ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-14219-2017Multi-model impacts of climate change on pollution transport
from global emission source regionsDohertyRuth M.ruth.doherty@ed.ac.ukhttps://orcid.org/0000-0001-7601-2209OrbeClaraZengGuanghttps://orcid.org/0000-0002-9356-5021PlummerDavid A.https://orcid.org/0000-0001-8087-3976PratherMichael J.https://orcid.org/0000-0002-9442-8109WildOliverhttps://orcid.org/0000-0002-6227-7035LinMeiyunhttps://orcid.org/0000-0003-3852-3491ShindellDrew T.https://orcid.org/0000-0003-1552-4715MackenzieIan A.School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3FF, UKGoddard Earth Sciences Technology and Research (GESTAR), Johns Hopkins University, Baltimore, MD 21218, USANational Institute of Water and Atmospheric Research, Wellington 6021, New ZealandCanadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Montréal, QC, CanadaDepartment of Earth System Science, University of California, Irvine, Irvine, CA 92697-3100, USALancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UKProgram in Atmospheric and Oceanic Sciences at Princeton University, Princeton, NJ 08540, USANOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08540, USANicholas School of the Environment, Duke University, Durham, NC 27708, USARuth M. Doherty (ruth.doherty@ed.ac.uk)30November20171723142191423719May20179October20178October201726June2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/14219/2017/acp-17-14219-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/14219/2017/acp-17-14219-2017.pdf
The impacts of climate change on tropospheric transport, diagnosed
from a carbon monoxide (CO)-like tracer species emitted from global
CO sources, are evaluated from an ensemble of four chemistry–climate
models (CCMs) contributing to the Atmospheric Chemistry and Climate
Model Intercomparison Project (ACCMIP). Model time-slice simulations
for present-day and end-of-the-21st-century conditions were
performed under the Representative Concentrations Pathway (RCP)
climate scenario RCP 8.5. All simulations reveal a strong
seasonality in transport, especially over the tropics. The highest
CO-tracer mixing ratios aloft occur during boreal winter when strong
vertical transport is co-located with biomass burning emission
source regions. A consistent and robust decrease in future CO-tracer
mixing ratios throughout most of the troposphere, especially in the
tropics, and an increase around the tropopause is found across the
four CCMs in both winter and summer. Decreases in CO-tracer mixing
ratios in the tropical troposphere are associated with reduced
convective mass fluxes in this region, which in turn may reflect
a weaker Hadley cell circulation in the future climate. Increases in
CO-tracer mixing ratios near the tropopause are largely attributable
to a rise in tropopause height enabling lofting to higher altitudes,
although a poleward shift in the mid-latitude jets may also play
a minor role in the extratropical upper troposphere. An increase in
CO-tracer mixing ratios also occurs near the Equator, centred over
equatorial and Central Africa, extending from the surface to the
mid-troposphere.
This is most likely related to localised decreases in
convection in the vicinity of the Intertropical Convergence Zone (ITCZ),
resulting in larger CO-tracer mixing ratios over biomass burning
regions and smaller mixing ratios downwind.
Introduction
The transport of pollutants from the atmospheric boundary layer is
governed by meteorological processes including deep convection,
Hadley-cell-driven overturning in the tropics, and mid-latitude cyclones, as well
as slow low-altitude airflow, small-scale turbulent mixing, and other
motions (e.g. Cooper et al., 2011; TF-HTAP, 2010). Climate change may
affect the large-scale circulation of the atmosphere through the above
processes and hence impact the intercontinental transport of
pollutants. In addition to influencing meteorological transport
processes, changes in climate will also modify the atmospheric
chemical environment and pollutant lifetimes. To understand how these
changes will influence future pollutant distributions, it is therefore
important to disentangle the relative impacts of changes in transport
and chemistry as well as future emission changes. The focus of this
study is to quantify climate change impacts on atmospheric transport.
In the tropics, the Hadley circulation determines the location of the
Intertropical Convergence Zone (ITCZ) (e.g. Kang et al., 2013). Deep
convection and mean upwelling associated with the Hadley circulation
control transport processes influencing pollutant distributions. For
example, satellite measurements from the MOPITT (Measurements of Pollution in the
Troposphere) instrument reveal that deep
convection during the Asian summer monsoon carries pollutants emitted
at the surface aloft into the upper troposphere (Kar et al.,
2004). Deep convective transport of biomass burning emissions into the
middle and upper troposphere was observed over Brazil during the TRACE
A atmospheric chemistry field campaign experiment (Pickering et al.,
1996).
Over the mid-latitudes, ascent of pollution from the surface to the
mid- to upper troposphere occurs along warm conveyor belt
airstreams embedded within synoptic-scale mid-latitude cyclones
(Cooper et al., 2004; Brown-Steiner and Hess, 2011; Lin
et al., 2012). Descent from the lower stratosphere and upper
troposphere to the mid-troposphere can occur in the dry intrusion
airstreams of cyclones (e.g. Langford et al., 2015; Knowland
et al., 2015). This is also the main mechanism for
stratosphere–troposphere exchange of ozone that occurs in the
mid-latitudes and which may extend to the surface in regions prone to
deep stratospheric ozone intrusions (Lin et al., 2015). Deep
convection is also important for lofting surface pollution in
mid-latitude regions in summer when the landmass is warm.
Changes in climate may influence many of these tropical and
mid-latitude transport processes, but the impact of these future
changes on chemical composition remains unclear. In the tropics and
subtropics a number of studies have shown a poleward expansion and
weakening of the Hadley cell circulation in response to future
increases in greenhouse gases (GHGs), which is most robust in boreal winter
(Vecchi and Soden, 2007; Lu et al., 2007; Ma et al., 2012; Levine and
Schneider, 2011; Williamson et al., 2013; Seo et al., 2014; Kang
et al., 2013). While these studies posit that the weakening of
the Hadley cell is related to a weakening of the meridional
temperature gradient between the tropics and subtropics, other
studies have invoked thermodynamic constraints to suggest that
convective mass fluxes throughout the tropics may decrease in response
to increasing greenhouse gases (Held and Soden, 2006).
