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

2010 ◽  
Vol 10 (11) ◽  
pp. 28399-28430 ◽  
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 years, while the transit times of the deep branch remain unchanged. This highlights the fact that a change in the upwelling across the tropical tropopause is not a direct indicator for changes of the whole Brewer-Dobson circulation.

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 Tracer measurements in the tropical tropopause layer during the AMMA/SCOUT-O3 aircraft campaign

2010 ◽  
Vol 10 (8) ◽  
pp. 3615-3627 ◽  
Author(s):  
C. D. Homan ◽  
C. M. Volk ◽  
A. C. Kuhn ◽  
A. Werner ◽  
J. Baehr ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Transport Processes ◽  
Gradual Transition ◽  
Tropical Tropopause Layer ◽  
Mixing Ratios ◽  
Tropical Tropopause ◽  
Multidisciplinary Analysis ◽  
The Tropics

Abstract. We present airborne in situ measurements made during the AMMA (African Monsoon Multidisciplinary Analysis)/SCOUT-O3 campaign between 31 July and 17 August 2006 on board the M55 Geophysica aircraft, based in Ouagadougou, Burkina Faso. CO2 and N2O were measured with the High Altitude Gas Analyzer (HAGAR), CO was measured with the Cryogenically Operated Laser Diode (COLD) instrument, and O3 with the Fast Ozone ANalyzer (FOZAN). We analyse the data obtained during five local flights to study the dominant transport processes controlling the tropical tropopause layer (TTL, here ~350–375 K) and lower stratosphere above West-Africa: deep convection up to the level of main convective outflow, overshooting of deep convection, and horizontal inmixing across the subtropical tropopause. Besides, we examine the morphology of the stratospheric subtropical barrier. Except for the flight of 13 August, distinct minima in CO2 mixing ratios indicate convective outflow of boundary layer air in the TTL. The CO2 profiles show that the level of main convective outflow was mostly located at potential temperatures between 350 and 360 K, and for 11 August reached up to 370 K. While the CO2 minima indicate quite significant convective influence, the O3 profiles suggest that the observed convective signatures were mostly not fresh, but of older origin (several days or more). When compared with the mean O3 profile measured during a previous campaign over Darwin in November 2005, the O3 minimum at the main convective outflow level was less pronounced over Ouagadougou. Furthermore O3 mixing ratios were much higher throughout the whole TTL and, unlike over Darwin, rarely showed low values observed in the regional boundary layer. Signatures of irreversible mixing following overshooting of convective air were scarce in the tracer data. Some small signatures indicative of this process were found in CO2 profiles between 390 and 410 K during the flights of 4 and 8 August, and in CO data at 410 K on 7 August. However, the absence of expected corresponding signatures in other tracer data makes this evidence inconclusive, and overall there is little indication from the observations that overshooting convection has a profound impact on gas-phase tracer TTL composition during AMMA. We find the amount of photochemically aged air isentropically mixed into the TTL across the subtropical tropopause to be not significant. Using the N2O observations we estimate the fraction of aged extratropical stratospheric air in the TTL to be 0.0±0.1 up to 370 K during the local flights. Above the TTL this fraction increases to 0.3±0.1 at 390 K. The subtropical barrier, as indicated by the slope of the correlation between N2O and O3 between 415 and 490 K, does not appear as a sharp border between the tropics and extratropics, but rather as a gradual transition region between 10° N and 25° N where isentropic mixing between these two regions may occur.

scholarly journals Quantifying transport into the lowermost stratosphere using simultaneous in-situ measurements of SF<sub>6</sub> and CO<sub>2</sub>

2009 ◽  
Vol 9 (16) ◽  
pp. 5905-5919 ◽  
Author(s):  
H. Bönisch ◽  
A. Engel ◽  
J. Curtius ◽  
Th. Birner ◽  
P. Hoor
Keyword(s):  
Mass Balance ◽  
Extreme Values ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Transit Times ◽  
Residual Transport ◽  
Tropopause Region ◽  
Passive Tracers ◽  
Extratropical Tropopause

