Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance of Dust Compared to Sea Surface Temperature Variations, Vegetation Changes, and Greenhouse Gas Warming

TL;DR: In this paper, the role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model.

Abstract: The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed in the African Sahel region in the last half of the twentieth century is investigated using simulations with an atmospheric general circulation model. The model simulations are conducted either forced by the observed sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant precipitation changes are restricted to over North Africa. These changes are considered to be due to the cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase of desert dust when allowing dust to respond to changes in simulated climate, even including changes in vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the direct radiative forcing by the increase of North African dust can explain up to 30% of the observed precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact. The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation.

Summary

1. Introduction

• The rainfall decrease and devastating droughts in the Sahel region during the last three decades of the twentieth century are among the largest recent climate changes recognized by the climate research community (e.g., Dai et al. 2004).
• In general, the studies examine either land–atmosphere interactions or forcing from sea surface temperatures (SSTs).
• This study represents the first time that the relative importance of dust forcing and other mechanisms for the changing Sahel precipitation are examined in the same modeling framework.
• The next section describes the model the authors use and the experimental designs they have performed.
• Section 4 summarizes the responses of precipitation and SSTs to dust radiative forcing at the global scale.

2. Methods

• The National Center for Atmospheric Research’s (NCAR’s) Community Climate System Model version 3 (CCSM3) is a coupled atmosphere, land, ocean, and sea ice model (Collins et al. 2006a).
• The dust model used in this study is described in more detail in Mahowald et al. (2006), including detailed comparisons of the model simulations of dust to available observations.
• The authors chose to use the Sahelian wet-period vegetation incorporating cultivated land based on Ramankutty and Foley (1999) for the simulations in this study.
• When SOM is coupled to CAM3, it simulates SST changes in response to dust radiative effects and atmospheric changes, although changes in ocean circulations are not included in an SOM simulation.
• To evaluate the statistical significance of the final estimates, the authors use confidence intervals.

3. Dust distribution and radiative forcing

• Mahowald et al. (2006) show that the model is able to capture the observed annually averaged optical depth and deposition patterns well.
• The annual average dust optical depth is high over the Sahel region (10°–20°N over Africa) in general and it maximizes over the central to eastern Sahel.
• As a result, the shortwave forcing is greater than the longwave forcing over the North Atlantic, but shortwave and longwave forcings have similar magnitudes over North Africa.
• The authors larger longwave forcing can be explained by their larger particle sizes.
• There may also be effects of differences in surface albedo and vertical dis- tribution of dust.

4. Atmospheric responses to dust radiative forcing

• A. Precipitation responses Figure 3 shows JJAS mean precipitations from observed estimates (Xie and Arkin 1997) and the difference between observed estimates and AMIP-simulated precipitation without dust radiative forcing.
• The precipitation response to dust radiative forcing is much larger, more consistent, and statistically more significant in SOM than AMIP simulations throughout the globe and especially in the Tropics, although the dust is largely confined to the North Africa and North Atlantic regions (Fig. 1).
• The smaller double ITCZ enhancement in the SOM.SL than in SOM.S means that including longwave forcing improves this bias compared to the case with shortwave forcing alone.
• In section 4a, it is shown that the sign of the precipitation response over Africa is the same in S and SL cases despite the fact that surface temperature responses over North Africa are opposite as a result of the opposite signs of the forcings at the surface (see bottom panels of Figs. 2a and 2c).
• Since the AEJ owes its existence to the meridional temperature gradient in the lower troposphere (e.g., Grist and Nicholson 2001), this change in AEJ is consistent with the lower-tropospheric temperature change due to the weakened monsoon and dust radiative cooling.

5. Impacts of dust on Sahel precipitation relative to those of SSTs, vegetation, and GHGs

• Figure 8 shows time series of observed (OBS) and AMIP-simulated precipitation over the Sahel.
• In the following sections, the authors analyze the impacts of changes in SSTs, vegetation, GHGs, and dust radiative forcing on Sahel precipitation by comparing different simulation cases and different periods as summarized in Table 3.
• These are calculated as differences between AMIP.SL and AMIP.ND over the entire period (1951–93) and between SOM.SL and SOM.ND over all 30 yr, respectively.
• SLVH simulation, dust emission from North Africa is reduced by a half in the wet period and the resulting increase of dust optical depth from wet to dry period over North Africa is by close to threefold (173% increase between wet and dry periods and 183% increase between least and most dusty 10-yr periods).
• In summary, their model results suggest that up to 50% of the observed precipitation change can be due to sea surface temperature changes, 10% from vegetation changes, and up to 30% due to dust radiative forcing and feedbacks.