As temperatures increase in the troposphere but decrease in the
stratosphere in response to enhanced CO2 concentrations, there
is a decrease in static stability close to the tropopause that leads
to an increase in its height (Manabe and Wetherald, 1975; Santer
et al., 2003; Lorenz and De Weaver, 2007; Kang et al., 2013; Vallis
et al., 2015). A higher tropopause may also be associated with
a poleward expansion or widening of the Hadley cell (Lu et al., 2007),
but the mechanisms underlying this change remain unclear.
Over the mid-latitudes, there is a general consensus that the storm
tracks will shift poleward in response to future increases in
greenhouse gases, at least in the zonal mean (Yin et al., 2005;
Bengtssen et al., 2006; Barnes et al., 2013; Christensen et al., 2013;
Shaw et al., 2016). This poleward shift in the mid-latitude storm
tracks has been dynamically linked to the weakening of the Hadley
circulation in the tropics (Vallis et al., 2015; Shaw et al., 2016)
and to the rise in tropopause height (Lorenz and De Weaver, 2007).
However, the zonally asymmetric and seasonally varying response of
mid-latitude storm tracks to forced climate change is much less robust
(Simpson et al., 2014; Shaw et al., 2016), partly due to interannual
variability (Deser et al., 2012; Shepherd, 2014).
In terms of ozone pollution transport, this shift in the mid-latitude
storm track position has been related to reduced mid-latitude cyclone
frequency leading to increased summertime surface O3 pollution
episodes over the eastern USA and Europe (Mickley et al., 2004; Forkel
and Knoche, 2006; Murazaki and Hess, 2006; Leibensperger et al., 2008;
Wu et al., 2008), although other studies do not report such changes in
frequency (Racherla and Adams, 2008; Lang and Waugh, 2011). The shift
in mid-latitude storm tracks has also been related to changes in
regional climate phenomena in particular the North Atlantic
Oscillation (Ulbrich et al., 2009; Christensen et al., 2013), blocking
anticyclone frequency (Masato et al., 2013), and the Pacific Decadal
Oscillation (Lin et al., 2014; Allen et al., 2014). Ozone transport
from the lower stratosphere to the troposphere will also be influenced
by future changes in stratosphere–troposphere exchange, which is
expected to increase under greenhouse gas warming owing to
a strengthening of the Brewer–Dobson circulation in the stratosphere,
leading to higher ozone mixing ratios in the mid- to upper troposphere
(Butchart and Scaife, 2001; Neu et al., 2014). Higher mixing ratios of
idealised tracers of stratospheric origin in the tropical and subtropical
troposphere have also been found due to enhanced
stratosphere–troposphere exchange in a future warmer climate (Orbe
et al., 2013a; Abalos et al., 2017).
Few studies have explicitly isolated the effects of climate change on
pollutant transport from its effects on chemical processes
(e.g. through enhanced chemical reaction rates or changes in natural
climate-sensitive emissions). Idealised tracers from surface sources
used by Holzer and Boer (2001) showed increases in
interhemispheric exchange times, mixing times, and mean transit times
between 2000 and 2100. This study also found a 25 % lower
tropospheric average tracer mixing ratio as well as a reduced
cross-tropopause tracer gradient, which was attributed to a slightly
higher tropopause. More recently, Orbe et al. (2015) used idealised
tracers of air-mass origin, as described in Orbe et al. (2013b), to
track how future increases in greenhouse gases modified transport
patterns extending from the northern mid-latitude boundary layer into
the Arctic. Using idealised tracers they diagnosed enhanced poleward
transport of the mid-latitude air arising from the poleward migration
of the mid-latitude storm tracks, as outlined above.
Using a carbon monoxide (CO)-like tracer, which has present-day fossil
fuel emissions as its source and loss by reaction with present-day OH,
Mickley et al. (2004) related 5–10 % enhancements at the high
percentile values of summer CO-tracer distributions in the United
States to reduced cyclone passage across southern Canada under
a future 2050 climate compared to present day. Several other studies
have used idealised CO tracers emitted from continental sources to
investigate climate variability and change impacts on pollutant
transport (Shindell et al., 2008; Doherty et al., 2013; Lin
et al., 2014; Monks et. al. 2015). Using a regional CO-like tracer
with surface emissions from Asia held constant, Lin et al. (2014)
examined the mean influence of Eurasian pollution over the subtropical
North Pacific, influenced by the position of the subtropical jet and
its decadal variability. Under the SRES A2 climate forcing scenario
for the 2090s compared to the 2000s, distinct dipole patterns in the
changes in surface CO-tracer mixing ratios were interpreted as
a response to future shifts in regional circulations within four
continental regions and their outflow locations (Doherty
et al., 2013).
A detailed analysis by Fang et al. (2011) used a global CO-like tracer
with a first-order 25-day lifetime and global anthropogenic CO
emissions to investigate changes in transport under the SRES A1B
scenario between 1981–2000 and 2081–2100 using the GFDL AM3
chemistry–climate model (CCM). They found that CO-tracer mixing
ratios increased at the surface and decreased in the tropical free
troposphere due to reduced convective mass fluxes and that reduced
CO-tracer mixing ratios in the Southern Hemisphere were most likely
a response to a weaker Hadley circulation and reduced interhemispheric
exchange (Fang et al., 2011). A large increase in CO-tracer mixing
ratios near the tropopause was suggested to arise from the upward
migration of the tropopause (Fang et al., 2011). This study focussed
on annual-mean distributions. Similarly, the trends in mixing ratios
of a 90-day e-folding (e90) tracer between 1955 and 2099 have been
related to trends in tropopause height (Abalos et al., 2017).
The aim of this paper is to explore the robustness of the changes in
transport found in the single-model study described above across an
ensemble of CCMs participating in the recent Atmospheric Chemistry and
Climate Model Intercomparison Project (ACCMIP) using a globally
emitted CO tracer (Lamarque et al., 2013) and to quantify for the
first time seasonal transport changes in response to climate change
and their dynamical attribution. Section 2 describes the models used
while Sect. 3 discusses future changes in CO-tracer mixing ratios
(with emissions held constant). Section 4 outlines the transport
processes and circulation changes that most likely drive CO-tracer
redistribution under climate change. Discussion and conclusions are
presented in Sect. 5.