Abstract. The seasonality of transport and mixing of air into the lowermost stratosphere (LMS) is studied using distributions of mean age of air and a mass balance approach, based on in-situ observations of SF6 and CO2 during the SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) aircraft campaigns. Combining the information of the mean age of air and the water vapour distributions we demonstrate that the tropospheric air transported into the LMS above the extratropical tropopause layer (ExTL) originates predominantly from the tropical tropopause layer (TTL). The concept of our mass balance is based on simultaneous measurements of the two passive tracers and the assumption that transport into the LMS can be described by age spectra which are superposition of two different modes. Based on this concept we conclude that the stratospheric influence on LMS composition is strongest in April with extreme values of the tropospheric fractions (α1) below 20% and that the strongest tropospheric signatures are found in October with α1 greater than 80%. Beyond the fractions, our mass balance concept allows us to calculate the associated transit times for transport of tropospheric air from the tropics into the LMS. The shortest transit times (<0.3 years) are derived for the summer, continuously increasing up to 0.8 years by the end of spring. These findings suggest that strong quasi-horizontal mixing across the weak subtropical jet from summer to mid of autumn and the considerably shorter residual transport time-scales within the lower branch of the Brewer-Dobson circulation in summer than in winter dominates the tropospheric influence in the LMS until the beginning of next year's summer.

scholarly journals Quantifying transport into the lowermost stratosphere using simultaneous in-situ measurements of SF<sub>6</sub> and CO<sub>2</sub>

2008 ◽  
Vol 8 (6) ◽  
pp. 21229-21264 ◽  
Author(s):  
H. Bönisch ◽  
A. Engel ◽  
J. Curtius ◽  
T. Birner ◽  
P. Hoor
Keyword(s):  
Mass Balance ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Transit Times ◽  
Residual Transport ◽  
Tropopause Region ◽  
Passive Tracers ◽  
Transport And Mixing ◽  
Extratropical Tropopause

Abstract. The seasonality of transport and mixing of air into the lowermost stratosphere (LMS) is studied using distributions of mean age of air and a~mass balance approach, based on in-situ observations of SF6 and CO2 during the SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) aircraft campaigns. Combining the information of the mean age of air and the water vapour distributions we demonstrate that the tropospheric air transported into the LMS above the extratropical tropopause layer (ExTL) originates predominantly from the tropical tropopause layer (TTL). The concept of our mass balance is based on simultaneous measurements of the two passive tracers and the assumption that transport into the LMS can be described by age spectra which are superposition of two different modes. Based on this concept we conclude that the stratospheric influence on LMS composition is strongest in April with tropospheric fractions (α1) below 20% and that the strongest tropospheric signatures are found in October with (α1 greater than 80%. Beyond the fractions, our mass balance concept allows to calculate the associated transit times for transport of tropospheric air from the tropics into the LMS. The shortest transit times (<0.3 years) are derived for the summer, continuously increasing up to 0.8 years by the end of spring. These findings suggest that strong quasi-horizontal mixing across the weak subtropical jet from summer to mid of autumn and the considerably shorter residual transport time-scales within the lower branch of the Brewer-Dobson circulation in summer than in winter dominates the tropospheric influence in the LMS until the beginning of next year's summer.

scholarly journals Tracer measurements in the tropical tropopause layer during the AMMA/SCOUT-O3 aircraft campaign

2009 ◽  
Vol 9 (6) ◽  
pp. 25049-25084 ◽  
Author(s):  
C. D. Homan ◽  
C. M. Volk ◽  
A. C. Kuhn ◽  
A. Werner ◽  
J. Baehr ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Transport Processes ◽  
Gradual Transition ◽  
Tropical Tropopause Layer ◽  
Mixing Ratios ◽  
Tropical Tropopause ◽  
Multidisciplinary Analysis ◽  
The Tropics

