in causing the polar summer TIL.

The Brewer-Dobson circulation in CMIP6

2021 ◽  
Author(s):  
Natalia Calvo ◽  
Marta Abalos ◽  
Samuel Benito-Barca ◽  
Hella Garny ◽  
Steven Hardiman ◽  
...  
Keyword(s):  
Climate Variability ◽  
Surface Temperature ◽  
Climate Model ◽  
Model Intercomparison ◽  
Residual Circulation ◽  
Careful Evaluation ◽  
Deep Branch ◽  
Surface Climate ◽  
Intercomparison Project ◽  
First Time

<div> <div> <div> <p>The Brewer-Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to improve the representation of surface climate variability. For the first time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in BDC trends between observations and models in the middle and upper stratosphere. The increasing CO2 simulations feature a robust acceleration of the BDC but also reveal large uncertainties in the deep branch trends. The very close connection between the shallow branch and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation.</p> </div> </div> </div>

scholarly journals On the structural changes in the Brewer-Dobson circulation after 2000

2011 ◽  
Vol 11 (8) ◽  
pp. 3937-3948 ◽  
Author(s):  
H. Bönisch ◽  
A. Engel ◽  
Th. Birner ◽  
P. Hoor ◽  
D. W. Tarasick ◽  
...  
Keyword(s):  
Structural Changes ◽  
Circulation Pattern ◽  
Tracer Transport ◽  
Residual Circulation ◽  
Tropical Tropopause ◽  
Transit Times ◽  
Ozone Profile ◽  
The Tropics ◽  
Year 2000

Abstract. In this paper we present evidence that the observed increase in tropical upwelling after the year 2000 may be attributed to a change in the Brewer-Dobson circulation pattern. For this purpose, we use the concept of transit times derived from residual circulation trajectories and different in-situ measurements of ozone and nitrous dioxide. Observations from the Canadian midlatitude ozone profile record, probability density functions of in-situ N2O observations and a shift of the N2O-O3 correlation slopes, taken together, indicate that the increased upwelling in the tropics after the year 2000 appears to have triggered an intensification of tracer transport from the tropics into the extratropics in the lower stratosphere below about 500 K. This finding is corroborated by the fact that transit times along the shallow branch of the residual circulation into the LMS have decreased for the same time period (1993–2003). On a longer time scale (1979–2009), the transit time of the shallow residual circulation branch show a steady decrease of about −1 month/decade over the last 30 yr, while the transit times of the deep branch remain unchanged. This highlights that changes in the upwelling across the tropical tropopause are not sufficient as an indicator for changes in the entire Brewer-Dobson circulation.

scholarly journals Extratropical age of air trends and causative factors in climate projection simulations

2019 ◽  
Vol 19 (11) ◽  
pp. 7627-7647 ◽  
Author(s):  
Petr Šácha ◽  
Roland Eichinger ◽  
Hella Garny ◽  
Petr Pišoft ◽  
Simone Dietmüller ◽  
...  
Keyword(s):  
Climate Change ◽  
Climate Model ◽  
Climate Projection ◽  
Residual Circulation ◽  
Wave Forcing ◽  
Open Questions ◽  
Transit Times ◽  
Causative Factors ◽  
Climate Model Simulations ◽  
Changes Over Time

Abstract. Climate model simulations show an acceleration of the Brewer–Dobson circulation (BDC) in response to climate change. While the general mechanisms for the BDC strengthening are widely understood, there are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for assessing changes in the BDC. Analyzing AoA from a subset of Chemistry–Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20 and 25 geopotential kilometers (gpkm) and 20–50∘ N and S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution, which is sensitive to an upward shift of the circulation in response to climate change. Also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore, we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers focusing on local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time.

scholarly journals Tropical convective transport and the Walker circulation

2012 ◽  
Vol 12 (20) ◽  
pp. 9791-9797 ◽  
Author(s):  
J. S. Hosking ◽  
M. R. Russo ◽  
P. Braesicke ◽  
J. A. Pyle
Keyword(s):  
Climate Model ◽  
Deep Convection ◽  
Walker Circulation ◽  
Reanalysis Data ◽  
Convective Transport ◽  
Boreal Winter ◽  
Transport Pathways ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
The Walker Circulation