Data sets and methods
In the ACCMIP model intercomparison, four global CCMs included a CO-like
tracer emitted from global sources: UM-CAM, GISS-E2-R, CMAM, and STOC-HadAM3.
A description of these models, including their chemistry, transport, and
configuration, can be found in Lamarque et al. (2013) and Young
et al. (2013). The horizontal resolution of the models varied between
1.875∘ by 2.5∘ and 5∘ by 5∘. Two of the
models, UM-CAM and STOC-HadAM3, have the same driving global climate model (GCM);
however, their
advection schemes differ substantially since STOC-HadAM3 is the only model to
use a Lagrangian approach to simulate transport processes. Deep convection
schemes used by the models are based on two main parameterisations: Gregory
and Rowntree (1990) for GISS-E2-R, UM-CAM, and STOC-HadAM3; and Zhang and
McFarlane (1995) for CMAM. In addition, STOC-HadAM3 uses Collins
et al. (2002) to derive using convective mass fluxes the probability of
a parcel being subject to convective transport. Although these two
parameterisations are based on a mass flux approach, there can be a wide
spread in simulated convective mass fluxes within a single parameterisation
(Scinocca and McFarlane, 2004; Lamarque et al., 2013). In addition, how the
transport of the CO tracer is implemented will influence the impacts of the
convection schemes. The simulations were performed using decadal-average
monthly sea surface temperature and sea ice concentration distributions for
two 10-year periods: present day (“acchist” simulations) as represented by
a period centred on the year 2000 (1996–2005) and a future projection under
the latest IPCC Representative Concentration Pathway (RCP) RCP 8.5 scenario
for 2090–2099. Note that each modelling group derived their own set of sea
surface temperature and sea ice fields, typically from a closely related
coupled-ocean GCM. Under RCP 8.5 the increase in global mean surface
temperature between 2081 and 2100, relative to 1986–2005, is projected to be
2.6–4.8 ∘C averaged across all (∼39) participating GCMs
(Collins et al., 2013). For the four models used here the global mean surface
temperature change between 1996–2005 and 2090–2099 is 3.1–4.6 ∘C.
Annual-mean anthropogenic CO emissions
(kgs-1)
from fossil fuels (a) and seasonal-mean biomass burning CO
emissions for the different seasons (kgs-1) (b
and c). The dashed line shows the approximate position of
the Intertropical Convergence Zone (ITCZ) in the different seasons.
The CO-like tracer was implemented as a chemically inert species with
monthly-varying emissions representing all global anthropogenic and
biomass burning CO sources with a first-order decay lifetime of
50 days (Shindell et al., 2008; Fang et al., 2011; Doherty
et al., 2013). This idealised tracer is relatively long lived such
that it can undergo interhemispheric transport and be used to diagnose
how changes in transport from source regions affect the distributions
of trace gas species with similar lifetimes (such as CO and
O3). Monthly CO-tracer fields were generated for two 10-year
periods (1996–2005 and 2090–2099), and the four models used the same
emissions data for 2001 for both time periods. Thus, for each CCM the
differences in CO-tracer mixing ratio distributions between these two
periods are due solely to how climate change affects transport from
global emission regions. To establish whether the CO-tracer
distributions in the present-day (1996–2005) and future (2090–2099)
periods are significantly different, a Student's t test was performed
using the 10 years of annual data for each period for each model grid
cell; a p value <0.05 was used to determine statistical
significance.
The Task Force on Hemispheric Transport of Air Pollution (TF-HTAP,
2010) CO-tracer emissions data
set was used in ACCMIP, which consists of annual-mean anthropogenic emissions
for the year 2001 from the RETRO project (Schultz and Rast, 2007) and monthly
average biomass burning emissions (injected into the models at, or near, the
surface) from GFED version 2 (van der Werf et al., 2006;
http://www.globalfiredata.org/). The major source regions for
anthropogenic CO emissions are in the northern mid-latitudes with peak levels
in East and South Asia (Fig. 1). Unlike the anthropogenic emissions, the
biomass burning emissions feature a strong seasonality, with high values over
equatorial Africa during December–January–February (DJF), peak values
southward of the Equator in South America and Central Africa in
June–July–August (JJA), and a stronger peak value in Southeast Asia during
JJA (Fig. 1). The location of these emission peaks in relation to the
position of the ITCZ can be clearly seen in Fig. 1.
Biomass emissions during March–April–May (MAM) and
September–October–November (SON) are significantly weaker (Fig. 1).
Top panels: present-day (1995–2006) boreal winter (DJF) 10-year
climatological mean zonal-mean CO-tracer distribution for (a)
UM-CAM, (b) STOC-HadAM3, (c) CMAM, and (d)
GISS-E2-R CCMs. The thick black solid line represents
the present-day (DJF) zonally averaged thermal tropopause. Bottom
panels: differences between 2090–2099 (RCP 8.5) and 1995–2006 (present day)
DJF climatological mean zonal-mean CO
distributions.
Thin black contours denote the present-day DJF
climatology. The thick solid and dashed lines represent the DJF
zonally averaged thermal tropopause for present-day and the 2090s
(RCP 8.5) climatologies. Grey shading indicates where results are not
significant at p<0.05 as evaluated with a Student's t test using
10 years of data for the 2090s (RCP 8.5) and present-day
simulations.
Temperature data from the four CCMs were used to calculate the thermal
tropopause following the World Meteorological Organization (WMO) lapse rate
definition as implemented by Reichler et al. (2003) for gridded reanalysis
data. The tropopause is defined separately as an average for the 2000s and
the 2090s as the lowest model level at which the lapse rate decreases to
2 ∘C km-1, provided that the average lapse rate between
this level and higher levels does not exceed this. Studies have shown the
lapse rate or thermal tropopause approximately coincides with the e90 tracer
tropopause which is used to distinguish stratospheric and tropospheric air
(Prather et al., 2011; Abalos et al., 2017). Convective mass flux and
zonal (u) wind data (available for three of the four
CCMs) were also used for qualitative attribution purposes.