6. Conclusions

• This study investigates the role of dust radiative forcing in the Sahelian drought observed in the last three decades of twentieth century using simulations with an atmospheric general circulation model (GCM).
• The authors model shows smaller shortwave and larger longwave forcing of dust than other studies.
• This bias is stronger when longwave forcing is neglected as in the default version of the CAM3 model.
• If dust emissions in the 1950–60s are reduced by half, which is approximately equivalent to a threefold increase of dust loading in the North Africa and North Atlantic regions, the Sahel precipitation response is estimated to be about 30% of the observed precipitation change between the 1950s and 1980s.
• The GHG warming acts to increase Sahel precipitation and hence does not explain the observed drying.

Figures

TABLE 1. List of simulations.

FIG. 7. (a), (b) Same as in Figs. 6a,b, but for atmospheric circulation (shading shows zonal wind component and arrows meridional and vertical components).

FIG. 2. (Continued)

FIG. 4. (Continued)

FIG. 2. (Continued)

FIG. 4. (Continued)

FIG. 3. (top) Observed JJAS precipitation and (bottom) difference between observed and AMIP.ND precipitations (1979–93) (mm day 1). The bottom panel shows statistically significant differences at 99% level only.

FIG. 8. Annual rainfall trend in the Sahel (10°–20°N, 18°W–20°E) in observations (black), AMIP.ND (blue solid), AMIP.NDV (blue dashed), AMIP.S (yellow), AMIP.SL (red solid), AMIP.SLV (red dashed), AMIP.SLVH (red dotted), AMIP.F (green solid), and AMIP.H (green dashed) simulations. (Precipitation values are 10-yr running means. For example, the abscissa value of 55 represents the period from 1951 to 1960.)

FIG. 6. (a) Meridional cross section of JJAS air temperature in (top) ECMWF and (bottom) AMIP.ND – ECMWF averaged over 15°W–15°E (abscissa is latitudes). (b) Meridional cross section of JJAS air temperature changes due to shortwave and longwave forcing by dust in (top) AMIP (AMIP.SL–AMIP.ND) and (bottom) SOM (SOM.SL–SOM.ND) simulations averaged over 15°W–15°E (abscissa is latitudes).

TABLE 2. Areal and annual averaged radiative forcing of prognostic dust in the SOM.SP simulation (top four rows) and from other studies (bottom four rows). TOA top of atmosphere, ATM within atmosphere, SFC surface, DOD dust optical depth at 670 m (W m 2). Positive downward at TOA and SFC and positive for absorption in ATM.

FIG. 2. (Continued)

TABLE 3. Differences of Sahel precipitation in different simulations and periods. D10 (W10): average for driest (wettest) 10-yr period; DP (WP): average for dry (wet) period [1971–93 (1951–70)]; ALL: average for the whole period of simulation: 1951–93 in AMIP, 9 yr for SOM.SP2, and 30 yr for all other SOM simulations. OD: dust optical depth at 670 nm averaged over North Africa; OD: change in dust optical depth.

FIG. 1. (a) Annual (top) mean optical depth and meridional cross sections of extinction cross section at (bottom left) 40°W and (bottom right) at the prime meridian of prognostic dust at 670 nm (case SOM.SP). (b) Same as in (a), but for the Sahel rainy season (JJAS).

FIG. 5. Zonal mean JJAS (left) observed and SOM simulated precipitations and (right) simulated changes due to dust radiative forcing averaged over (top) all global longitudes and (bottom) 15°W–15°E. Observations are averaged for 1979–2001 and the simulation mean is for all 30 yr.

Full text

UC Irvine
Faculty Publications
Title
Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance of Dust
Compared to Sea Surface Temperature Variations, Vegetation Changes, and Greenhouse Gas
Warming
Permalink
https://escholarship.org/uc/item/4ft4n4nj
Journal
Journal of Climate, 20(8)
ISSN
0894-8755 1520-0442
Authors
Yoshioka, Masaru
Mahowald, Natalie M
Conley, Andrew J
et al.
Publication Date
2007-04-01
DOI
10.1175/JCLI4056.1
Copyright Information
This work is made available under the terms of a Creative Commons Attribution License,
availalbe at https://creativecommons.org/licenses/by/4.0/
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University of California