Abstract. We introduce a methodology to visualise rapid vertical and zonal tropical transport pathways. Using prescribed sea-surface temperatures in four monthly model integrations for 2005, we characterise preferred transport routes from the troposphere to the stratosphere in a high resolution climate model. Most efficient transport is modelled over the Maritime Continent (MC) in November and February, i.e., boreal winter. In these months, the ascending branch of the Walker Circulation over the MC is formed in conjunction with strong deep convection, allowing fast transport into the stratosphere. In the model the upper tropospheric zonal winds associated with the Walker Circulation are also greatest in these months in agreement with ERA-Interim reanalysis data. We conclude that the Walker circulation plays an important role in the seasonality of fast tropical transport from the lower and middle troposphere to the upper troposphere and so impacts at the same time the potential supply of surface emissions to the tropical tropopause layer (TTL) and subsequently to the stratosphere.

scholarly journals Extratropical Age of Air trends and causative factors in climate projection simulations

2019 ◽  
Author(s):  
Petr Šácha ◽  
Roland Eichinger ◽  
Hella Garny ◽  
Petr Pišoft ◽  
Simone Dietmüller ◽  
...  
Keyword(s):  
Climate Change ◽  
Climate Model ◽  
Climate Projection ◽  
Residual Circulation ◽  
Wave Forcing ◽  
Open Questions ◽  
Transit Times ◽  
Causative Factors ◽  
Climate Model Simulations ◽  
Changes Over Time

Abstract. Climate model simulations show a Brewer-Dobson circulation (BDC) acceleration in the course of climate change. While the mechanisms for the BDC strengthening are well understood, there are still open questions concerning its dynamical driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for accessing changes of the BDC. Analysing AoA from a subset of Chemistry Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20 gpkm and 25 gpkm and 20°–50°N/S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution being sensitive to an upward shift of the circulation in response to a climate change. But also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers like local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. Namely, we postulate a possible influence of stratospheric shrinkage on RCTT, AbM and therefore also on AoA trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time.

scholarly journals The role of the winter residual circulation in the summer mesopause regions in WACCM

2018 ◽  
Vol 18 (6) ◽  
pp. 4217-4228 ◽  
Author(s):  
Maartje Sanne Kuilman ◽  
Bodil Karlsson
Keyword(s):  
Planetary Wave ◽  
Climate Model ◽  
Zonal Flow ◽  
Global Scale ◽  
Wave Activity ◽  
Latitudinal Gradients ◽  
Residual Circulation ◽  
Mean Flow ◽  
Vertically Propagating

Abstract. High winter planetary wave activity warms the summer polar mesopause via a link between the two hemispheres. Complex wave–mean-flow interactions take place on a global scale, involving sharpening and weakening of the summer zonal flow. Changes in the wind shear occasionally generate flow instabilities. Additionally, an altering zonal wind modifies the breaking of vertically propagating gravity waves. A crucial component for changes in the summer zonal flow is the equatorial temperature, as it modifies latitudinal gradients. Since several mechanisms drive variability in the summer zonal flow, it can be hard to distinguish which one is dominant. In the mechanism coined interhemispheric coupling, the mesospheric zonal flow is suggested to be a key player for how the summer polar mesosphere responds to planetary wave activity in the winter hemisphere. We here use the Whole Atmosphere Community Climate Model (WACCM) to investigate the role of the summer stratosphere in shaping the conditions of the summer polar mesosphere. Using composite analyses, we show that in the absence of an anomalous summer mesospheric temperature gradient between the equator and the polar region, weak planetary wave forcing in the winter would lead to a warming of the summer mesosphere region instead of a cooling, and vice versa. This is opposing the temperature signal of the interhemispheric coupling that takes place in the mesosphere, in which a cold and calm winter stratosphere goes together with a cold summer mesopause. We hereby strengthen the evidence that the variability in the summer mesopause region is mainly driven by changes in the summer mesosphere rather than in the summer stratosphere.

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