CO-tracer redistribution in response to climate change
The distribution of the CO tracer in the troposphere under present-day
conditions and its redistribution under the RCP 8.5 climate scenario
are now discussed. Similarities and differences across the four CCMs
and with season are highlighted. Note that the monthly average
atmospheric burden of the CO tracer is nearly identical for the 2000s
and 2090s, as expected given the specified emissions and lifetime, so
that the differences in mixing ratio discussed here result purely from
a redistribution due to changes in transport.
Present-day distributions
For the present-day period (1995–2006), the CO-tracer distributions
show the effect of deep convection in the tropics and synoptic and
convective lifting over mid-latitudes. There is a strong seasonality
in the CO-tracer distributions, which is driven by both the
seasonality of CO source emissions (Fig. 1) and seasonal changes in
transport. In the tropics, the largest CO-tracer mixing ratios occur
during boreal winter (DJF), hereinafter winter (Fig. 2), compared to
boreal summer (JJA), hereinafter summer (Fig. 3). In the northern
mid-latitudes, CO-tracer mixing ratios are largest in boreal spring and
in the southern mid-latitudes in boreal autumn. Elsewhere CO-tracer
mixing ratios have a fairly uniform seasonal cycle.
Same as Fig. 2 but for boreal summer (JJA).
During winter, large CO-tracer mixing ratios are found near the
Equator, with decadal-average values of more than 60 ppb
extending from the surface to ∼700hPa and up to
40 ppb in the mid-upper tropical troposphere in all four CCMs
(Fig. 2). In summer, CO-tracer mixing ratios are lower in the tropics
and northern extratropics extending to ∼40∘ N (∼40–50 ppb at the surface and 30–40 ppb in the
mid- to upper troposphere in the Northern Hemisphere; Fig. 3) than in winter
(Fig. 2). In contrast, in the northern middle and high latitudes,
CO-tracer mixing ratios are higher in summer (up to 50 ppb
near the surface; Fig. 3) than in winter (Fig. 2). Note that, while
the CO-tracer distribution patterns are fairly similar between the
models, there are some differences. In particular, CMAM simulates
slightly lower values in the tropical upper troposphere in winter
compared to the other CCMs (Fig. 2c), while the GISS-E2-R simulation
features larger values above 700 hPa over northern
mid-latitudes during summer (Fig. 3d).
The spatial pattern of the CO tracer averaged over the lower to mid-troposphere (400–800 hPa; Fig. 4) is zonally relatively
uniform in winter and is similar across the four CCMs. This pattern
also highlights the influence of strong vertical transport in the
tropics and subsequent horizontal transport of the CO tracer from its
major anthropogenic surface sources over East and South Asia as well as
biomass burning sources in equatorial and Central Africa. During
summer, the lower- to mid-tropospheric CO-tracer patterns are more closely
confined to the source region locations over East and South Asia and
Central Africa (Fig. 5) suggesting weaker transport around the middle
troposphere than in winter.
Top panels within subplots (a–d): present-day
(1995–2006) DJF 10-year climatological mean CO-tracer distribution, averaged over
400–800 hPa. Bottom panels: differences between 2090–2099 (RCP 8.5)
and 1995–2006 (present day) DJF climatological mean CO-tracer distributions,
wherein black contours denote the present-day climatology. Results
are presented for (a) UM-CAM and (b) STOC-HadAM3
(top panels) as well as (c) CMAM and (d) GISS-E2-R
(bottom panels). Grey shading indicates where results are not
significant at p<0.05 as evaluated with a Student's t test using
10 years of data for the 2090s (RCP 8.5) and present-day
simulations. Note the different scales for (i) UM-CAM and STOC-HadAM3
and (ii) CMAM and GISS-E2-R for the difference plots.
Same as Fig. 4 but for JJA. Note the different scales for
(i) UM-CAM and STOC-HadAM3 and (ii) CMAM and GISS-E2-R for the difference
plots.
The seasonal differences in CO-tracer mixing ratios in the tropics
reflect the combined influence of seasonal differences in biomass
burning emissions and in tropical convection. In particular, during
winter, when the ITCZ is located in
the Southern Hemisphere, the largest emissions from biomass burning
originate over Africa near the Equator (Fig. 1). In contrast, during
summer, when the ITCZ is located north of the Equator, the peak
biomass burning emissions are located further southward (∼20∘ S) (Fig. 1). Thus, in summer tropical convection is
weakly co-located with biomass burning emissions, resulting in lower
CO-tracer mixing ratios in the tropical mid-troposphere as a whole and
a more confined region of peak CO-tracer mixing ratios over Central
Africa (Fig. 5), compared to winter (Fig. 4). The influence of the
ITCZ position is also seen in the CO-tracer vertical gradient (the
ratio of the CO tracer relative to its value at the surface) which is
shallower in the southern tropics during winter and in the northern
tropics in summer (not shown) due to greater vertical mixing from
convective lofting along the ITCZ (Sect. 4.1). Note that seasonal
differences related to the transport of anthropogenic sources in the
mid-latitudes may also be important, with strong surface lofting in
winter storm tracks resulting in larger CO-tracer mixing ratios over
the northern extratropics during winter (Fig. 4) than in summer
(Fig. 5).
Note that the CO-tracer mixing ratios in this study are higher than
those reported by Fang et al. (2011) since we use a longer lifetime of
50 days rather than 25 days. CO-tracer mixing ratios are typically
lower than modelled or observed real CO (which is usually more than
100 ppb over source regions) as there is no chemical
production of the CO tracer, which accounts for about half of atmospheric
CO (Shindell et al., 2006). The seasonality of the CO tracer and CO
is fairly similar, with a more pronounced winter peak in the tropics
in the CO tracer in the mid-troposphere. The relative changes in
CO-tracer mixing ratios are largest in the tropics and during winter
and smaller in summer. Henceforth, the focus is on findings for boreal
winter and summer.