Abstract. We present airborne in situ measurements made during the AMMA (African Monsoon Multidisciplinary Analysis)/SCOUT-O3 campaign between 31 July and 17 August 2006 on board the M55 Geophysica aircraft, based in Ouagadougou, Burkina Faso. CO2 and N2O were measured with the High Altitude Gas Analyzer (HAGAR), CO was measured with the Cryogenically Operated Laser Diode (COLD) instrument, and O3 with the Fast Ozone ANalyzer (FOZAN). We analyze the data obtained during five local flights to study the dominant transport processes controlling the tropical tropopause layer (TTL) above West-Africa: deep convection up to the level of main convective outflow, overshooting of deep convection, horizontal inmixing across the subtropical tropopause, and horizontal transport across the subtropical barrier. Except for the flight of 13 August, distinct minima in CO2 indicate convective outflow of boundary layer air in the TTL. The CO2 profiles show that the level of main convective outflow was mostly located between 350 and 360 K, and for 11 August reached up to 370 K. While the CO2 minima indicate quite significant convective influence, the O3 profiles suggest that the observed convective signatures were mostly not fresh, but of older origin. When compared with the mean O3 profile measured during a previous campaign over Darwin in November 2005, the O3 minimum at the main convective outflow level was less pronounced over Ouagadougou. Furthermore O3 mixing ratios were much higher throughout the whole TTL and, unlike over Darwin, rarely showed low values observed in the regional boundary layer. Signatures of irreversible mixing following overshooting of convective air were scarce in the tracer data. Some small signatures indicative of this process were found in CO2 profiles between 390 and 410 K during the flights of 4 and 8 August, and in CO data at 410 K on 7 August. However, the absence of expected corresponding signatures in other tracer data makes this evidence inconclusive, and overall there is little indication from the observations that overshooting convection has a profound impact on TTL composition during AMMA. We find the amount of photochemically aged air isentropically mixed into the TTL across the subtropical tropopause to be not significant. Using the N2O observations we estimate the fraction of aged extratropical stratospheric air in the TTL to be 0.0±0.1 up to 370 K during the local flights, increasing above this level to 0.2±0.15 at 390 K. The subtropical barrier, as indicated by the slope of the correlation between N2O and O3 between 415 and 490 K, does not appear as a sharp border between the tropics and extratropics, but rather as a gradual transition region between 10 and 25° N latitude where isentropic mixing between these two regions may occur.

scholarly journals More evidence for very short-lived substance contribution to stratospheric chlorine inferred from HCl balloon-borne in situ measurements in the tropics

2010 ◽  
Vol 10 (2) ◽  
pp. 397-409 ◽  
Author(s):  
Y. Mébarki ◽  
V. Catoire ◽  
N. Huret ◽  
G. Berthet ◽  
C. Robert ◽  
...  
Keyword(s):  
In Situ Measurements ◽  
Tunable Diode Laser ◽  
Vertical Profiles ◽  
Tropical Tropopause Layer ◽  
Laser Absorption ◽  
Tropical Tropopause ◽  
Product Gas ◽  
Gas Measurements ◽  
The Tropics

Abstract. Volume mixing ratio (vmr) vertical profiles of hydrogen chloride (HCl) are retrieved from in situ measurements performed by a balloon-borne infrared tunable diode laser absorption spectrometer (SPIRALE) during two balloon flights in the tropics (Teresina, Brazil, 5.1° S–42.9° W) in June 2005 and June 2008. HCl vertical profiles obtained from 15 to 31 km are presented and analysed to estimate the contribution of very short-lived substances (VSLS) to total stratospheric chlorine. Both retrieved vertical profiles of HCl from these flights agree very well with each other, with estimated overall uncertainties of 6% on vmr between 23 and 31 km. Upper limits of HCl vmr as low as 20 pptv in June 2008 and 30 pptv in June 2005 are inferred in the upper part of the tropical tropopause layer (TTL). Backward trajectory calculations and such low amounts suggest that the air masses sampled correspond to typical background conditions, i.e. neither influenced by recent tropospheric nor stratospheric air. Taking into account the recently reported VSL source gas measurements obtained in similar conditions (Laube et al., 2008) and the main intermediate degradation product gas COCl2 (Fu et al., 2007), a total VSLS contribution of 85±40 pptv to stratospheric chlorine is inferred. This refines the WMO (2007) estimation of 50 to 100 pptv, which was not taking into account any HCl contribution. In addition, comparisons of HCl measurements between SPIRALE and the Aura MLS satellite instrument in the tropical lower and middle stratosphere lead to a very good agreement. The previous agreement between MLS-deduced upper stratospheric total chlorine content and modelled values including 100 pptv of VSLS (Froidevaux et al., 2006) is thus supported by our present result about the VSLS contribution.

scholarly journals Time varying changes in the simulated structure of the Brewer Dobson Circulation

2016 ◽  
Author(s):  
Chaim I. Garfinkel ◽  
Valentina Aquila ◽  
Darryn W. Waugh ◽  
Luke D. Oman
Keyword(s):  
Structural Changes ◽  
Climate Models ◽  
Climate Model ◽  
Volcanic Eruptions ◽  
Residual Circulation ◽  
The Past ◽  
Earth Observing System ◽  
Long Time ◽  
System Chemistry