Impact of Desert Dust Radiative Forcing on Sahel Precipitation: Relative Importance
of Dust Compared to Sea Surface Temperature Variations, Vegetation Changes, and
Greenhouse Gas Warming
MASARU YOSHIOKA AND NATALIE M. MAHOWALD
Donald Bren School of Environmental Science and Management, and Institute of Computational Earth System Science, University of
California, Santa Barbara, Santa Barbara, California, and Climate and Global Dynamics Division, National Center for Atmospheric
Research,* Boulder, Colorado
ANDREW J. CONLEY AND WILLIAM D. COLLINS
Climate and Global Dynamics Division, National Center for Atmospheric Research,* Boulder, Colorado
DAVID W. FILLMORE
Climate and Global Dynamics Division, National Center for Atmospheric Research,* and Program in Atmospheric and Oceanic
Sciences, University of Colorado, Boulder, Colorado
CHARLES S. ZENDER
Department of Earth System Science, University of California, Irvine, Irvine, California
DANI B. COLEMAN
Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder,* Colorado
(Manuscript received 26 January 2006, in final form 3 July 2006)
ABSTRACT
The role of direct radiative forcing of desert dust aerosol in the change from wet to dry climate observed
in the African Sahel region in the last half of the twentieth century is investigated using simulations with an
atmospheric general circulation model. The model simulations are conducted either forced by the observed
sea surface temperature (SST) or coupled with the interactive SST using the Slab Ocean Model (SOM). The
simulation model uses dust that is less absorbing in the solar wavelengths and has larger particle sizes than
other simulation studies. As a result, simulations show less shortwave absorption within the atmosphere and
larger longwave radiative forcing by dust. Simulations using SOM show reduced precipitation over the
intertropical convergence zone (ITCZ) including the Sahel region and increased precipitation south of the
ITCZ when dust radiative forcing is included. In SST-forced simulations, on the other hand, significant
precipitation changes are restricted to over North Africa. These changes are considered to be due to the
cooling of global tropical oceans as well as the cooling of the troposphere over North Africa in response to
dust radiative forcing. The model simulation of dust cannot capture the magnitude of the observed increase
of desert dust when allowing dust to respond to changes in simulated climate, even including changes in
vegetation, similar to previous studies. If the model is forced to capture observed changes in desert dust, the
direct radiative forcing by the increase of North African dust can explain up to 30% of the observed
precipitation reduction in the Sahel between wet and dry periods. A large part of this effect comes through
atmospheric forcing of dust, and dust forcing on the Atlantic Ocean SST appears to have a smaller impact.
The changes in the North and South Atlantic SSTs may account for up to 50% of the Sahel precipitation
reduction. Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect
is statistically insignificant because of the small number of years in the simulation. Greenhouse gas warming
* The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Corresponding author address: Masaru Yoshioka, School of Geographical Sciences, University of Bristol, University Road, Bristol,
BS8 1SS, United Kingdom.
E-mail: m.yoshioka@bristol.ac.uk
15 A
PRIL 2007 Y O S H I O K A E T A L . 1445
DOI: 10.1175/JCLI4056.1
© 2007 American Meteorological Society
JCLI4056

seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although
the estimated values of impacts are likely to be model dependent, analyses suggest the importance of direct
radiative forcing of dust and feedbacks in modulating Sahel precipitation.
1. Introduction
The rainfall decrease and devastating droughts in the
Sahel region during the last three decades of the twen-
tieth century are among the largest recent climate
changes recognized by the climate research community
(e.g., Dai et al. 2004). The large temporal and spatial
coherence of the dry (and wet) conditions are the ex-
ceptional characteristics of rainfall variability in the Sa-
hel (Nicholson and Grist 2001). A number of studies
have investigated the possible causes or mechanisms of
the Sahelian drought in the last 30 yr. In general, the
studies examine either land–atmosphere interactions or
forcing from sea surface temperatures (SSTs). The land–
atmosphere investigators include Xue and Shukla
(1993), Xue (1997), Clark et al. (2001), Taylor et al.
(2002), and Xue et al. (2004), and they demonstrate that
land surface degradation leads to reduced precipitation
in North Africa using numerical simulations. The forc-
ing studies by Folland et al. (1986), Lamb and Peppler
(1992), Rowell et al. (1995), Bader and Latif (2003),
Giannini et al. (2003), and Hoerling et al. (2006) show
that large-scale sea surface temperature patterns are
closely related to precipitation patterns over North Af-
rica. Many of these studies have identified the role of
the interhemispheric SST contrast (i.e., SST difference
between Northern and Southern Hemispheres) in the
Atlantic basin as the most important mechanism, and
the dry condition in Sahel is associated with lower
North and higher South Atlantic SSTs compared to the
climatological mean. Xue and Shukla (1998), Zeng et
al. (1999), and Wang et al. (2004) argue that both the
land–atmosphere interaction and SST control are re-
quired to reproduce the magnitude and duration of ob-
served precipitation variability.
On the other hand, some authors such as Nicholson
(2000) and Prospero and Lamb (2003) hypothesize that
dust may play a role in the changes in Sahel climate.
They note that dust radiative forcing is significant in
this region and can modulate the interhemispheric SST
gradient that may be responsible to the precipitation
variability in the Sahel. Dust can also provide a mecha-
nism necessary for the interannual coherence since dust
entrainment into the atmosphere is limited by vegeta-
tion, which is correlated with precipitation in the pre-
vious year (e.g., Prospero and Lamb 2003). Miller and
Tegen (1998) and Miller et al. (2004, hereafter MTP04)
have examined the climatic effects of dust using general
circulation model (GCM) simulations and found pre-
cipitation reductions over the tropical North Atlantic
and adjacent continental areas including the Sahel and
Guinea Coast regions. They emphasize the importance
of surface radiative forcing in modulating hydrologic
cycles.
This study investigates the possible effects of direct
radiative forcing of dust on Sahel precipitation through
its impacts on the atmosphere and the surface. We also
characterize relative roles of dust and other processes
such as SSTs, vegetation change, and greenhouse gas
(GHG) warming using simulations with an atmospheric
general circulation model. This study represents the
first time that the relative importance of dust forcing
and other mechanisms for the changing Sahel precipi-
tation are examined in the same modeling framework.
The next section describes the model we use and the
experimental designs we have performed. Section 3
presents the radiative forcing of dust simulated in the
model and compares it with previous studies. Section 4
summarizes the responses of precipitation and SSTs to
dust radiative forcing at the global scale. Section 5 in-
vestigates the Sahel precipitation responses to SSTs,
vegetation change, greenhouse gas warming, and dust
radiative forcing, and evaluates their relative roles. Sec-
tion 6 summarizes our findings and concludes the study.
2. Methods
The National Center for Atmospheric Research’s
(NCAR’s) Community Climate System Model version
3 (CCSM3) is a coupled atmosphere, land, ocean, and
sea ice model (Collins et al. 2006a). This model is used
for simulating past, present, and future climate changes,
such as for Houghton et al. (2001). Here we describe
the mineral aerosol and surface vegetation changes in-
corporated in the Community Land Model version 3
(CLM3; Dickinson et al. 2006) and Community Atmo-
sphere Model version 3 (CAM3; Collins et al. 2006b) to
examine the effects of dust, sea surface temperature,
vegetation change, and greenhouse gases.
a. Dust modeling
The dust model used in this study is described in
more detail in Mahowald et al. (2006), including de-
tailed comparisons of the model simulations of dust to
available observations. The model resolution is about
1446 JOURNAL OF CLIMATE VOLUME 20