Response to greenhouse gas increases
The response of CO-tracer mixing ratios to climate change shows robust
features that are statistically significant across the four CCMs
(Figs. 2 and 3). This suggests a general consistency in changes in
transport between 1995–2006 and 2090–2099 under the RCP 8.5 climate
scenario. In general, in both winter and summer, CO-tracer mixing
ratio decreases are ubiquitous throughout most of the
troposphere. The largest changes occur in boreal winter, with decreases
of ∼2–6 ppb (∼5–10 %) in CO-tracer mixing
ratios near the surface at the Equator, and especially in the middle to
upper tropical troposphere in the tropics and the northern
mid-latitudes (Fig. 2). In contrast, there is a narrow region of
increases in CO-tracer mixing ratios of up to 6 ppb (∼10 %) at ∼5–10∘ N reaching from the surface to
the mid-troposphere (and into the upper troposphere in CMAM). This
feature is also seen in the annual-mean CO-tracer distributions for
the four CCMs (not shown). Fang et al. (2011) also find substantial
decreases in annual-mean CO-tracer concentrations in the free
troposphere (-2 to -12 % at 400 hPa) but more
widespread increases (2–7 %) in annual-mean CO-tracer mixing
ratios near the surface that extend from the Equator to the northern
mid-latitudes. Since the same emission data are used in both studies,
this difference is likely to arise from differences in CO-tracer
lifetime used as well as model differences in representing shallow
convection and/or advection processes.
Future CO-tracer mixing ratios also increase substantially by ∼2–6 ppb (∼10–25 %) near the present-day tropopause and
into the lower stratosphere (where the relative changes can reach 50 %),
especially in the tropics and northern mid-latitudes in all the CCMs
(Fig. 2). The contribution of the rise in the tropopause to the increase in
CO-tracer mixing ratios is explored further in Sect. 4.2. This
near-tropopause increase in CO-tracer mixing ratios in the northern mid- to
high latitudes is also consistent with future increases in poleward transport
reported by Orbe et al. (2015) based on tracers of air-mass origin. Increases
in the vertically integrated CO-tracer column between the 2000s and 2090s
between 30 and 40∘ N in all models also suggest an increase in
advective transport poleward, since vertical redistribution alone would not
produce an increase in the vertical column. Similar difference patterns occur
in summer, except that the narrow region of increase is above the surface, is
smaller in magnitude and vertical extent, and is centred south of the
Equator, where present-day CO-tracer concentrations peak (coinciding with the
summer biomass burning peak) (Fig. 3). The fractional or relative changes in
CO-tracer concentrations between winter and summer are fairly similar (not
shown).
Examining the spatial changes in tropical CO-tracer concentrations in
the lower to mid-troposphere in relation to the future increases and
decreases described above, a clear dipole pattern emerges across all
four CCMs (Figs. 4 and 5). In particular, during winter, there is
a large increase centred over equatorial and Central Africa (the
regions with peak biomass burning) and a decrease south of this of
similar magnitude of ∼15ppb (and up to 30 ppb
for the region of decrease in UM-CAM and STOC-HadAM3) (Fig. 4). The
area of increase coincides with the increase near the Equator
extending from the surface to the mid-troposphere, seen in Figs. 2 and
3 described above. This dipole pattern reflects stronger confinement
of CO-tracer mixing ratios near regions of emissions, flanked by
smaller concentrations downwind to the south. Note that, while the
zonally varying pattern of the response in the CO tracer is
characterised by high internal variability, this dipole pattern is
statistically significant across most of its extent in all of the
CCMs. However, there are small differences across models. Similar
patterns of change are simulated by the UM-CAM and STOC-HadAM3 CCMs
(Figs. 4e–f and 5e–f) since they use the same driving GCM. A stronger
and more extensive area of increase and a weaker area of decrease is
simulated by the CMAM model in both seasons (Figs. 4c and 5c), while
GISS-E2-R simulates a weaker area of increase and a more extensive
area of decreases that extends latitudinally across to South America in
winter (Figs. 4d). Both CMAM and GISS-E2-R also depict an area of
decrease over East Asia, although these changes are not statistically
significant.
Potential drivers of changes in transport in a future
climate
The impact of climate change under the RCP 8.5 scenario on deep
convection and on jet stream locations is outlined here in relation
to the seasonal CO-tracer redistributions described above. The
increase in tropopause height under CO2 warming and its
influence on CO-tracer distributions is also elucidated. Modifications
to these transport processes have implications for pollutant transport
from major source regions in the future.
Convection and jet streams
The meteorological and physical drivers of the CO tracer mixing ratios
and changes due to climate change are examined using data from the
four ACCMIP model simulations. For present day, during both winter and
summer, deep convection in the tropics extends from the surface to
∼300hPa or higher in all four CCMs (Figs. 6
and 7). While the magnitude of the parameterised convective mass
fluxes simulated by CMAM and especially the GISS-E2-R CCM (up to
30×10-3kgm-2s-1) is larger than for
UM-CAM and STOC-HadAM3, which are driven by the same GCM (up to
8×10-3kgm-2s-1), the spatial patterns
and seasonality of convection are consistent across all models
(Figs. 6 and 7). For example, in the tropics and subtropics, the
strongest convective mass fluxes shift location from
10–20∘ S in winter to 10–20∘ N in summer as the
ITCZ migrates south and northwards of
the Equator. Convective mass fluxes are also large in the northern
mid-latitudes in winter and in the southern mid-latitudes in summer
when the mid-latitude jet streams are strongest, but their vertical
extent is shallower than in the tropics. Substantial differences of
a factor of 2–3 in annual-mean zonal convective mass fluxes simulated
across three models (including STOC-HadAM3) were also reported in
Doherty et al. (2005). Since the same parameterisation is used by
UM-CAM, HadAM3, and GISS-ER-2, it may be the specific details of its
implementation and interactions with internal parameters (Scinocca and
McFarlane, 2004) that cause this large difference in magnitudes across
the four CCMs.