Abstract. A series of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model are analyzed in order to assess changes in the Brewer-Dobson Circulation (BDC) over the past 55 years. When trends are computed over the past 55 years, the BDC accelerates throughout the stratosphere, consistent with previous modeling results. However, over the second half of the simulations (i.e. since the late 1980s), the model simulates structural changes in the BDC as the temporal evolution of the BDC varies between regions in the stratosphere. In the mid-stratosphere in the mid-latitude Northern Hemisphere, the BDC decelerates in a simulation despite increases in greenhouse gas concentrations and warming sea surface temperatures. This deceleration is reminiscent of changes inferred from satellite instruments and in-situ measurements. In contrast, the BDC in the lower-stratosphere continues to accelerate. The main forcing agents for the recent slowdown in the mid-stratosphere appear to be declining ODS concentrations and the timing of volcanic eruptions. Changes in both age of air and the tropical upwelling of the residual circulation are similar. We therefore clarify that the statement that is often made that climate models simulate a decreasing age throughout the stratosphere only applies over long time periods, and is not the case for the past 25 years when we have most tracer measurements.

scholarly journals Ice supersaturations exceeding 100% at the cold tropical tropopause: implications for cirrus formation and dehydration

2004 ◽  
Vol 4 (6) ◽  
pp. 7433-7462
Author(s):  
E. Jensen ◽  
J. B. Smith ◽  
L. Pfister ◽  
J. V. Pitman ◽  
E. M. Weinstock ◽  
...  
Keyword(s):  
Water Saturation ◽  
Accommodation Coefficient ◽  
Ice Nucleation ◽  
Cloud Formation ◽  
High Threshold ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
Accommodation Coefficients ◽  
The Tropics

Abstract. Recent in situ measurements at tropical tropopause temperatures as low as 187 K indicate supersaturations with respect to ice exceeding 100% with little or no ice present. In contrast, models used to simulate cloud formation near the tropopause assume a supersaturation threshold for ice nucleation of about 65% based on laboratory measurements of sulfate aerosol freezing. The high supersaturations reported here, along with cloud simulations assuming a plausible range of temperature histories in the sampled air mass, indicate that the vast majority of aerosols in the air sampled on this flight must have had supersaturation thresholds for ice nucleation exceeding 100% (i.e. near liquid water saturation at these temperatures). Possible explanations for this high threshold are that (1) the expressions used for calculating vapor pressure over supercooled water at low temperatures give values at least 20% too low, (2) most of the available aerosols had a composition that makes them much more resistant to ice nucleation than aerosols used in laboratory experiments, and (3) organic films on the aerosol surfaces reduce their accommodation coefficient for uptake of water, resulting in aerosols with more concentrated solutions when moderate-rapid cooling occurs and correspondingly inhibited homogeneous freezing. Simulations of in situ cloud formation in the tropical tropopause layer (TTL) throughout the tropics indicate that if these decreased accommodation coefficients and resulting high thresholds for ice nucleation prevailed throughout the tropics, then the calculated occurrence frequency and areal coverage of TTL cirrus would be significantly suppressed. However, the simulations also show that even if in situ TTL cirrus form only over a very small fraction of the tropics in the western Pacific, enough air passes through them due to rapid horizontal transport such that they can still effectively freeze-dry air entering the stratosphere.

scholarly journals The observation of nitric acid-containing particles in the tropical lower stratosphere

2005 ◽  
Vol 5 (5) ◽  
pp. 10097-10124
Author(s):  
P. J. Popp ◽  
T. P. Marcy ◽  
E. J. Jensen ◽  
B. Kärcher ◽  
D. W. Fahey ◽  
...  
Keyword(s):  
Nitric Acid ◽  
Lower Stratosphere ◽  
Nucleation Process ◽  
Eastern Pacific Ocean ◽  
Number Density ◽  
Tropical Tropopause ◽  
Process Understanding ◽  
Trajectory Models ◽  
The Tropics

Abstract. Airborne in situ measurements over the eastern Pacific Ocean in January 2004 have revealed a new category of nitric acid (HNO3)-containing particles in the tropical lower stratosphere. These particles are most likely composed of nitric acid trihydrate (NAT). They were intermittently observed in a narrow layer above the tropopause (18±0.1 km) and over a broad geographic extent (>1100 km). In contrast to the background liquid sulfate aerosol, these particles are solid, much larger (1.7–4.7 µm vs. 0.1 µm in diameter), and significantly less abundant (<10-4 cm-3 vs. 10 cm-3). Microphysical trajectory models suggest that the NAT particles grow over a 6–14 day period in supersaturated air that remains close to the tropical tropopause and might be a common feature in the tropics. The small number density of these particles implies a highly selective or slow nucleation process. Understanding the formation of solid NAT particles in the tropics could improve our understanding of stratospheric nucleation processes and, therefore, dehydration and denitrification.

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