2.8° horizontally near the equator (T42) and 26 levels
vertically. The dust source mechanism follows the Dust
Entrainment and Deposition Module (Zender et al.
2003a) and work conducted in the offline Model of At-
mospheric Transport and Chemistry (MATCH; Ma-
howald et al. 2002, 2003; Luo et al. 2003; Mahowald and
Luo 2003). The sources of dust are assumed to be dry,
unvegetated regions subject to strong winds. The mag-
nitude of the dust source is calculated within the CLM3.
In the default version of the model, the satellite veg-
etation climatology of vegetation is the same as that
used for other land surface calculations in the model
(Bonan et al. 2002). Sensitivity studies using modified
vegetation data are described below. When the total
leaf area index plus the stem area index is below 0.1, the
area of the grid box available for dust generation is
assumed to increase linearly with decreasing vegetation
cover (Mahowald et al. 2006).
The source scheme parameterizing dust entrainment
into the atmosphere is described in detail in Zender et
al. (2003a). The model calculates a wind friction thresh-
old velocity based on the surface roughness and soil
moisture, and dust is entrained into the atmosphere
when the friction velocity exceeds this threshold. The
model assumes that the optimum size distribution of
soil particles is available for saltation and subsequent
vertical flux (⬃75
␮
m). However, after the dust flux is
calculated, the dust source magnitude is multiplied by a
soil erodibility factor to include the impact of differ-
ences in soil size and texture following the “preferential
source” concept (Ginoux et al. 2001). We use the geo-
morphic soil erodibility factor described by Zender et
al. (2003b). A wind friction threshold is calculated fol-
lowing Iversen and White (1982). This threshold is
modified for two different processes in the model. Fol-
lowing Fecan et al. (1999), the threshold wind friction
velocity increases with increasing soil moisture. The
fetch of the winds over this erodible surface is allowed
to modify the wind friction velocity threshold as well
(Gillette and Passi 1988). Once the wind friction thresh-
old velocity is calculated, the horizontal saltation fluxes
are calculated (White 1979); vertical fluxes are a small
fraction of the horizontal flux (Marticorena and Berga-
metti 1995). In this scheme, the effects of subgrid-scale
variation of wind in dust emission and drying of top soil
(affecting threshold velocity) by strong wind will not be
simulated.
We use four aerosol size bins with boundaries at 0.1,
1.0, 2.5, 5.0, and 10.0
␮
m in diameter. The transported
aerosols are assumed to have a subbin distribution
based on a lognormal distribution just like Zender et al.
(2003a; see Fig. 1) although we use a mass median di-
ameter (MMD) of 3.5
␮
m as reported by Reid et al.
(2003), which is larger than the value used in Zender et
al. (2003a; 2.5
␮
m). Hand et al. (2004) have shown the
model using MMD of 2.5
␮
m underestimates fine and
overestimates coarse particles (Hand et al. 2004, p. 16),
and Arimoto et al. (2006) recently reported even larger
MMD of 5.5
␮
m for the long-range transported Asian
dust. Mass fractions at source in the four bins are 3.8%,
11%, 17%, and 67% as predicted in Grini and Zender
(2004) using the saltation–sandblasting model. This size
distribution tends toward larger particles than many
previous studies (e.g., Tegen and Fung 1994; Ginoux et
al. 2001; Mahowald et al. 2002; Zender et al. 2003a; Luo
et al. 2003; MTP04).
Deposition processes include dry gravitational set-
tling, turbulent dry deposition, and wet deposition dur-
ing precipitation events. Both dry depositional pro-
cesses are modeled using parameterizations described
in Zender et al. (2003a), with a mass flux advection
scheme in order to parameterize vertical fall rates cor-
rectly (Rasch et al. 2001; Ginoux 2003). Wet deposi-
tional processes are parameterized within the CAM3
similar to Rasch et al. (2001). The dust determined us-
ing the processes described above is referred to as the
prognostic dust (prescribed dust is described below).
Shortwave radiative effects are calculated within
CAM3 every hour. A delta-Eddington approximation
is adopted for the shortwave using 19 discrete intervals
(Collins 1998) for each vertical layer in the model (Col-
lins et al. 2004). Longwave effects are calculated every
12 h in CAM3, which uses an absorptivity/emissitivity
formulation for longwave heating (Ramanathan and
Downey 1986). A broadband approach with seven
bands is used, which accounts for the water vapor win-
dow regions (Collins et al. 2002). The indices of refrac-
tion have been derived from Patterson (1981) for the
visible wavelengths, Sokolik et al. (1993) for the near
infrared, and Volz (1973) for the infrared. The imagi-
nary part of indices of refraction in the visible wave-
length were scaled to match the new estimates of
Sinyuk et al. (2003) and Dubovik et al. (2002) for the
region 0.33 to 0.67
␮
m. These estimates are based on
satellite- and surface-based field observations and may
still be susceptible to biases due to measurement errors
and contaminations. Scattering of longwave radiation
by dust is neglected in radiative calculations. This may
lead to underestimates of longwave radiative forcing by
up to 50% at top of the atmosphere (TOA) and 15% at
the surface (SFC; Dufresne et al. 2002).
b. Vegetation change
We do not include the Dynamic Global Vegetation
Model (DGVM) version of the CLM3 because when
coupled to the CAM3, it produces unrealistically large
15 APRIL 2007 Y O S H I O K A E T A L . 1447

dust plumes (not shown) due to dry biases in both
CAM3 and CLM3 (Collins et al. 2006a; Dickinson et al.
2006). However, vegetation dynamics is thought to
have a strong control on precipitation and dust emis-
sions. Vegetation itself is controlled by precipitation
during previous months and years. For example, the
observed increase of dust in the Atlantic since the 1970s
may be partly due to vegetation loss near the Sahara
FIG. 1. (a) Annual (top) mean optical depth and meridional cross sections of extinction
cross section at (bottom left) 40°W and (bottom right) at the prime meridian of prognostic
dust at 670 nm (case SOM.SP). (b) Same as in (a), but for the Sahel rainy season (JJAS).
1448 JOURNAL OF CLIMATE VOLUME 20