Top panels: DJF 10-year climatological mean zonally averaged
convective mass fluxes for present day (1995–2006) for (a)
UM-CAM, (b) STOC-HadAM3, (c) CMAM, and (d)
GISS-E2-R. The thick black solid line represents the present-day
(DJF) zonally averaged thermal tropopause. Bottom panels: differences between 2090–2099
(RCP 8.5) and 1995–2006 (present day) DJF climatological mean
zonal-mean convective mass fluxes. The thick solid and dashed lines
represent the DJF zonally averaged thermal tropopause for the
present day and the 2090s (RCP 8.5) climatologies. Grey shading
indicates where results are not significant at p<0.05 as evaluated
with a Student's t test using 10 years of data for the 2090s
(RCP 8.5) and present-day simulations. Note the different
scales for (i) UM-CAM and STOC-HadAM3 and (ii) CMAM and GISS-E2-R.
Same as Fig. 6 but for JJA. Note the different scales for
(i) UM-CAM and STOC-HadAM3 and (ii) CMAM and GISS-E2-R.
The spatial patterns of convective mass fluxes averaged over the lower
to upper troposphere (800 to 300 hPa) highlight the zonal
symmetry of deep convection across the ITCZ and depict the seasonal
migration of the ITCZ within the subtropics (Figs. 8 and 9). Hence,
as discussed in Sect. 3.1, the large CO-tracer mixing ratios over
equatorial and Central Africa during winter (Fig. 4) reflect the
strong co-location of biomass burning emissions and convection in the
Southern Hemisphere subtropics when the ITCZ has shifted south of the
Equator. By comparison, lower CO-tracer mixing ratios over equatorial
Africa during summer reflect both a southward migration in emissions
and a northward migration of the ITCZ, resulting in weaker convective
lofting in this region (Fig. 5). The larger CO-tracer mixing ratios at
higher altitudes over northern mid-latitudes during summer simulated
by the GISS-E2-R CCM (Fig. 2d) may be related to stronger convective
mass fluxes at this altitude (Fig. 7d).
Top panels within subplots (a–d): DJF 10-year
climatological mean convective mass fluxes, averaged over
300–800 hPa for 1996–2005 (present day). Bottom panels: differences between 2090–2099 (RCP 8.5) and 1996–2005
(present day) DJF climatological mean convective
mass fluxes, wherein black contours denote the
present-day climatology. Results are presented for UM-CAM and
STOC-HadAM3 (top panels) and CMAM and GISS-E2-R (bottom
panels). Grey shading indicates where results are not significant at
p<0.05 as evaluated with a Student's t test using 10 years of
data for the 2090s (RCP 8.5) and present-day
simulations. Note the different scales for (i) UM-CAM and STOC-HadAM3
and (ii) CMAM and GISS-E2-R.
Same as Fig. 8 but for JJA. Note the different scales for
(i) UM-CAM and STOC-HadAM3 and (ii) CMAM and GISS-E2-R.
A robust feature across all of the models is an overall reduction in
convection, as reported by Held and Soden (2006), in response to climate
change in the 2090s throughout most of the troposphere in both winter and
summer (up to 3–5×10-3kgm-2s-1; ∼10–30 %) that is slightly larger for UM-CAM and STOC-HadAM3 than the
other two CCMs (Figs. 6 and 7). Absolute and relative changes in convective
mass fluxes between winter and summer are similar (not shown). This reduction
occurs both in the tropics and in the extratropics, extending to about
40∘ N and 40∘ S. The UM-CAM and STOC-HadAM3 CCMs also
feature strong decreases in convection centred at 60∘ N and
60∘ S extending vertically from the surface to 700 hPa in
both seasons. The spatial pattern of the magnitude of convective mass fluxes
averaged over the mid-upper troposphere shows the largest decreases occurring
along the ITCZ (including the ITCZ portion over Africa) in both seasons for
all CCMs (Figs. 8 and 9).
Although convective mass fluxes predominately decrease under
greenhouse gas warming, there are small areas of increase. Close to
the surface in the tropics increases in convective mass fluxes are
simulated by the UM-CAM and STOC-HadAM3 CCMs, but not by the other two
CCMs (Figs. 6a–b and 7a–b). The GISS-E2-R CCM depicts a small band
of increased convective mass fluxes at the Equator extending from the
surface to the upper troposphere in winter. A strong increase in
convective mass fluxes in the Northern Hemisphere polar latitudes at
the surface extending upwards is also a consistent model feature in
winter, which is most prominent in UM-CAM and STOC-HadAM3. Since this
region is not co-located with major emissions, these increases in
convection have a very small influence on the CO-tracer distributions.
Increases in convective mass fluxes are also simulated over the east
Pacific portion of the ITCZ by GISS-E2-R and, to a lesser extent, by
the CMAM model (Figs. 8c–d and 9c–d). Again, since this area of
increase is over the ocean and hence not co-located with emissions,
there is little influence on the spatial patterns of CO-tracer concentration changes (Figs. 4c–d and 5c–d).
The decrease in convective mass fluxes in the tropics under climate
change described above is consistent with the reduced convective
lofting of biomass burning emissions and, hence, decreased CO-tracer
mixing ratios in the tropical mid- to upper troposphere in all of the
CCMs. Furthermore, reduced convection in the tropics may also explain
the dipole pattern of change in CO-tracer mixing ratios over
equatorial and Central Africa in the lower to mid-troposphere through
greater confinement of CO-tracer concentrations to the region directly
aloft of the surface emissions source (Figs. 4 and 5). These changes
in CO-tracer concentrations are primarily determined by the extent of
co-location between convective and biomass burning source regions and
how convection changes over these emission source regions.
Reduced convective mass fluxes in the future may also partly explain
decreases in CO-tracer concentrations in the mid-latitude mid- to upper
troposphere (Figs. 2 and 3), although changes in the mid-latitude
storm tracks may also play an important role in modulating the
CO-tracer changes at these higher altitudes. In particular, all of the
models feature a poleward shift in their zonal-mean zonal winds under
climate change, as found in previous studies (e.g. Yin et al., 2005;
Orbe et al., 2015), leading to reductions in zonal-mean winds in
adjacent regions of the mid-latitude troposphere in both seasons,
although they are generally largest in winter (Fig. 10). However, although the
broad patterns of change in zonal-mean winds are similar, there are
different magnitudes and patterns of responses across latitude bands
for the different models with UM-CAM and CMAM featuring substantially
weaker (∼5ms-1; Fig. 10a–b) zonal-mean winds
poleward of ∼35∘ S in winter, while the GISS-E2-R CCM
shows weaker (5 ms-1; Fig. 10c) zonal-mean winds poleward
of ∼35∘ N in winter and ∼35∘ N in
summer. In the tropics, the zonal-mean wind response to climate change
is rather variable. Other wind component fields were not archived in
the ACCMIP simulations, which prohibits further investigation into the
relationship between mid-latitude jet stream changes and CO-tracer
responses over the mid-latitudes.
Zonal-mean zonal (u) wind differences
between 2090–2099 (RCP 8.5) and 1996–2005
(present day) for DJF (top panels) and JJA (bottom
panels). Results are shown for (a) UM-CAM and STOC-HadAM3
(same driving GCM), (b) CMAM, and (c)
GISS-E2-R. Black contours denote the present-day climatology.
Tropopause height
Robust increases in CO-tracer mixing ratios near the tropopause in the
tropical and northern mid-latitudes in response to climate change are seen in
all four CCMs (Figs. 2 and 3). The annual average multi-model mean tropopause
in the 2090s moves upward by 12 hPa in the tropics and 27 hPa
in the mid-latitudes relative to its position in the 2000s. Previous studies
have attributed similar CO-tracer changes to changes in tropopause height
(Fang et al., 2011), which is a robust feature of greenhouse gas warming
(e.g. Kang et al., 2013) and shown by all four CCMs.
Following Fang et al. (2011), we elucidate the role of an increase in
tropopause height in modulating CO-tracer concentrations by comparing
annual-mean profiles of the CO tracer, plotted relative to the thermal
tropopause, for both present-day and future periods
(Fig. 11). Comparison of annual-mean CO-tracer profiles reveals that
when vertical CO-tracer profiles are compared in tropopause relative
coordinates there is generally less difference between present day
and future, unlike when the CO-tracer profiles are plotted relative to
pressure. Therefore, much of the CO-tracer increase near the
tropopause that occurs in the future arises from a rise in tropopause
height, as reported in Fang et al. (2011) and also by Abalos
et al. (2017) using the e90 tracer. This is evident for all models,
especially in the northern mid-latitudes (40∘ N) near the
tropopause (Fig. 11). Hence the increase in CO-tracer mixing ratios
arises from a transition between low-CO stratospheric air for
present day and higher CO in tropospheric air in the future.
CO-tracer (COt) mixing ratio annual-average profiles (ppb)
averaged over various latitude bands (40∘ N, 0,
40∘ S) for the 2000s (present day) and 2090–2099
(RCP 8.5) for (a) UM-CAM, (b) STOC-HadAM3,
(c) CMAM, and (d) GISS-E2-R plotted against
altitude in pressure units (hPa) (green)
and with distance from the tropopause
for the respective time period (blue).
This also suggests that the impacts of enhanced poleward and upward
transport in the northern mid-high latitudes near the tropopause on
CO-tracer mixing ratios (Sect. 3.2) are largely outweighed by the
impact of the rise in the tropopause, although these effects may be
interrelated. However, the tropopause relative profiles in the 2090s
also show a slightly weaker vertical gradient between the mid-upper
troposphere in the NH extratropics (40∘ N) compared to the
2000s (Fig. 11, top panels). This reduced vertical gradient has been
noted in previous studies and also related to a rise in tropopause
height (Holzer and Boer, 2001). At the same time, a reduced vertical
gradient may also reflect an overall increase in eddy mixing
associated with the upward and poleward shifts in zonal-mean winds (Wu
et al., 2011). Similar transport responses to GHG-induced changes in
eddy mixing have been documented in other studies (Orbe et. al. 2015;
Abalos et al., 2017). Since changes in eddy mixing near the tropopause
are also linked to changes in tropopause height, it is not possible to
disentangle the separate imprint of these changes on the CO-tracer
distributions.
Discussion and conclusions
This study quantifies the seasonal variation and the robustness of
changes in transport under climate change. In response to future
increases in greenhouse gases in the 2090s under the RCP 8.5 scenario,
changes in mixing ratios of a CO-like tracer with a 50-day lifetime
exhibit robust features across four chemistry–climate models
participating in the ACCMIP model intercomparison. These include
a decrease in CO-tracer mixing ratios throughout most of the
troposphere, especially in the tropics (5–10 %), and an increase
in CO-tracer mixing ratios near the tropopause (10–25 %),
especially over the tropics and northern mid-latitudes. Underlying
these changes there is a strong seasonality in transport patterns
between winter and summer, with higher CO-tracer mixing ratios aloft
in winter when biomass burning emissions source regions are located
along the ITCZ.
These absolute changes in CO-tracer mixing ratios due to climate
change are generally larger in boreal winter than in summer, although
relative changes are similar. The relative changes in annual-mean
CO-tracer mixing ratios at the surface and in the free troposphere are
of similar magnitude to those reported by Fang et al. (2011) using the
GFDL AM3 model. Somewhat larger decreases in tropospheric-average
idealised tracer mixing ratios of 25 % were reported in 2100 by
Holzer and Boer (2001) under a different climate change scenario.
In addition, all four CCMs simulate a small region of increase in
zonal-mean surface CO-tracer mixing ratios at ∼5–10∘ N
that extends to the mid-troposphere, with decreases southwards, which
arise from a dipole pattern of adjacent increases and decreases in
CO-tracer mixing ratios over equatorial and Central Africa – the
largest biomass burning emission source regions.
Convective mass fluxes consistently decrease throughout most of the
troposphere in the future in all four CCMs in both seasons, with the
strongest decreases occurring within the tropics along the ITCZ, in agreement
with tropical convective mass flux reductions diagnosed by Held and
Soden (2006). However, in contrast to our findings, Abalos et al. (2017)
suggests decreases in convection mass fluxes are limited to ∼5km. Decreased convective mass fluxes in our study are consistent
with a weakening of the Hadley cell in winter when this feature is robust
(Hwan-Seo et al., 2014; Kang et al., 2013; Vallis et al., 2015). The
decreases in CO-tracer mixing ratios in the tropical troposphere therefore
most likely reflect reduced convection in the future. Reduced convection in
the tropics may also explain the dipole in CO-tracer response that occurs
near equatorial and Central Africa, since the seasonal patterns of changes in
CO-tracer concentrations in the tropics are in essence determined by how
seasonal changes in convection project onto seasonally varying biomass
burning emissions. Biomass burning emissions were held constant for present
day and future in this study.
The higher tropopause reported here is a robust finding across climate change
studies (e.g. Kang et al., 2013; Vallis et al., 2015). The strong increases
in CO-tracer concentrations in the vicinity of the tropopause can be largely
attributed to a higher tropopause under greenhouse gas warming, whereby this
region has low-CO stratospheric air for present day and higher CO in
tropospheric air in the future, in accord with Fang et al. (2011). A poleward
and upward shift in zonal-mean winds is consistent across the four models and
noted in previous studies (e.g. Orbe et al., 2015). Resultant enhanced
poleward transport may also minorly contribute to CO-tracer increases in the
future near the tropopause in the northern mid-latitudes, and changes in eddy
mixing may also have an impact. However, all these processes may be
interrelated such that it is not possible to discern the impacts of
individual processes on CO-tracer mixing ratios.
Overall, large-scale dynamical responses linked to changes in the
Hadley cell circulation and their impact on convection, mid-latitude
jets, and tropopause height appear to govern the main features of
the redistribution of CO-tracer mixing ratios between present day and
future simulated by four CCMs in this study. Further diagnostics to
allow more detailed dynamical insights would be most useful to probe
the relative contributions of different large-scale dynamical
processes including stratosphere–troposphere exchange, alongside
other aspects of the Hadley circulation such as its broadening or
poleward expansion under climate change.
One further limitation of this study is that the CO-tracer data were
not archived at a higher temporal resolution than monthly. Hence, it
is not possible to examine how CO-tracer concentrations in the
mid-latitudes are influenced by changes in synoptic-scale storm or
blocking frequency. Our ability to look further into relationships
between the mid-latitude storm tracks and CO-tracer distributions over
middle and high latitudes is also limited by the fact that only
zonally averaged zonal wind fields were archived. Another limitation is
that, while the use of a single tracer emitted from all global
CO-sources highlights transport associated with the global continental
emission source regions, it precludes an in-depth analysis of regional
changes in transport patterns under climate change.
Nevertheless, this multi-model study presents a clear and robust
picture of the effect of climate change on the transport of pollution
from major emission source regions, in particular from biomass burning
regions in the tropics that are strong CO sources, and how this effect
varies seasonally as governed by the seasonal location of the ITCZ and
biomass burning emissions sources. Furthermore, the key roles of
reduced convection consistent with a weakened Hadley circulation in
winter and a higher tropopause in governing transport changes are
confirmed. Overall, a reduction in tropical deep convection under
climate change will confine pollution more closely to its surface
source regions, potentially reducing intercontinental transport in
upper-level winds aloft. In the mid-latitudes transport of pollution
aloft may be impacted by a poleward shift in storm track
pathways. Hence, transport changes alone in the absence of stricter
emissions controls may reduce future air quality in the vicinity of
emission source regions especially in the tropics, due to reductions
in vertical transport and dispersion by deep convection. However, this
study examines the impacts of climate change on transport alone,
whilst future air quality will also be greatly influenced by
climate-driven changes in chemistry and by future changes in
emissions. Future multi-model comparison studies would benefit from
a larger suite of meteorological variables that enable a more detailed
diagnosis of the large-scale dynamical responses to climate
change. Such improved dynamical attribution in tandem with tracer
transport studies will enable better quantification of the response of
global air pollution transport to greenhouse gas warming.
The ACCMIP data used in this study can be obtained
from the British Atmospheric Data Centre (BADC) upon request.
RD, CO, GZ, MP, DP, and ML contributed to conception
and design. GZ, DP, DS, and IM performed ACCMIP simulations with the
UM-CAM, CMAM, GISS-ER-2, and STOC-HadAM3 CCMs respectively. CO, RD,
GZ, and IM contributed to processing of data. All co-authors
contributed to the analysis and interpretation of data. RD and CO
drafted the article aided by revisions by GZ, DP, ML, IM, and OW. All
co-authors approved the submitted version for publication.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Global and regional
assessment of intercontinental transport of air pollution: results from HTAP,
AQMEII and MICS”. It is not associated with a conference.
Acknowledgements
Ruth M. Doherty and Ian A. MacKenzie acknowledge the ARCHER UK National Supercomputing Service
(http://www.archer.ac.uk)
and funding under the UK Natural Environment Research Council grant
NE/I008063/1, NE/M003906. Guang Zeng acknowledges the contribution of NeSI
high-performance computing facilities to the results of this
research. NZ's national facilities are provided by the NZ eScience
Infrastructure and funded jointly by NeSI's collaborator
institutions and through the Ministry of Business, Innovation &
Employment's Research Infrastructure programme
(https://www.nesi.org.nz). ACCMIP is organised under the
auspices of Atmospheric Chemistry and Climate (AC&C), a project
of International Global Atmospheric Chemistry (IGAC) and
Stratospheric Processes And their Role in Climate (SPARC) under the
International Geosphere–Biosphere Programme (IGBP) and World
Climate Research Program (WCRP). The authors are grateful to the
British Atmospheric Data Centre (BADC), which is part of the NERC
National Centre for Atmospheric Science (NCAS), for collecting and
archiving the ACCMIP data and to Jean Francois Lamarque for overall
ACCMIP coordination.
Edited by: Stefano Galmarini
Reviewed by: three anonymous referees
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